«A Microquasar Odyssey: Unveiling the Complexities»

15 Sept 2025, 09:00 → 19 Sept 2025, 16:00 Europe/Rome

Palazzo Municipale - Sala delle Capriate (Cefalù (Palermo))

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The 11th Microquasar Workshop «A Microquasar Odyssey: Unveiling the Complexities»  will take place at Cefalù, Sicily, Italy, on 15-19 September 2025.

Similarly to the past Microquasar Workshops, the aim is to gather theorists, observers and specialists of the field, discussing the latest results and reviewing the current state of our understanding of the physics of stellar mass compact objects in the context of the instruments devoted to this science (such as XRISM, IXPE, NICER, NuSTAR, MeerKAT, etc.).

Large dedicated time will be devoted to the following specific points:

• Multi-wavelength observations: broad band SED, (high resolution X-ray) spectroscopy, (X-ray) polarimetry ;

• Accretion-ejection connection;

• Outflows (jet, winds) signatures, properties and modelling;

• Formation, evolution and merging;

• Temporal behavior and Timing properties.

 

The program will be organised in 8 sessions centred around a limited number of invited reviews, with contributed presentations (oral and flash talks 2 min long) and time for discussion.

The number of participants cannot exceed 120, because of the size of the conference room. Note that Cefalù is a very popular tourist destination, therefore we strongly recommend that you book your flight and hotel as soon as possible. 

On 14 September 2025, registration will take place at the Sala Ottagonale of City Hall from 18:00 to 22:00. A welcome cocktail will be offered to participants as a warm reception.

Poster Poster_ MQW_2025.pdf

Monday 15 September

09:00 → 09:15 Welcome

09:15 → 11:00 Broadband Spectroscopy (modelling and observation)

Convener: Nick Kylafis (Institute of Astrophysics, FORTH, Crete)

09:15 Broadband Spectroscopy of Microquasar Systems

I will present an overview of broadband spectral data for microquasar systems, with a focus on black hole X-ray binaries. In particular, I will describe different broadband X-ray spectral data sets which are fitted to infer properties of the disk, corona, and/or jet, including e.g., the accretion geometry or black-hole spin. I will discuss the connection of X-ray spectra to other wavelengths and describe prospects for advancement in our understanding of these systems with current and next-generation spectroscopic capabilities.

Speaker: James ("Jack") Steiner (Smithsonian Astrophysical Observatory)

09:45 Broadband spectra of microquasars: a modelling perspective

Microquasars, with their complex interplay of accretion disks, relativistic jets, and coronae, emit across the electromagnetic spectrum. This talk will review the ongoing attempts to perform broadband, radio to gamma-ray, spectral decomposition in microquasars. I will summarise the key observables that help us distinguishing between different spectral models and physical components.

Speaker: Alexandra Veledina (University of Turku)

10:15 Understanding the Enigmatic Nature of the Transient X-ray Binary 4U 1630–47

4U 1630-47 is a peculiar transient X-ray binary believed to host a black hole as its compact object. Its behavior differs from that of other BH-XRBs, as it shows recurrent outbursts with a fairly regular recurrence period and lacks bright hard states. In contrast, its soft-state spectra above 10 keV are characterized by a strong power-law component extending up to 200 keV without any cutoff.
A large multiwavelength campaign was carried out during the 2022/2023 outburst, involving NICER, NuSTAR, INTEGRAL, and IXPE observations.
In particular, IXPE revealed a higher level of polarization than typically observed in other X-ray binaries which is difficult to explain as the result of scattering, even for a high-inclination system or a highly asymmetric corona.
Here, I will review the main characteristics of this enigmatic source and discuss possible explanations for its peculiar behavior in light of the new spectro-polarimetric results.

Speaker: Fiamma Capitanio (Istituto Nazionale di Astrofisica (INAF))

10:30 Multi-wavelength outburst evolution of the black hole transient Swift J1727.

I will present the results of a unique multi-wavelength campaign focused on the recently discovered black-hole transient Swift J1727. This system exhibited the brightest X-ray source in the sky ever observed, and it was the first to be observed with the X-ray polarimeter (IXPE) throughout an entire outburst. Our observations covered simultaneously the full frequency range from X-ray to radio and were provided by observatories including HST, XMM, VLT/X-Shooter, NICER, and MeerKAT. Totalling 12 different visits throughout the outburst, we covered all the distinct spectral states, including the hard-to-soft transition and back to the low-hard state.
Our simultaneous spectroscopy covering from the far-UV to the near-IR reveals the presence of a massive multi-phase outflow emerging during the soft-to-hard transition from the inner parts of the disk, signalling a large-scale change in the structure of the disk triggered by the state transition. The modelling of the observed spectral energy distribution allowed us to infer the geometry of the system and track the evolution of the different emitting regions throughout the entire outburst. I will put these results in context with the X-ray polarimetric observations and compare them with previous multi-wavelength campaigns of low-mass X-ray binaries (LMXBs; e.g. Maxi 1820 and Swift J1858).

Speaker: Noel Castro Segura (University of Warwick)

10:45 Aql X-1 "from dawn 'til dusk": observing an entire outburst from early rise to quiescence with Einstein Probe

Outbursts from transient Low-Mass X-ray Binaries (LMXBs) have been historically caught by All-Sky X-ray monitors at relatively high X-ray luminosities, i.e. above ~1e36 erg/s; observations of these sources during the early rise of the outbursts, the stage where the X-ray luminosity increases by three or more orders of magnitude with respect to the quiescence regime, have been so far very limited. However, the launch of the Einstein Probe (EP) has significantly improved our ability to detect fainter X-ray activity, unlocking access to the outburst early rise. In September 2024, EP detected the early onset of a new outburst from the archetypal neutron star LMXB Aql X-1, catching the source at a luminosity below 1e35 erg/s. I will present results from a comprehensive, multi-wavelength campaign following this event, combining data from EP, NICER, and NuSTAR, covering the full outburst from its early rise through its return to quiescence. By comparing X-ray and optical light curves during the initial rise, we show that the outburst proceeds on similar timescales at different wavelengths. Time-resolved X-ray spectroscopy allows us to track how the geometry and the physical properties of the accretion flow evolve during the outburst and across the canonical spectral states - hard, intermediate and soft. In particular, our data show that in the rise, the thermal emission comes from an extended boundary layer between the disk and the neutron star, while during the decay it likely originates from a small hot spot on the neutron star surface.

Speaker: Alessio Marino (Insitute of Space Sciences (ICE-CSIC))

11:00 → 11:30 Coffee Break

11:30 → 13:00 Broadband Spectroscopy (modelling and observation)

Convener: Melania Del Santo (Istituto Nazionale di Astrofisica (INAF))

11:30 Completing the accretion-ejection picture of MAXI J1348-630 from a multiwavelength perspective

MAXI J1348-630 is a black hole X-ray transient discovered in January 2019 that underwent a reflare and several mini-outbursts at the end of its main outburst. In this talk, we present the first multiwavelength study of the system during outburst, reflare and mini-outbursts based on the long-term optical monitoring of the system with the Las Cumbres Observatory (LCO) network. The full optical coverage, including mini-outbursts, represents one of the best-sampled datasets for black hole transients. We show that the outer parts of the accretion disc dominate the optical emission of MAXI J1348-630. In particular, the slope in the spectral energy distributions and the colour-magnitude diagram suggest that the optical emission originates from viscous-thermal processes, with a spectral index of ~0.3. However, the optical/X-ray flux correlation is consistent with both viscous-thermal emission and that expected from an irradiated accretion disc (slope ~0.3-0.4). We also show that during the decay of the reflare and mini-outbursts, the disc temperature is below 7000 K, which suggests that we are observing the cooling front propagate through the disc. Finally, we show that the radio/optical correlation, one of the first reported for an X-ray binary, is steeper than expected. Since during the reflare and mini-outbursts the source remained in the hard state, these results will help to constrain models of mini-outbursts and failed-transition outbursts.

Speaker: Kevin Alabarta Jativa (New York University Abu Dhabi)

11:45 What are the spins of stellar-mass black holes?

In recent years, spins of merging black holes have been relatively accurately measured based on their gravitational-wave signals. Their are generally low, with the estimated average effective spin parameter as low as 0.06. On the other hand, spins of many accreting black-hole binaries have been measured to be high, some close to the maximum spin parameter of 1, e.g. greater than 0.9985 at 3 sigma in Cyg X-1. I will present our recent results regarding this discrepancy. In particular, I will discuss possible systematic effects affecting the spin measurements in accreting systems and whether they can be reconciled with those based on gravitational waves.

Speaker: Mr Andrzej Zdziarski (N. Copernicus Astronomical Center)

12:00 A Self-Shadowing Black Hole Accretion Disc during a Soft State Decay

The geometry and physical conditions of black hole accretion discs are expected to change dramatically as a function of mass accretion rate and the resulting X-ray luminosity. Theoretical models predict that these changes become particularly relevant as the luminosity approaches the Eddington limit (L_Edd), a regime where the standard thin-disc solution is thought to break down in favour of alternative configurations, many of which predict vertically extended, radiation-pressure-supported structures in the inner regions. However, the threshold for this transition, and its observational signatures, remain poorly constrained.

I will present simultaneous X-ray, optical, and near-infrared spectroscopic observations during the decay of a black hole outburst, focusing on the soft accretion state. Below ~0.1L_Edd, the optical and infrared spectra are consistent with a highly irradiated outer disc, characterised by weak high-excitation emission. In contrast, a substantially cooler spectrum is observed at the outburst peak (~0.2L_Edd), dominated by strong double-peaked recombination lines of hydrogen and helium. This behaviour can be explained by self-shadowing effects, which shield the outer disc (where these low-ionisation lines form) from the high-energy emission produced near the black hole. In this context, the inner accretion flow would deviate from the standard thin-disc structure already at moderate luminosities, providing an observational link between inner flow geometry and outer disc spectral behaviour in stellar-mass black holes. I will discuss how these structural changes can leave a measurable imprint far beyond their immediate surroundings, with important implications for our understanding of spectral variability, disc winds, and state transitions in accreting black holes.

Speaker: Dr Montserrat Armas Padilla (Instituto de Astrofísica de Canarias (IAC))

12:15 Constraining the geometry of the corona with relativistic reflection modeling

The irradiation of the innermost accretion disk by X-rays emitted by the corona produces characteristic reflection features. Besides depending on the density and ionization of the disk itself, also the spin of the black hole and the corona itself is imprinted on the reflection spectrum due to strong relativistic effects. When fitting observational data of black hole X-ray binaries and AGN, relativistic reflection models assuming a point-like lamp post corona were very successful, which is challenged by recent polarization measurements. We present a new reflection model in the relxill framework, which implements a radially extended disk-like corona. We show how a more self-consistent approach to relativistic reflection modeling allows to better constrain the accretion flow, including the geometry of the corona. This self-consistent approach means that starting from the corona we directly predict the continuum emission and the irradiation of the accretion disk, including the ionization gradient and the resulting reflection including returning radiation. To obtain constrains on the radial extent of the corona, the newly developed model is applied to observed spectra of well known black holes with strong reflection

Speaker: Thomas Dauser (Remeis Observatory & ECAP, FAU)

12:30 Is there disk truncation in the hard states of Swift J1727.8-1613?

The 2023/24 NICER monitoring campaign of the 7 Crab bright black hole X-ray binary Swift J1727.8-1613 covered the outburst in almost all states. High-quality data are available in the high-Eddington-fraction bright hard state, the soft intermediate state, the soft state decay, and the poorly understood back-transition into the dim hard state, making it an ideal dataset to compare the accretion flow at vastly different accretion rates. We deploy continuum fitting techniques to investigate the evolution of the inner disk radius throughout the outburst. Taking a temperature-dependent color-correction factor into account, we see apparent evolution of the disk inner radius by a factor of a few comparing the hard states to the thermal/soft state; we are investigating to discern whether this trend is real or a modeling artifact, and how it differs between the bright and dim hard state. Large-scale truncation with r >> r_ISCO is incompatible with our results. In the high-luminosity soft-intermediate state, standard thin-disk models do not fit the data well, which may be an indication of "slim-disk" manifestations. I will discuss model systematics, the impact of these constraints on the hard state disk truncation debate, and the IXPE measurements of this source.

Speaker: Ole Koenig (Center for Astrophysics | Harvard & Smithsonian)

12:45 Sub-second vibrations from a vertical corona in a black-hole transient.

The geometry of the accretion flow around accreting black holes is highly variable and remains highly unconstrained. In their hard states, the X-ray spectrum is dominated by a hot inflow located above or within the accretion disk. Conversely, in their soft states, the spectrum is dominated by soft emission from the geometrically thin accretion disk. The geometry of the hot inflow and its evolution across the hard-to-soft transition are still debated. Correlated fast multilambda variability can be a powerful tool for studying the physics of accretion and ejection in X-ray binaries, providing quantitative constraints that X-ray data alone cannot offer. Here, we report the detection of a characteristic variability feature observed during the hard-to-soft transition of the accreting black hole GX 339-4, simultaneously at X-ray and infrared wavelengths. Supported by an extensive multi-wavelength campaign, these data allow us to constrain the geometry of the emitting region, suggesting the presence of a vertical structure.

Speaker: Piergiorgio Casella (Istituto Nazionale di Astrofisica (INAF))

13:00 → 13:14 Broadband Spectroscopy (modelling and observation)

Convener: Melania Del Santo (Istituto Nazionale di Astrofisica (INAF))

13:00 High-density reflection spectroscopy of black X-ray binaries and its implication of accretion geometry

X-ray reflection spectroscopy is a powerful tool to study the accretion geometry near black holes and spacetime properties. The spirit of this method is to study the reprocessed corona emission by the cold accretion disk that extends close to the black hole. Previous X-ray reflection models consider a disk density of 10^-15 cm^-3, which is only appropriate for very massive black holes (> 10^7 M_Sun). In this talk, I will present a systematic study of high-density reflection effects for black hole XRBs. The study also provides insights into their accretion geometry in the bright hard and intermediate states.

Speaker: Dr Honghui Liu (Institute for Astronomy and Astrophysics, University of Tuebingen)

13:02 RTFAST-S: emulation of x-ray reflection around black holes

Bayesian analysis has begun to be more widely adopted in X-ray spectroscopy, but it has largely been constrained to relatively simple physical models due to limitations in X-ray modelling software and computation time. As a result, Bayesian analysis of numerical models with high physics complexity have remained out of reach. This is a challenge, for example when modelling the X-ray emission of accreting black hole X-ray binaries, where the slow model computations severely limit explorations of parameter space and may bias the inference of astrophysical parameters. We present RTFAST-Spectra: a neural network emulator that acts as a drop-in replacement for the spectral portion of the RTDIST flavour of the reltrans black hole X-ray reverberation model suite. This is the first emulator for the reltrans model suite and the first emulator for a state-of-the-art x-ray reflection model incorporating relativistic effects with 17 physically meaningful model parameters. We use Principal Component Analysis to create a light-weight neural network that is able to preserve correlations between complex atomic lines and simple continuum, enabling consistent modelling of key parameters of scientific interest. We achieve a O(100) times speed up over the original model in the most conservative conditions with O(1%) precision over all 17 free parameters in the original numerical model, taking full posterior fits from months to hours on a laptop.

Speaker: Benjamin Rickett

13:04 A new connection between the accretion disk and corona revealed by simultaneous NICER and HXMT observations

The X-ray flux from black hole X-ray binaries in the hard state is highly variable on a broad range of timescales. On a timescale of seconds, such broadband noise is thought to arise in the turbulent accretion disk and propagated to the corona, which emits harder X-rays in the spectral shape of a powerlaw with a cut-off. The nature and geometry of the corona is subject to debate, but there is general consensus that its emission arises from Compton-upscattering of seed photons. In this picture, the cut-off represents the coronal temperature. If the accretion disk is the main source of seed photons, it may modulate the coronal temperature as the number of seed photons and heating of the corona varies.

To test this scenario, we analysed the variability properties of black hole X-ray binary MAXI J1820+070 with simultaneous NICER and Insight-HXMT observations to study disk-corona interactions across an unprecedented X-ray energy range of 0.3-250 keV. We also show simulations to help interpret the data and find that the disk modulates the coronal cut-off energy. We discuss possible explanations for our findings, concluding that the complex variability places novel constraints on the origin of the hard X-ray emission from the corona.

Speaker: Niek Bollemeijer (University of Amsterdam)

13:06 Accretion Disk evolution across State Transitions in GX 339–4: Broad-Band Spectral Insights from NuSTAR, NICER, and Insight–HXMT

I will present results from a coordinated spectral study of the black hole X-ray binary GX 339–4 during its 2021 outburst, capturing its transition from the hard to soft accretion state. Using simultaneous observations from NuSTAR, NICER, and Insight–HXMT, we applied relativistic reflection models to track changes in the accretion disk, particularly the inner disk radius (𝑅𝑖𝑛). Interestingly, in models with only a hard Comptonization component, 𝑅𝑖𝑛 appears smaller in the hard state than in the soft—contrary to standard expectations. However, when we include a warm Comptonization component, this trend reverses, revealing a physically consistent picture with a larger 𝑅𝑖𝑛 in the hard state. Assuming the disk extends to the ISCO in the soft state, we estimate the black hole mass as a function of distance. Across all cases, we observe a clear increase in both accretion rate and Eddington ratio as the source softens. These findings highlight the importance of spectral model choice and demonstrate the value of broad-band coverage for understanding accretion physics during state transitions.

Speaker: Ruchika Dhaka (Indian Institute of Technology Kanpur, India)

13:08 X-ray spectral and timing evolution during the 2023-2024 outburst of GX 339-4

GX 339–4 is a low-mass X-ray binary often used as an archetype, showing typical source behaviors. The source undergoes a full outburst every 2-3 years, passing through all known accretion
states. The 2023-2024 outburst was monitored as part of a multiwavelength campaign, which
included X-ray observations by IXPE and NICER. We
used NICER data to analyze the spectral-timing properties of GX 339-4 and compare them
to those of previous outbursts. This spectral-timing information is used to characterize the
evolution of the source during the outburst and to search for time-lags in the X-ray light curves.
We will present the preliminary results of this analysis.

Speaker: Mr Federico Ferretti (Università degli Studi Roma Tre)

13:10 nDspec: a new Python-based modelling software for multi-dimensional model fitting

The wealth and complexity of X-ray data provided by modern observatories has seen a dramatic improvement in recent years, in part due to new facilities like NICER and IXPE. This trend will accelerate further with new techniques like polarimetry timing, as well as future missions like eXTP and Athena. On the other hand, the software modelling tools utilized by the community to model spectral timing data are mostly legacy stand-alone packages tailored to one-dimensional spectral analysis. In mu talk, I will discuss the development and alpha release of nDspec, a new Python-based modeling package designed to enable Xspec-style forward folding of models with multi-dimensional data in a computationally efficient fashion, and at the same time enabling users to utilize modern inference libraries for model fitting. I will demonstrate a sample application to a set of exising NICER data, and highlight how nDspec allows users to easily constrain a model impulse response function. Finally, I will discuss planned features for the software and the development time table.

Speaker: Matteo Lucchini (University of Amsterdam)

13:12 the dimming GRS 1915+105 observed with NICER

The BHB GRS~1915+105 was bright for 26 years since its discovery and is well known for its disk instabilities, quasi-periodic oscillations, and disk wind signatures. We report a long-term spectral-timing tracing of this source from mid-2017 until the onset of the so-called obscured state based on the complete data from NICER and other potential X-ray missions. In the soft state predating 2018, we observed highly ionized winds. However, in the hard state shortly before transitioning into the obscured state on May 14, 2019 (MJD 58617), the winds exhibited a discernible reduction in ionization degree ($\log \xi$), which decreased from above 4 to approximately $3$. Our analysis involves the measurement of the frequencies of the quasi-periodic oscillations and the estimation of the properties of the ionized winds and the intensities of different spectral components through spectroscopy during the decay phase. We studied the origin of these infrequently observed warm outflows in the hard state. The launching radius of the winds in the hard decay phase is similar to that in the soft state, which indicates that the launching mechanism of these winds is likely the same in both states. The presence of the ionized winds is preferentially dependent on the periphery of the accretion disk, but it is not directly related to the coronal activities in the center of the binary system.

Speaker: Menglei Zhou (Institut für Astronomie & Astrophysik (IAAT), Universität Tübingen, Germany)

13:14 → 16:00 Lunch

16:00 → 17:30 Polarimetry (X-ray, Optical, Radio…)

Convener: Dr Michal Dovciak (Astronomical Institute of the Czech Academy of Sciences)

16:00 X-ray polarimetry in Microquasars

X-ray polarimetry provides a powerful new window into the geometry and physical conditions of the innermost regions of microquasars. In this review, I will present recent results from the IXPE mission that have revealed the X-ray polarimetric properties of accreting compact objects across different accretion states. These polarimetric signatures offer direct insight into scattering environments and emission mechanisms near the event horizon, placing constraints on coronal geometry, its optical depth, system inclination, and potential misalignment with the binary orbit. I will also discuss how complementary polarimetric observations in the optical and radio bands contribute to a unified, multiwavelength view of accretion and ejection in microquasars. Case studies of key black hole systems will illustrate how X-ray polarimetry is reshaping our understanding of relativistic accretion flows.

Speaker: Jiří Svoboda

16:30 Variability of X-ray polarization of Cyg X-1

In this talk, I’ll present the results of a comprehensive, 3-year-long multiwavelength polarimetric campaign on the prototypical black hole X-ray binary Cygnus X-1, conducted between 2022 and 2024. Using data from the Imaging X-ray Polarimetry Explorer (IXPE), we measured X-ray polarization 13 times across both hard and soft spectral states. We found that the polarization degree in the hard state is significantly higher — about 4% compared to 2.2% in the soft state — and increases with energy in both cases. At the same time, being on average well-aligned with the radio-jet orientation, as well as with optical and radio polarizaiton angle (PA), the X-ray PA showed long-term variations with amplitude of about 5 deg. By combining hard state observations, we find indications of orbital variability of the X-ray polarization. This variability may arise from the scattering of the central source X-ray emission by the circumstellar medium. The orbital profile of these variations requires the presence of asymmetry of scattering/emitting medium. Future high-precision X-ray polarimetric observations of Cyg X-1 with high temporal resolution are encouraged to shed light on the nature of this variability.

Speaker: Mr Vadim Kravtsov (University of Turku)

16:45 X-ray Polarimetry of Neutron Star Low-Mass X-ray Binaries throughout the Z-track with IXPE

Accreting, weakly magnetized Neutron Stars in Low-mass X-ray binaries (NS-LMXBs) are a fundamental laboratory to study radiation processes in the strong gravity regime. Z-sources are NS-LMXBs characterized by a wide Z-like three branches track in the Color-Color Diagram, typically accreting close to the Eddington limit.

In this talk, we discuss the spectro-polarimetric results obtained by IXPE (most often in coordination with other facilities) on NS-LMXBs in Z-state. Most of the known Z-sources were observed by IXPE during its two-year baseline phase, being the most polarized NS-LMXBs in the full 2-8 keV band and with a strong correlation between the observed polarization and the position along the Z-track. We will also provide an in-depth interpretation of these results, including also numerical simulations.

Speaker: Andrea Gnarini (Università degli Studi Roma Tre)

17:00 IXPE’s First Look at the Newly Discovered Black Hole Binary MAXI J1744–294

MAXI J1744–294 is a bright X-ray transient discovered in early January 2025 within 20″ of Sgr A* by MAXI/GSC. Multi-instrument follow-up observations established it as a black hole low-mass X-ray binary in outburst, with a soft-state spectrum characterized by strong absorption, a disk component, a steep power-law tail, and a prominent Fe K$\alpha$ line, albeit with possible contamination from diffuse Galactic center emission. We report on the first Imaging X-ray Polarimetry Explorer (IXPE) observation of MAXI J1744–294, carried out over April 5–8, 2025, for a total exposure of 150 ks. In the 2–8 keV band, the measured polarization degree remained below the observation Minimum Detectable Polarization, resulting in an upper limit of PD < 1.3%. This non-detection is in line with expectations for a thermal-dominated accretion disk viewed at low‐to‐intermediate inclination. We discuss how polarimetric measurements can help constrain key disk parameters such as inclination and the BH spin.

Speaker: Lorenzo Marra (Istituto Nazionale di Astrofisica (INAF))

17:15 X-ray Reverberation and X-ray Polarization: reconciling contradictory views of the corona

The geometry of the corona in accreting black hole binaries remains a central puzzle in high-energy astrophysics, particularly during their outburst phases when dramatic structural changes occur. Two powerful observational tools available—X-ray reverberation mapping and X-ray polarization—now offer complementary but seemingly conflicting insights into the innermost regions around black holes. While reverberation studies suggest a vertically extended corona close to the black hole, recent polarization measurements point toward an horizontally extended corona structure. In this talk, I will present the recent IXPE polarization detection of GX 339-4 and place it in the broader context of polarization measurements of stellar-mass black holes. I will discuss the apparent inconsistency between polarization and reverberation results, and explore the implications for our understanding of the physical and geometric configuration of the corona. How can we reconcile these observables, and what might this tension reveal about the complexity of black hole accretion physics?

Speaker: Guglielmo Mastroserio (Università Statale di Milano)

17:30 → 18:00 Coffee Break

18:00 → 19:00 Polarimetry (X-ray, Optical, Radio…)

Convener: Alexandra Veledina (University of Turku)

18:00 Highs, Lows, and Flares: A Polarized View of Shocks and Outflows in PSR J1023+0038

Transitional millisecond pulsars (tMSPs) bridge the gap between accreting neutron stars in low-mass X-ray binaries and millisecond radio pulsars, offering a unique laboratory to study the interplay between accretion and pulsar activity. These systems exhibit a subluminous X-ray state characterized by alternating high, low and flaring emission modes.
Multi-wavelength campaigns on the prototype tMSP, PSR J1023+0038, have helped establish a solid understanding of how tMSPs are powered. A complex interplay between a compact jet, discrete ejecta, accreting matter and the pulsar wind seem to be regulating the behaviour of this puzzling source.
Recently, using polarimetric data from the Imaging X-ray Polarimetry Explorer (IXPE), the Very Large Telescope (VLT), and the Karl G. Jansky Very Large Array (VLA), we conducted the first multiwavelength polarimetric study of PSR J1023+0038.
A linear polarization of (12±3)% in the 2–6 keV band was observed during the high mode. The polarization angle aligns with the optical polarization observed by the VLT, suggesting a shared physical mechanism. During the low mode, the significance was insufficient for detailed analysis, resulting in an upper limit of 26% (90% confidence) on the polarization degree. The results strongly indicate that both optical and X-ray polarization originate from synchrotron radiation at the shock formed by the interaction of the pulsar wind with the inner accretion disc.
Finally, simultaneous radio, optical, and X-ray observations obtained as part of this campaign have, for the first time, shed light on the poorly understood flaring mode emission, emphasizing the critical role of outflows in tMSPs.

Speaker: Maria Cristina Baglio (Istituto Nazionale di Astrofisica (INAF))

18:15 Spectro-polarimetric modelling of XrB observations in the JED-SAD/MONK framework

Swift J1727.8–1613 is a recently discovered X-ray binary system entered in outburst in August 2023 and observed by SWIFT, IXPE, NiCER, NuSTAR and INTEGRAL. In this work, we analyze the source within the framework of the Jet Emitting Disk–Standard Accretion Disc (JED-SAD) model, which provides a self-consistent description of accretion and ejection processes in X-ray binaries. We focus on the bright (at the end of the rising phase) and the dim (in the decaying phase) hard state of the source. We deduce from the JED-SAD spectral fits the thermal structure of the accretion flow.The dominant radiative process appears to be Bremsstrahlung Self Compton and Synchrotron Self Compton in the bright and dim state respectively. The JED-SAD thermal structure is then used as input in the monte-carlo code MONK to compute the associated polarisation and compare it to the IXPE observations. Our simulated polarisation degree and angle are not consistent with the observations in the bright state but are consistent in the dim state. These results highlight the diagnostic power of joint spectral and polarimetric observations with physically motivated models. We will discuss our results and their implication for our understanding of the close environment of black holes in X-ray binaries.

Speaker: rafan muhammad (IPAG-UGA)

18:30 Probing the X-ray polarization properties of Atoll sources: the case of GX 9+1

The Imaging X-ray Polarimetry Explorer (IXPE) is opening a new observational window into the physics of weakly magnetized neutron stars in low-mass X-ray binaries (LMXBs). In particular, coordinated multi-instrument campaigns have enabled the first systematic studies of the polarization signatures from bright Atoll-type sources. In this contribution, we focus on the source GX 9+1 and present recent spectro-polarimetric results, underlining the energy dependence of the polarization degree. We also compare these results with those obtained for other Atoll sources, discussing the emerging trends and their implications for the geometry and emission mechanisms in these systems.

Speaker: Dr Antonella Tarana (INAF-IAPS)

18:45 Investigating the Polarimetric and Spectral Dynamics of Cygnus X-3 through IXPE Observations

X-ray binaries are key laboratories for studying accretion and jet ejection mechanisms in strong gravitational fields. Among them, Cygnus X-3 stands out due to its distinctive properties and long-standing observational interest. In this talk, I will present results from the second and third observations of Cygnus X-3 by the Imaging X-ray Polarimetry Explorer (IXPE), focusing on the evolution of its spectral and polarimetric characteristics. Both observations reveal significant polarization, with notable differences in spectral shape and polarization behavior, offering a dynamic view of the system across time.

A particularly novel aspect of this study is the exploration of a possible correlation between the observed polarization and the orbital phase of the source, with an emphasis on the variability seen during the third observation. The results suggest a complex interplay between orbital geometry and emission properties, with the inferred geometry appearing especially peculiar for a system of this kind. Unlike more typical black hole binaries in hard state that feature a compact corona as the primary emission region, Cygnus X-3 appears to involve an outflow, responsible for reflection, which fundamentally alters the interpretation of the polarimetric signal. As such, these findings may not be directly applicable to standard black hole binary systems, further underscoring the unique nature of this source.

These findings contribute to a deeper understanding of black hole accretion physics and jet formation, and they underscore the potential of X-ray polarimetry to probe the high-energy behavior of compact binary systems.

Speaker: Romana Mikusincova (Istituto Nazionale di Astrofisica (INAF))

19:00 → 19:27 Polarimetry (X-ray, Optical, Radio…): Flash Talk

19:00 X-ray spectro-polarimetric models for XRBs and AGN

Recent discoveries by the IXPE mission since 2022 have highlighted the need for more sophisticated spectro-polarimetric models to interpret photon reprocessing in various components of X-ray binaries (XRBs) and active galactic nuclei (AGN). To address this, our group has developed several models compatible with XSPEC, a widely used tool for X-ray data analysis. These models include STOKES tables for the reprocessing of power-law radiation in ionised constant-density slabs, models of relativistic reflection from the inner accretion disc, non-relativistic reflection from distant material, and polarisation models of thermal disc emission. In this flash talk, I will provide an overview of the available models, highlighting recent updates and enhancements.

Speaker: Dr Michal Dovciak (Astronomical Institute of the Czech Academy of Sciences)

19:02 Radiation effects on jets and disks in the MAD state

I will discuss recent results of the radiation effect on magnetically-arrested disks (MAD) around rotating black holes and the resulting jets, obtained using the general-relativistic radiative magneto-hydrodynamic (GR-R-MHD) code cuHARM. I will show that (i) The jet width, MAD parameter and jet efficiency increase with the accretion rate, before saturation caused by gas-pressure to magnetically supported disk ; (ii) at high accretion rates, magnetic field pressure plays an important role in stabilizing the disk. (iii) At higher accretion rates, more angular momentum is transferred through radiation. Finally, (iv) I will discuss posibble mechanism for entaining the jet with external particles, via the Kelvin-Helmholtz instability.

Speaker: Asaf Pe'er (Bar Ilan University)

19:04 Peeling back the layers: a stripped companion star revealed

Studying the chemical composition of accretion discs around compact objects, such as neutron stars in low-mass X-ray binaries (LMXBs), provides vital information about their formation and evolutionary history. This touches on a range of important topics, including key uncertainties in binary evolution such as mass transfer efficiency and common-envelope phases, that in turn impact the occurrence rates of various types of energetic transients, such as supernovae, gamma-ray bursts, and gravitational wave mergers. Finally, it can improve our understanding of the demographics and connections among different classes of binaries containing (accreting) compact objects.

In this talk, I will present an interesting case study to demonstrate the impact that can be made by exploiting UV and optical spectroscopy to study the composition of the transferred material in accretion discs. The LMXB UW CrB hosts a relatively small companion star, but its abundances reveal that this should be the CNO-core of a more massive, stripped companion star. Combined with its short orbital period, this suggests the system potentially underwent a common-envelope phase in its evolution. I will present UV and optical spectroscopy of UW CrB revealing the characteristics of the companion, discuss the possible formation paths, and put the system in context of the sample of accreting compact objects.

Speaker: Stefanie Fijma (University of Amsterdam)

19:06 Probing magnetic fields and accretion physics through pulsed emission in accreting X-ray pulsars

We analyze all NuSTAR observations of the accreting X-ray pulsar 4U 1538–52 to investigate the energy-dependent pulse profiles and the phase-dependent spectral variability of its fundamental cyclotron resonant scattering feature (CRSF). Fourier decomposition of energy-resolved pulse profiles reveals a broad bump in the pulsed fraction spectrum (PFS) near the CRSF energy, contrasting with the dip-like features observed in other systems (Ferrigno et al. 2023). Phase-resolved spectroscopy shows that the CRSF depth varies strongly with spin phase, becoming deeper during low-flux intervals and shallower at pulse maximum. We interpret the PFS morphology across different sources as the result of the interplay between spin-dependent flux variations and phase-dependent CRSF behavior. These findings establish the PFS as a sensitive, model-independent diagnostic for CRSF detection and characterization, providing new constraints on the magnetic field structure and accretion geometry of X-ray pulsars. To interpret the observations, we perform physical modeling of the pulsed X-ray emission from a neutron star, favoring a geometry with high inclination, large magnetic obliquity, and a ~15° asymmetry between magnetic poles. This configuration naturally reproduces the observed pulse profiles and PFS features, offering a framework linking CRSF behavior, system geometry, magnetic topology, and potential jet activity. Given the possible existence of jets in strongly magnetized HMXBs (van den Eijnden et al. 2021), detailed knowledge of magnetic field configurations through model-independent methods such as the PFS and pulse profile modeling is essential to understand jet-launching mechanisms in these systems.

Speaker: Mr Dimitrios Konstantinos Maniadakis (Istituto Nazionale di Astrofisica (INAF))

19:08 PPANDA: Pulse Profiles of Accreting Neutron Stars Deeply Analyzed: the LMXB 4U 1626-67

The spectral modelling of accreting X-ray pulsars is difficult owing to the anisotropy linked to the strong magnetic field and the geometrical properties such as the inclination of the line of sight and the location of the magnetic poles. This strongly influences the observed phase variability at the spin frequency. In Ferrigno et al. (2023), we introduced a method to compute pulsed profiles with the intrinsic energy resolution of the NuSTAR instruments optimized for the S/N, enabling detailed analysis of the Pulsed Fraction across different energy bands—referred to as the Pulsed Fraction Spectrum (PFS). This approach revealed characteristic features in the PFS corresponding to well-known spectral signatures, such as the iron fluorescence line and cyclotron resonance scattering features (CRSFs), superimposed on an overall increasing trend of pulsed fraction with energy.
Furthermore, a detailed analysis of the pulse profiles themselves—such as cross-correlation studies and phase lags—provides complementary insights that contribute to a more complete understanding of their energy dependency.
In this talk, I will give an overview of our method and focus on the results of its application to the Low Mass X-ray Binary (LMXB) 4U 1626-67, which exhibits a CRSF at $\approx$ 38 keV in the phase averaged spectrum. We found evidence for the CRSF in the PFS, phase lags and cross-correlation, but no signatures of the iron line, which is present in the energy spectra.
Our method paves the way for systematic searches for CRSFs and other spectral features, providing additional information to be considered in spectral modelling.

Speaker: Elena Ambrosi (Istituto Nazionale di Astrofisica (INAF))

19:10 Disentangling Comptonization processes and mapping emission regions in accreting neutron stars

Understanding the dominant mechanisms behind radiation production and energisation, as well as the spatial distribution of emission sources near an accreting compact object, remains one of the fundamental problems in high-energy astrophysics. While it is generally agreed that multi-temperature plasma with emitting and Comptonizing components is involved, the exact mechanisms of photon production and reprocessing remain debated across various accreting systems. A similar uncertainty applies to the geometry and spatial distribution of emission regions and their interplay — specifically, which region acts primarily as a source and which serves predominantly as a site of reprocessing. While this class of problems is well known for X-ray binaries in general, high-mass X-ray binaries with neutron stars introduce additional complexity and unique diagnostic potential due to the presence of strong magnetic fields. At high and intermediate mass-accretion rates, a major challenge lies in disentangling the emission from the accretion column and its reprocessing by the neutron star atmosphere, as well as identifying the respective roles of bulk and thermal Comptonization in shaping the observed spectrum. The magnetic field induces strong anisotropy and introduces resonances into radiative processes — most notably, in Compton scattering. Using radiative transfer in a highly magnetised plasma combined with relativistic ray tracing near the neutron star, we show that observational signatures from resonant Compton scattering vary for different scenarios in the accretion channel. This enables the development of diagnostic tests to distinguish between competing models. We discuss the limitations of this approach and its relevance for interpreting observational data.

Speaker: Ekaterina Sokolova-Lapa (Remeis Observatory & ECAP, FAU Erlangen-Nuernberg)

Tuesday 16 September

09:00 → 10:45 Wind (Observation, Theory and modelling)

Convener: Dr pierre-olivier petrucci (Institute of Planetary science and Astrophysics of Grenoble)

09:00 Towards a Broader View of Accretion Disc Winds in X-ray Binaries

Accreting stellar-mass black holes in X-ray binaries offer a unique opportunity to study black hole accretion in relatively clean environments and on human-accessible timescales—much shorter than those typical of AGN and quasars. Over the past few decades, the coupling between accretion and ejection processes in these systems has been extensively investigated, primarily through X-ray and radio continuum observations. More recently, systematic studies of their X-ray, ultraviolet, optical, and infrared spectral lines have revealed the presence of accretion disc winds, showing that substantial amounts of both hot and cold gas are expelled from these systems. In this talk, I will review this increasingly broad field and discuss how the different types of disc winds observed in X-ray binaries relate to one another, with emphasis on the likely multi-phase nature of the phenomenon.

Speaker: Dr Teo Muñoz-Darias (Instituto de Astrofisica de Canarias)

09:30 Winds Theory and Modeling: Characteristics and Spectral Signatures

Powerful ionized winds are often observed during episodic outbursts in black hole X-ray binaries (aka. microquasars). Focused on X-ray band, the observed highly ionized absorbers of multiple ions are thought to be launched and accelerated from an accretion disk by some means. The current disk wind theory suggests multiple driving mechanisms with distinct characteristics, which may well manifest themselves in absorption features. In this presentation, I will review the X-ray wind physics of some leading scenarios and their spectral signatures.

Speaker: Keigo Fukumura (James Madison University)

10:00 Ultra-fast outflows in the black hole candidate MAXI J1810-222

The X-ray transient MAXI J1810-222 was discovered in 2018 and has been active ever since. A deep, ongoing, multiwavelength monitoring campaign suggests a black hole accretor, although the unusual outburst behaviour. Through a NICER monitoring we detected a strong spectral absorption feature around 1 keV which can be described with a photoionised absorber. A deep scan of the parameters space showed evidence for a spectral-state dependent outflow, with mildly relativistic speeds at 0.05-0.15 c. An XMM-Newton observation was recently triggered in which the high-resolution RGS detector confirmed the presence of a broad absorption line. Further investigation of NUSTAR data also indicated the presence of a hotter Fe K counterpart to such a relativistic outflow. These findings would make MAXI J1810-222 the first X-ray binary with an ultra-fast outflow, most-likely of MHD nature. In this talk, I will present an overview of these exciting results.

Speaker: Dr Ciro Pinto (Istituto Nazionale di Astrofisica (INAF))

10:15 Modelling wind signatures in the optical and infrared spectrum of XRBs

Accretion disc winds are thought to play a significant role in shaping the outbursts of X-ray binaries (XRBs) by extracting mass and momentum from these systems. The study of cold, low-ionisation winds, detected through hydrogen and helium lines at optical and infrared (OIR) wavelengths, has recently provided substantial insights into their properties. The analysis of the optically thick wind regime, identified through blue-shifted absorptions, has offered further evidence for the equatorial geometry of the winds and provided information on the multiphase structure and velocity of the ejecta. Studies of winds detected in the optically thin regime (i.e. broad emission components) have enabled the determination of the ejected mass, providing evidence that winds can deplete a significant fraction of the disc.

By employing radiative transfer and photoionisation models, it is possible to reproduce the OIR signatures of accretion disc winds across both the optically thin and optically thick regimes. In this talk, I will provide a brief overview of the modelling of low-ionisation wind signatures. I will discuss the insights that can be gained from this approach, including the impact of ionisation on the detectability of wind features, the necessity for a clumpy wind structure, and the wind’s efficiency in removing mass from the system. I will conclude by exploring the implications of these findings for the underlying wind launching mechanisms.

Speaker: Alessandra Ambrifi (Instituto de Astrofísica de Canarias)

10:30 Tracking outflows evolution across spectral states of the black hole X-ray binary MAXI J1820+070

In this talk, I will present a study of the optical counterpart of the black hole X-ray binary MAXI J1820+070 during its main 2018 outburst and subsequent three reflares. The goal is to track the evolution of outflows across different spectral states and investigate the origin of minute-timescale optical variability. We collected high-cadence multi-filter optical light curves using the Las Cumbres Observatory network of telescopes and the Al Sadeem Observatory (UAE) telescope from 2018 to 2020. These were complemented by low-resolution optical spectra obtained with telescopes in Mexico and Italy, and by archival X-ray data from Swift (BAT and XRT) and MAXI. We find a positive correlation between the fractional optical rms and the X-ray spectral hardness, particularly at low frequencies. A similar correlation is observed between optical and X-ray rms variability. Optical spectra show double-peaked emission lines and evidence of cold wind outflows only during the hard state, including during reflares, constituting one of the first detections of such winds in this phase. These findings support a scenario where the optical variability is linked to jet activity, which dominates during the hard state and is quenched in the soft state. The variability may be driven by fluctuations in the inflow velocity at the jet base, consistent with internal shock models. Alternatively, the observed variability might arise from a hot magnetised accretion flow. The lack of wind features in the soft state could indicate over-ionisation of the outflow.

Speaker: Marco Maria Messa (Istituto Nazionale di Astrofisica (INAF))

10:45 → 11:15 Coffee Break

11:15 → 13:15 High resolution spectroscopy (BH, NS)

11:15 High Resolution X-ray Spectroscopy in Black Hole X-ray Binaries

Black hole X-ray binaries sometimes show powerful outflows along the accretion disk, which is called the disk winds. They often appear in X-ray spectra as blueshifted, highly ionized absorption lines. Previous observations suggest that the mass loss rate due to disk winds is comparable to the mass accretion rate onto black holes, and therefore the disk winds are thought to have a significant impact on the disk structure and the surrounding environment. However, the their launching mechanism and changes in the wind structure associated with variations in mass accretion rate are not yet fully understood.
The X-ray Imaging and Spectroscopy Mission (XRISM), launched in September 2023, carries Resolve, a new technology detector (X-ray micro-calorimeter) that achieves unprecedented energy resolution. Resolve has achieved a resolution of ~5 eV @ 6 keV, which is an order of magnitude better than previous detectors. Resolve enables us to measure the fundamental wind parameters such as the velocity and the wind launching radius and is expected to dramatically advance our understanding of the wind launching mechanism and affects of the winds to the accretion disk and environment.
In this talk, I will review previous studies of outflows in black hole X-ray binaries, present the latest results from XRISM observations, and discuss future prospects.

Speaker: Megumi Shidatsu (Ehime University)

11:45 High resolution X-ray spectroscopy in neutron star X-ray binaries

Winds from accretion discs around compact objects, both in X-ray binaries and Active Galactic Nuclei, have been extensively studied in the past two decades with X-ray spectroscopy. However, to date, the launching mechanism of these winds remains a highly controversial topic. A distinct advantage of studying these winds in high inclination, dipping, neutron star X-ray binaries is that highly ionised plasmas are detected in a vast majority of cases and that the orbital periods and ephemeris of the systems are well known, thus enabling to precisely test the predictions of different models for sources with a range of sizes.

While observations previous to XRISM have already provided a relatively good characterisation of the plasmas, the exquisite spectral resolution of XRISM/Resolve has now allowed to precisely detect low velocity outflows and to resolve the width of the spectral lines in the outflows, thus disentangling different velocity components.

In this talk I will review how XRISM observations have already advanced our knowledge of accretion processes and in particular winds in neutron star X-ray binaries and I will give an outlook on the prospects of future observations.

Speaker: Dr Maria Diaz Trigo (ESO)

12:15 The XRISM view of outflows in Black Hole Low-Mass X-ray Binaries

The first year and a half of XRISM science has led to a select number of observations of Black Hole Low Mass X-ray Binaries (BHLMXB), most of which in very unusual states. We will briefly present the context of all these observations, and notably results for :
-A faint soft state exposure of the Black Hole candidate 4U 1630-47 taken at the end of its 2022-2024 outburst. Despite the closest to a "standard" observation with wind signatures, the sensitivity and resolution of XRISM reveal a variety of time-dependent absorbers, whose origin remain puzzling at such a low Eddington fraction.
-An extremely faint, soft, obscured exposure of the BHLMXB V4641Sgr, taken at the tail-end of its weak, fully obscured 2024 outburst. Despite an apparent Eddington fraction around 2e-4, the spectral shape and many strong emission lines hint at much higher intrinsic accretion rate.
-A low hard state exposure of the wind-emitting BHLMXB candidate IGR J17091-3624 from February 2025.
-A high-soft state exposure of the new BHLMXB candidate MAXI J1744-294, discovered in 2025. The very crowded field of view makes for a very complex analysis, with a very strong diffuse emission from the Supernova Remnant Sagittarius A East, and the dipping Neutron Star Low-Mass X-ray Binary AX J1745.6-2901.

Speaker: Maxime Parra (Ehime University, Japan)

12:30 Observing the Hypersoft State of Cygnus X-3 with XRISM and Chandra

In March 2024 Cygnus X-3 (Cyg X-3) was in a quenched/hypersoft state (soft/high). During this state a 67 ksec XRISM observations was triggered along with ToO observations with the SMA. During this period there were also supporting observations being made with AMI (radio), NuSTAR (hard X-ray), and Fermi (gamma-ray). The SMA submillimeter observation taken during the XRISM observation revealed that Cyg X-3 was at an extremely low flux level (~3.8 mJy @ 225.53 GHz) indicating that the jet had basically turned off. SMA observations done 2 and 3 days later showed flux density exceeded 1 Jy (@1.3 mm) indicating a major flare was underway. The XRISM observations showed a rich spectrum with the detection of multiple kinematic and ionization components in absorption and emission whose superposition leads to complex line profiles, including strong P Cygni profiles on resonance lines. The prominent Fe XXV He alpha and Fe XXVI Ly alpha emission complexes are clearly resolved into their characteristic fine-structure transitions. In this presentation we will present some of the XRISM spectral results and put them in the context of the multi-wavelength campaign which was in place for these observations. Also we present a comparison of the XRISM observations with previously made Chandra HETG observations of the hypersoft state.

Speaker: MICHAEL MCCOLLOUGH (CXC/SAO/CfA)

12:45 Probing accretion disk plasmas in LMXBs with XRISM

Low mass X-ray binaries (LMXBs) are systems composed of a compact object (black hole or neutron star) and a low mass (< mSun) companion star. The compact object accretes matter from the companion, forming an accretion disk. The gas in the disk is ionized by the strong X-ray emission from the inner disk region, leading to the appearance of photoionized plasmas. Characterizing their properties (e.g. plasmas’ velocity, column density, and ionization) is essential for fully understanding the accretion disk structure of LMXBs.

X-ray dipping provides an excellent opportunity to study the properties of the plasmas in the accretion disk of these systems. The dipping phenomenon consists of periodic drops in X-ray flux that result from the interaction between the dense stream of colder matter accreted from the companion star and the impact region of the accretion disk. To date, this phenomenon has been observed in about two dozen high-inclination systems, however, the specifics of the structure and plasma properties in this impact region of the accretion disk are not yet understood.

I will present exciting new XRISM Resolve data of the dipping LMXB 4U1624-490, showcasing some of the ongoing analysis of the prominent X-ray dips, which have never been studied at the high spectral resolution that the microcalorimeter of the Resolve instrument allows. With this study, we aim to provide insight into the dipping phenomenon, any potential connection to disk winds, as well as getting a comprehensive picture of the plasmas in the accretion disk of LMXBs.

Speaker: Eleonora Caruso (API/SRON)

13:00 Using high resolution mm and X-ray images to constrain distance to black hole 4U1630-47 and the molecular clouds in the line of sight

We report new constraints on the distance to 4U 1630−47 by analyzing its dust scattering halo (DSH) using high-resolution X-ray (Chandra) and millimeter (APEX) observations. DSHs, produced when X-rays scatter off interstellar dust, encode information about both the source distance and the distribution of intervening material. Previous distance estimates for 4U 1630−47 (4.9 kpc and 11.5 kpc) remained uncertain due to limited-resolution molecular gas maps. Accurate distance measurements are especially important for this source due to its surprisingly high X-ray polarization amplitude.

We developed a new methodology combining a machine learning approach to reconstruct the 3D distribution of molecular clouds from APEX data with synthetic DSH image modeling based on X-ray flux measurements. By fitting both radial and azimuthal profiles of these synthetic halos to Chandra observations, we simultaneously constrained the source distance and identified the near/far distances of the intervening molecular clouds.

Our analysis still favors a distance of 11.5 kpc, rejecting a 13.6 kpc solution—despite its lower χ² fit—because it would produce a bright halo feature beyond the observed field of view. The 4.9 kpc estimate was also ruled out due to poor fit quality and inconsistencies in cloud positioning. The primary source of uncertainty remains a systematic ~1 kpc error associated with molecular cloud distance estimates.

Speaker: Emrah Kalemci (Sabanci University)

13:15 → 16:00 Lunch

16:00 → 17:30 Timing properties (Observations, modelling)

Convener: Luciano Burderi (University of Cagliari)

16:00 Timing Properties: Observations

The power of timing is that it can strongly constrain source geometry and causal mechanisms. I will review how multi-wavelength timing - from radio to X-rays and beyond - reveals the structure and scales of microquasars and the causal interplay of accreting components on a variety of physical scales. I will survey some lessons learnt from campaigns over past six decades of timing, and then discuss new parameter space that forthcoming facilities across the electromagnetic spectrum will enable, as well as gaps that remain to be addressed. Finally, I will outline community-driven actions and collaborative efforts needed to accelerate transformational discoveries.

Speaker: Poshak Gandhi (University of Southampton)

16:30 Modelling the broadband X-ray variability of black hole X-ray binaries

In the last 8 years, NICER and Insight-HXMT have opened up new windows to study X-ray variability from the softest to hardest X-rays. With a focus on insights from these instruments, I will review our current understanding of the origin of the rapid (minutes to milliseconds) X-ray variability in black hole X-ray binaries and how we can model the Fourier properties of the variability, to learn about the innermost regions and the disk-corona geometry.

Speaker: Prof. Phil Uttley (UVA NL)

17:00 Subsecond O-IR QPOs from the black hole transient Swift J1727.8-1613 during the state transition

Fast multi-wavelength studies of black hole transients are revealing a complex and rich phenomenology, opening a new era in the study of disc-jet coupling. In this talk, I will show the latest results regarding an X-ray/Optical and infrared (O-IR) campaign on the bright transient Swift J1727.8-1613. By following the source with NICER, Ultracam HAWKI, and HIPERCAM, for the first time, we were able to track the evolution of a QPO in X-ray and O-IR band during the state transition. In particular, during our three observations, we observed a drift in the QPO  from 1.4 Hz to 4 Hz. These are among the highest O-IR QPO frequencies detected for a black hole X-ray transient. During the first two epochs, the X-ray and O-IR emissions are correlated, with an optical lag (compared to the X-rays) varying from +70 ms to 0 ms. Finally, during the third epoch, we measured for the first time, a lag of the $z_s$-band respect to the $g_s$-band at the QPO frequency ($\approx$+10 ms). By estimating the variable O-IR SED we find that the emission is most likely non-thermal. I will show how these observations challenge our state-of-the-art models showing that neither the jet nor the hot flow model can easily explain the observed evolution of the QPOs.

Speaker: Federico Maria Vincentelli (INAF IAPS)

17:15 Hidden Variability and the Link Between Type-B QPOs and Jet Ejections in Swift J1727.8−1613 and MAXI J1820+070

It is widely accepted that Type-B quasi-periodic oscillations (QPOs) are causally connected to discrete ejections of relativistic jets in black hole X-ray binaries. Here, I present new results from a bright flare of Swift J1727.8−1613, during which the source evolved from the hard intermediate state (HIMS) into the soft intermediate state (SIMS) and almost immediately back to the HIMS, without progressing into the high-soft state (HSS).

During this brief excursion into the SIMS, the broadband X-ray variability drops and discrete, optically thin radio ejections are observed, as typically seen during transitions into the SIMS.

Using a novel joint power and cross-spectrum fitting technique, I show that the Type-B QPO is already present during the HIMS, and hence not physically associated with the discrete jet ejections. Initially, the Type-B QPO is hidden beneath the stronger Type-C QPO and dominant broadband variability. As the Type-C QPO disappears, broadband noise weakens, and the steady compact radio jet vanishes, the Type-B QPO becomes detectable and dominates the variability, and the power spectrum changes from Type-C to Type-B.

These results challenge models that interpret the power spectrum as a single broadband component, favoring instead a picture where distinct variability components coexist, each with their own QPOs. The disappearance of the Type-C power spectrum alongside the steady radio jet suggests a close link between them.

I will present these findings for Swift J1727.8−1613, complemented by results from MAXI J1820+070, and discuss their implications for the structure of variability and the formation of jets in black-hole binaries.

Speaker: Prof. Mariano Mendez (Kapteyn Astronomical Institute, University of Groningen)

17:30 → 18:00 Coffee Break

18:00 → 19:00 Timing properties (Observations, modelling)

Convener: Phil Uttley (University of Amsterdam)

18:00 A NICER view of accretion onto Black Hole X-ray binaries

Black hole X-ray binaries (BHXBs) are key laboratories for studying accretion physics, relativistic jets, and strong gravity. Since its launch in 2017, NASA’s Neutron Star Interior Composition Explorer (NICER) has provided unprecedented X-ray timing and spectral data, enabling new insights into accretion phenomenology and the physics behind. In this talk, I will review what we have learned about BHXBs from NICER observations in recent years, highlighting key discoveries related to quasi-periodic oscillations (QPOS) and their link to relativistic jets. I will also summarize NICER major contributions to multi-wavelength campaigns.

Speaker: Diego Altamirano (University of Southampton)

18:15 First sub-second Infrared/X-ray observations of low inclination black hole X-ray transient 4U 1543-475

In recent years, fast optical-infrared (O-IR) variability of LMXBs has opened a new window in the study of the jet and accretion flow properties. All objects observed thus far are known to have an intermediate-high inclination, biasing our interpretations and hampering the possibility of adequately testing models. Here, I present the first sub-second multiwavelength analysis of low inclination black hole X-ray transient 4U 1543-475 (i<30 deg.). Observations were taken during its 2022 outburst using sub-second observations from HAWK-I@VLT (IR) and NICER (X-ray). Similarly to higher inclination objects, cross-correlation analysis shows a strong correlation between the IR and X-ray bands with IR lagging the X-rays by ~100 ms. More interestingly, an IR QPO is detected at 0.07 Hz with an X-ray counterpart at twice the IR frequency (0.14 Hz), indicating a possible harmonic relation. These results provide the first constraints on jet timing signatures at low viewing angles, representing a key dataset for our understanding of these collimated outflows.

Speaker: Ms Sian Akua Woahene-Demehin (University of Southampton)

18:30 A Unified Framework for Type B and C QPOs via Disk Warping in X-ray Binaries

Low-frequency quasi-periodic oscillations (LFQPOs) are among the most striking timing features in black hole X-ray binaries, yet their physical origin and diversity remain unresolved. In this talk, I will present a new framework that naturally explains the transition from type C to type B QPOs through the progressive warping of the accretion disk. I will show that when the transition radius of the hot inner flow drops below a critical break radius, the outer disk begins to warp. This leads to several observational signatures: a sharp drop in QPO amplitude, the disappearance of broadband noise, and a modification of lag behavior—hallmarks of the hard-to-soft transition. This framework also accounts for the consistent properties of type B QPOs across different systems and offers a physical explanation for the lack of simultaneous type B and type C QPOs. If time permits, I will also discuss how this geometry-dependent behavior may generate the flip-flop variability observed in several X-ray binaries. Overall, this model provides a predictive, physically grounded framework to interpret LFQPO behavior across spectral states and offers new insights into the interplay between accretion geometry and variability.

Speaker: Gregoire Marcel (University of Turku)

18:45 Disc–Corona Geometry Modelling and Lense–Thirring Precession in QPOs

Based on the AGNSED model (Kubota & Done 2018), we developed a spectral model, SSsed, which describes the disc–corona geometry in stellar-mass black hole binaries. We applied this model to RXTE spectral datasets of XTE J1550–564 and compared the estimated coronal radii
$R_{\rm cor}$ with independently determined centroid frequencies of low-frequency QPOs in their power density spectra. In the intermediate states (HIMS and SIMS), the relation between $R_{\rm cor}$ and the QPO frequencies is in good agreement with predictions from the Lense–Thirring precession scenario for the QPO origin (Kubota et al. 2024). In that work, we could not explain the QPOs in the hard state using the LT scenario, as the assumed sandwich-type disc–corona structure was unsuitable for reproducing the hard spectra ($\Gamma<1.9$). We have now modified the SSsed model to better represent the hard state, incorporating an inner hot corona region (as in the original AGNSED model), with allowance for two-temperature Comptonisation, in addition to the sandwich-type disc–corona region. The updated model appears capable of explaining the QPOs observed in the bright hard state as well.

Speaker: Aya Kubota (Shibaura Institute of Technology)

19:00 → 19:20 Timing properties (Observations, modelling): Flash Talk

Convener: Phil Uttley (University of Amsterdam)

19:00 Unveiling hidden variability: an imaginary QPO in the soft-to-hard transition of MAXI J1820+070

Black hole X-ray binaries (BHXBs) show strong variability in their X-ray light curves over a broad range of time scales. Fast X-ray variability in BHXBs is key to understanding the properties of the innermost regions of the accretion flow in these sources. Among the most notable features of this variability are quasi-periodic oscillations (QPOs), which provide a unique insight due to their well-defined frequencies.
A recent study shows that signals with a large Imaginary and a small Real part of the cross spectrum (CS) can get hidden in the power spectrum (PS) when overlapping in frequency with other signals. For the last four decades, the power spectrum (PS) has been the primary tool for identifying the variability components, therefore, we have been missing significant signals. In this talk, I will show how using a novel technique, we revealed a narrow hidden variability component in the PS of the BHXB MAXI J1820+070. We call this component "imaginary QPO”, given its large Imaginary part in the CS. Moreover, we found that this imaginary QPO evolves smoothly along the soft-to-hard transition of MAXI J1820+070, decreasing its characteristic frequency together with other variability components. We also found that this imaginary QPO morphs from a type-C QPO detected in earlier observations when the source just left the High-Soft state. The presence of a similar imaginary QPO in Cygnus X-1, and its possible link to the type-C QPO, suggests a deeper connection between these timing features and the accretion dynamics during state transition.

Speaker: Candela Abril Bellavita (Kapteyn Astronomical Institute (RUG) - Instituto Argentino de Radioastronomía (IAR-CONICET))

19:02 Unveiling hidden variability components in Cyg X-1 and their connection to the radio jet

The study of fast variability properties in X-ray binaries advances our understanding of the properties of the accretion flow around the compact object. As most timing studies focus on the power spectrum (PS), the phenomenology of the cross spectrum (CS), especially the coherence, is less understood. In this talk, I will present the results of a systematic study of the evolution of both PS and CS of Cygnus X-1 with AstroSat/LAXPC, during the transition of the source from the hard to soft state in 2017. Using a novel technique to fit simultaneously the PS and CS with a multi-Lorentzian model, and predicting the phase-lags and coherence-frequency spectra, we study the evolution of the frequency and power of the five main variability components that are present throughout all the states. Additionally, we identify previously undetected variability components, one of which manifests as a narrow dip in the coherence function and a broad drop in the phase-lag spectrum at the same frequency. This dip in coherence, which we detected for the first time in Cygnus X-1 at energies above 3 keV, had only been previously detected with NICER data when a soft reference band (< 2 keV) was used. Moreover, these dips appear in a state in which the source shows high-amplitude radio variability and significant hard X-ray polarization. While the contribution of the compact jet in X-rays is debated in the literature, this study provides a new avenue for investigating jet properties as well as the geometry of the Comptonizing medium.

Speaker: Sandeep Kumar Rout (New York University Abu Dhabi)

19:04 Narrow coherence drops in Black-Hole X-ray Binaries: evidence for additive QPOs?

Recent timing studies of black hole X-ray binaries have revealed a narrow drop in the coherence between soft (<2 keV) and hard (>2 keV) bands in the 1–5 Hz range during soft-to-hard transitions in outburst decay, at lower luminosities than the hard-to-soft transitions observed during outburst rise. A sudden jump in the phase lags accompanies this coherence drop at the same frequency. Using a novel approach that models the power and cross spectra with combinations of incoherent Lorentzians, we found compelling evidence that these features are caused by a narrow quasi-periodic oscillation (QPO) with intrinsic lags distinct from the underlying broadband noise.
In-depth studies of this phenomenon across multiple sources—MAXI J1348-630, MAXI J1820+070, Cygnus X-1 (NICER), and Swift J1727.8-1613 (XMM-Newton)—yield consistent results. The coherence drops, phase-lag jumps, and the energy-dependent rms and lag behaviour of a weak narrow Lorentzian (often insignificant in the full-band power spectrum) all point to a QPO origin, likely corresponding to the Type-C QPO broadly seen in Low-Hard and Hard-Intermediate States during outburst rise.
Our findings support a model in which the QPO is an additive process, rather than a modulation of broadband noise or a multiplicative variability pattern, such as precession-induced modulation. This interpretation has important implications for understanding the geometry and dynamics of accretion flows, suggesting that distinct variability components arise from independent physical mechanisms.
In this talk, I will briefly present the observational evidence and then focus on the implications of these results on spectral-timing studies of accreting compact objects.

Speaker: Federico Garcia (Instituto Argentino de Radioastronomia (IAR-CONICET))

19:06 The First Coherent Census of Optical Variability in X-Ray Binaries with Gemini

X-ray binaries (XRBs) are systems containing accreting black holes and neutron stars that launch relativistic jets. Recently, it has been shown that we can move beyond spectral and imaging studies and use time-domain observations to measure fundamental properties of the jets and accretion flow, and in turn, begin to address key open questions in compact object research. However, XRB timing studies in the optical regime currently lag behind those performed at other wavelengths. To remedy this, we launched the GOFAST-XRB (Gemini Optical FAST timing of X-Ray Binaries) Large Program, which uses Gemini's `Alopeke and Zorro instruments to perform the first coherent census of XRB optical variability across different compact objects, timescales (sub-seconds to months), and accretion regimes. In this talk, I will present the first results of the GOFAST-XRB program, including observations of persistent systems (Cyg X-2 and Sco X-1), and recent outbursts of transient systems (Aql X-1 and 4U 1608-52).

Speaker: Efren James Elomina (University of Lethbridge)

19:08 Spectral-timing made easy with Stingray

Spectral-timing analyses are a key tool to investigate the innermost region in accreting X-ray sources.
However, when encounter them, such techniques can appear strenuous to implement and use in an easy way. An advantageous way is to rely on well-maintained and reliable software such as Stingray.
Stingray is an open-source Python library that is able to compute the main Fourier analyses techniques, comprising not only power and cross-spectral analyses, but also supporting data handling of the main X-ray missions.
In this brief talk, I will illustrate how Stingray is able to perform advanced techniques just with a few lines of code, effectively easing the learning curve for young researchers and accelerating the analysis process for experienced astronomers.

Speaker: Dr Eleonora Veronica Lai (Istituto Nazionale di Astrofisica (INAF))

19:10 Testing the Lense–Thirring Precession Origin of QPOs in Swift J1727.8–1613: Constraints from Disc–Corona Radii and QPO Frequencies

We present a comprehensive spectral and timing analysis of the newly discovered black hole transient Swift J1727.8--1613, based on broadband (2--150 keV) observations from Insight-HXMT during its 2023 outburst. Using the flexible, energy-conserving SSsed model, we identify the presence of both thermal and non-thermal Comptonization components in the hard component dominated state. We track the evolution of the truncated accretion disc radius, $r_{cor}$, which decreases from 45\,$R_{\rm g}$ to 9\,$R_{\rm g}$, consistent with the transition from the hard to the intermediate state. Additionally, we explore the correlation between $r_{cor}$ and the centroid frequency of quasi-periodic oscillations (QPOs; $\nu_{\rm c}$) to test the hot flow Lense-Thirring (LT) precession model. The overall slope of the observed trend is in strong agreement with the predictions of LT precession, despite the complexities of accretion behavior. Nonetheless, slight deviations from the expected $\nu_{\rm c}$-$r_{cor}$ relation suggest the presence of additional influencing factors, such as uncertainties in system parameter measurements, inner disc flipping, or jet power losses.

Speaker: Ruican Ma

19:12 Timing Analysis of the Black-Hole Candidate Swift J1727.8–1613: Detection of a Dip-like Feature in the High-Energy Cross Spectrum

The energy-dependent variability of black hole X-ray binaries (BHXRBs) provide crucial insights into the physical processes at play in the accretion flow of these systems. We analyze the rapid X-ray variability of the BHXRB Swift J1727.8-1613 during its 2023 outburst, using observations from the Hard X-ray Modulation Telescope. We detect, for the first time, a dip-like feature in the Real part of the Fourier cross spectrum of this source from the >25 keV and <10 keV X-ray light curves in the 3-15 Hz frequency range. This feature implies a phase lag between π/2 and π. We modeled the variability and found that a Gaussian phase-lag model provides a better fit than simpler models, reducing ambiguities in interpreting the data. Interestingly, both the fractional-rms and phase-lag spectra of the dip exhibit a change in trend around 15 keV, closely aligning with predictions from the time-dependent Comptonization model vKompth under a low feedback factor scenario. I will discuss these findings, and their consequences upon the current understanding of the mechanisms that produce the fast X-ray variability.

Speaker: Pei Jin (Kapteyn Astronomical Institute)

19:14 X-ray timing analysis of Swift J1727.8-1613 during its soft-to-hard transition

The timing analysis of X-ray binaries (XBs) is crucial for understanding the structure and dynamics governing these systems. Since the memorable 2023 outburst of the Low-Mass X-ray binary Swift J1727.8–1613, observations have been conducted across the entire electromagnetic spectrum, progressively constraining the source parameters. Here, we present the first XMM-Newton timing analysis of Swift J1727.8–1613 during the transition from the disk-dominated soft state to the hard state – dominated by the Comptonized emission from the hot plasma. When applying a multi-Lorentzian model to the power spectra (PS) and the Real and Imaginary parts of the cross spectra (CS), we identified quasi-periodic oscillations (QPOs) in the two harder observations, while no significant variability was observed in the softer observation. As the system approaches the hard state, the QPOs become narrower, shift to lower frequencies, and the rms amplitude in the PS increases in the 0.3–2 keV energy band. At the QPO frequency, the phase lags drop while the coherence function increases. All these features, previously observed in other XBs, suggest the presence of a type-C QPO.

Speaker: Maïmouna Brigitte (Astronomical Institute of the Czech Academy of Sciences)

19:16 Unveiling State Transitions and Inner Disk-Corona Dynamics in Black Hole X-ray Binaries Using Power-Color and QPO Analysis

Black hole low-mass X-ray binary (BH-LMXB) systems are exceptional laboratories for studying accretion physics under strong gravity. The variability of these systems, characterised by the power spectra changes systematically in different states. The power color analysis offers a robust way of classifying the spectral states, as an alternative to the traditional HID (Hardness-intensity diagram) method. We consider the ratio of variability amplitude across selected frequency ranges, defined as the hue parameter, to identify the states. We conducted a comprehensive timing analysis of all the archival data of ten BH-LMXB spanning from 2016 to 2021. We investigated the energy dependence of QPO (Quasi-Periodic Oscillations) variability and time lag to probe the physical properties of the QPO emitting region, which are yet to be fully understood. Notably, we detect a sign reversal in the average QPO time lag between hard and soft photons around 2 Hz during the hard to hard-intermediate state transition. We found that the slope of the RMS variability spectra and the time lag spectra also change signs at the same frequency and hue value, highlighting a major change in accretion flow geometry. During the hard lag phase, time lags of 40–50 milliseconds suggest an extended corona modulating the QPOs. In the soft lag phase, much shorter and constrained lags of around 10 milliseconds, indicating a more compact corona surrounding the disk, producing soft lags through reverberation. This study offers critical insights into the transformation of the coronal structure in association with the state evolution of BH-LMXBs.

Speaker: Mr Biki Ram (Indian Institute of Technology Indore)

Wednesday 17 September

09:00 → 10:55 Jets (Observation, theory and modelling)

Convener: Alessio Marino (Insitute of Space Sciences (ICE-CSIC))

09:00 Relativistic jets from black hole X-ray binaries: an observational review

Black hole X-ray binaries (BH XRBs) exhibit relativistic jets whose properties evolve with the accretion state. Compact jets, causally connected to the accretion flow, are observed during the hard state and are absent during the soft state, while discrete ejecta are launched during hard-to-soft state transitions. The physical mechanisms driving jet formation, quenching, and ejection remain poorly understood. In this talk, focused on recent observations primarily at radio wavelengths, I will review some of the most interesting results obtained over the past few years concerning both types of jets launched from BH XRBs. More in detail, I will discuss our current efforts for understanding the jet coupling to the accretion flow, variability timescales, launching conditions and interactions with the ambient medium, emphasizing results from high-resolution imaging and coordinated multi-wavelength campaigns.

Speaker: Francesco Carotenuto (Istituto Nazionale di Astrofisica (INAF))

09:30 Understanding the emission properties of microquasar jets

I will discuss models for the spectral and variability properties of jets.

Speaker: Julien Malzac (IRAP (CNRS / Universite de Toulouse))

10:00 Accretion and jets in X-ray binaries: the revolution brought by MeerKAT

The inflow of matter onto astronomical objects is linked to the generation of outflows across the Universe, ranging from proto-planetary disks to merging neutron stars and gamma-ray bursts, to stellar-mass and supermassive black holes.
Low-mass X-ray binaries, hosting either stellar-mass black holes or neutron stars, are common accreting systems in the Galaxy and exhibit bright jets detected in the radio band.

Compact jets from X-ray binaries, typical of hard accretion states remain unresolved at most angular scales, with only a few cases spatially resolved by milli-arcsecond observations. For years, such high-resolution observations were considered the only way to detect and track the transient jets launched in intermediate accretion states, although they are technically demanding and must be carried out shortly after launch.

The advent of MeerKAT has transformed our view of X-ray binary jets. It has more than doubled the number of black hole jet detections in the hard state, most with simultaneous X-ray coverage, revealing new features of the accretion-jet coupling and associated physical processes.
Even more remarkably, MeerKAT has shown that transient relativistic jets, when observed with sufficient surface brightness sensitivity, can be tracked from launch to terminal deceleration in the interstellar medium over months to years. In six years, it has tripled the sample of such jets, providing the most comprehensive data set available to constrain black hole jet power, which has been instrumental in proposing a new jet paradigm.

I will review the key results obtained with MeerKAT, highlighting the most significant discoveries and their implications.

Speaker: Sara Elisa Motta (Istituto Nazionale di Astrofisica (INAF))

10:25 The outflow model of black-hole X-ray binaries: predictions

The outflow model is a very simple model, yet it explains quantitatively a number of very stringent correlations observed in black-hole X-ray binaries, some of which have not been explained by any other model. It is interesting that all these correlations are explained with only two parameters with values in two narrow ranges. What is more interesting is that the model makes three quantitative predictions for the source GX 339-4. These are: 1) the time-lag of the hard X-ray photons (6 – 15 keV) with respect to softer ones (2 – 6 keV) should increase at all Fourier frequencies as the source moves from the low-hard state, to the hard state proper, to the hard-intermediate state (HIMS). 2) The radio flux as a function of the X-ray photon-number spectral index Gamma, during the rise of an outburst, should be a bell-shaped curve with its maximum in the HIMS. 3) The well-known radio – X-ray correlation should break down at the end of the hard state and after that the radio flux should first increase suddenly and then decrease suddenly within a small range of the X-ray luminosity. The first prediction has been confirmed and I will discuss it. The other two predictions are awaiting publication of MeerKAT observations taken in 2024. Hopefully, these observations will be public by the time of the workshop and the model will either survive or it will be killed.

Speaker: Prof. Nick Kylafis

10:40 Jet Formation in Transitional Millisecond Pulsars: Insights from ALMA Observations of 4FGL J0427.8−6704

Transitional millisecond pulsars (tMSPs) offer a unique opportunity to study jet formation in a low-accretion regime around fast-spinning, magnetized neutron stars. We analyzed archival spectral energy distributions of confirmed and candidate tMSPs to investigate their jet properties. In this presentation, I will focus on the candidate tMSP 4FGL J0427.8−6704, for which ALMA data revealed a low-frequency jet break, providing direct estimates of the jet base properties. Additionally, I will compare these properties with those of other neutron star accretors. These findings place new constraints on jet formation in tMSPs and highlight the need for multi-wavelength studies to fully characterize jet behavior in low-accretion systems.

Speaker: Karri Koljonen (Norwegian University of Science and Technology)

10:55 → 11:25 Coffee Break

11:25 → 13:10 Jets (Observation, theory and modelling)

Convener: Julien Malzac (IRAP (CNRS / Universite de Toulouse))

11:25 The Z-shape parsec-scale jets of the microquasar GRS1758-258 as seen by MeerKAT

Jets from black-hole X-ray binary systems (BH-XRBs) are powerful outflows that release a large fraction of the accretion energy to the surrounding environment, providing a feedback mechanism that alters the properties of the interstellar medium (ISM). Studying accretion processes alongside their feedback on the environment is crucial, as it enables to estimate the energy input/output around accreting BHs.
Signatures of jet feedback from BH-XRBs have been confirmed in few sources, such as GRS1758-258. At radio frequencies extended structures around the binary position were first observed in archival VLA data. These features are believed to originate from the jet-ISM interaction, leading to the formation of parsec-scale jets that create a Z-shape structure.
We present the recent results obtained from ~9hr of observations taken in 2024 at 1.28GHz using the MeerKAT interferometer. We identified several regions forming the extended structures seen in the radio image, characterized by synchrotron or bremsstrahlung emission. Comparing these data with those from VLA we observed the jet’s motion and measured a velocity of ~6000 km/s. Following a calorimetry-based method originally proposed for AGN, later applied to XRBs, we estimated the time-averaged jet power transferred to the ISM to be on the order of ~10e36 erg/s, comparable to the accretion energy released by XRBs in outburst. The morphological resemblance to the jet-ISM interaction structures seen in other cases, GRS1915+105 and Cygnus X-1, combined with the lower energy transfer observed in GRS1758-258, suggests that the latter may represent an earlier evolutionary stage of large-scale jets interacting with the environment.

Speaker: Isabella Mariani (Istituto Nazionale di Astrofisica (INAF))

11:40 Giant radio flares from microqusars

One of the most amazing microquasars is the Cyg X-3 binary system with a black hole and a massive Wolf-Rayet star. In 2024, during multi-frequency monitoring with the RATAN-600 radio telescope, we detected five giant (>10Jy) flares from Cyg X-3. The onsets of flares was clearly associated with the detection of significant Gamma-ray radiation at 0.1-300 GeV (Fermi space observatory). The flares occurred just after the hyper-soft X-ray state, when the flux of hard X-ray radiation (15-50 keV) strongly increased from zero, and a flux of soft radiation of 4-10 keV (MAXI monitor) decreased. The flares trigger by change of supercritical rate of matter accretion onto black hole, variations in the state of the corona and the accretion disk, and changes in the collimation process of jets. Radio flares evolved on different time scales from 10-60 days: at first, the flux grew linearly with time, and the radio emission was optically thick at frequencies below 2 GHz, then after the maximum of the flare, there was an exponential attenuation with a ''softening'' of the spectrum (spectral index varied from -0.1 to -0.9). After period of the almost zero radio fluxes GRS 1915+105 wake up in 2023 and then maximum flux reached 5.5 Jy at 2.3 GHz, unexampled for measurements before. We followed this giant flare between August 1-9, 2023 simultaneously on two frequencies 4.7 and 8.2 GHz in the daily multi-azimuthal mode observations, when flux were measured every 8.6 minutes within interval +-2.5 hours from the local culmination time.

Speaker: Sergei Trushkin (Special Astrophysical Observatory)

11:55 Precessing relativistic jets and dynamic shocks powered by an accreting neutron star

Circinus X-1 is a peculiar neutron star X-ray binary system (XRB) which defies conventional classification despite being studied for over 50 years. Surrounded by its natal supernova remnant aged at ~4600 years, Circinus X-1 is the youngest known XRB. However paradoxically, it displays many features common to older, low mass, low magnetic field neutron star XRBs. This provides a unique laboratory to test our knowledge of accretion, jets, and jet interactions with surrounding media.

We present the results of an intensive radio campaign on Circinus X-1. We use the excellent sensitivity of the MeerKAT telescope combined with the improved resolution of the new S-band receivers to produce the most detailed images to date of the jets and the surrounding radio nebula. We show that the jets of Circinus X-1 are able to propagate relativistically to parsec scales, and we identify a curved S-shape jet morphology. We observe drastic swings in the jet axis over time, over a total range of 110 degrees on the sky. Both of these features can be explained by extreme precession of the jet launch axis. Furthermore, using over 20 years of archival radio data we have found dynamic large scale, polarised, decelerating relativistic shocks being driven into the ambient medium, and identify them as a potential site of PeV particle acceleration. These results all illustrate that Cir X-1 is the southern, younger analogue of the famous microquasar SS 433.

Speaker: Fraser Cowie (University of Oxford)

12:10 The Homogeneous MeerKAT & Swift XRT Radio:X-ray Plane

The radio:X-ray plane is a valuable tool to better understand the connection between accretion and jet production in X-ray binaries during their hard spectral states. This correlation was originally believed to be universal and observed to span many orders of magnitude in X-ray flux. It was later extended to active galactic nuclei by including an additional mass term. However, doubt has since been cast over its universality, as many ‘outliers’ have been found that do not follow the so-called ‘standard’ correlation, and instead seem to populate a separate track.

Thus far, all large studies of the radio:X-ray plane have combined data from various telescopes, thereby introducing errors when the fluxes are converted to a common frequency. In addition, these results are impacted by differing systematic errors across telescopes, and mistakes that have been propagated in the literature. Therefore, questions remain regarding whether there truly is a separate track, or whether the outliers are simply manifestations of a scatter in the correlation.

ThunderKAT was a five-year programme on the MeerKAT telescope which monitored outbursting X-ray binaries. It ran in conjunction with SwiftKAT, which collected quasi-simultaneous Swift XRT observations of these sources. Using these data, we have compiled the largest, homogeneous radio:X-ray plane for X-ray binaries to date. I will present the results of this study, addressing whether multiple populations truly exist, the possible influence of Doppler boosting, and what the refined correlation reveals about accretion and jet physics. This will be accompanied by a public data release.

Speaker: Justine Crook-Mansour (University of OXford)

12:25 Tracking Jet Evolution in Black Hole X-ray Binaries: First Results from the PITCH-BLACK Survey

Relativistic jets launched from accreting compact objects are thought to play an important role in our Universe, influencing large-scale processes such as galaxy evolution and star formation. However, the connection between their properties and those of the accretion flow remains poorly understood. Multi-wavelength time-domain observations of black hole X-ray binaries now offer a new avenue to study this connection in detail, as different wavelengths probe different components of the system, in turn making it possible to track material as it moves from inflow to outflow. Recently, we launched the PITCH-BLACK survey, a first-of-its-kind large global observing program designed to yield multi-wavelength time-domain observations spanning across an entire X-ray binary outburst. In this talk, I will present the first results of the PITCH-BLACK survey, including sub-millimetre time-domain and polarimetric observations of Swift J1727.8-1613.

Speaker: Élodie Lescure (University of Lethbridge)

12:40 High sensitivity radio observations of neutron star X-ray binary jets: variability, spectra, and polarization

Neutron star X-ray binary jets are observed to be less luminous than those of black holes, and are also less observed and thus less understood. Modern radio facilities are sensitive enough to investigate these neutron star jets in detail and begin to look at their properties as has been done for black hole jets. I will present an archival radio data project that compares rapid variability properties of black hole versus neutron star jets, as well as compact versus discrete jet types. I will also present new radio data of Eddington accretion neutron star X-ray binary jets that provide spectral information, and for the first time detect linear radio polarization in neutron star X-ray binaries that can also be compared to X-ray polarization information, investigating both neutron star and Eddington jet parameter space and laying the foundation for more complex investigations of neutron star jets in the future.

Speaker: Eliot Pattie (Texas Tech University)

12:55 Jet modeling with ISHEM of the BH transient MAXI J1535-571

The multi-wavelength spectral energy distributions of low-mass X-ray binaries (LMXBs) in the hard state are thought to be determined by the emission from a jet (up to mid-infrared frequencies) and the emission from the accretion flow from optical to X-ray up to (possibly) the soft gamma-ray domain. In recent years, the flat radio-to-mid-IR spectra of black hole (BH) X-ray binaries have been described using the internal shocks model. This model assumes that the fluctuations in the velocity of the ejecta along the jet are driven by fluctuations in the accretion flow, as described by the X-ray Power Density Spectrum (PDS).
In this work, we aim to apply an updated version of the internal shocks code for the ejection to a BH LMXB, specifically MAXI J1535-571. We used a multi-wavelength data set obtained in 2017, comprising data from radio to gamma-ray. The old version of the code already allowed for changes in the geometry of the jet, accommodating non-conical jets, and took into account adiabatic cooling. The important change in the new version is the inclusion of the radiative cooling.
We fit the spectral energy distribution (SED) of MAXI J1535 collected in different spectral states. Our results seem to favour a parabolic jet geometry over a conical one, unlike the results for other BHs analyzed previously, but similar to the findings for the only NS examined thus far. Additionally, we investigated the possible contribution of the jet to the gamma-ray emission using data from the SPI instrument onboard INTEGRAL.

Speaker: Ms Carlotta Miceli (Istituto Nazionale di Astrofisica (INAF))

13:10 → 13:28 Jets (Observation, theory and modelling): Flash Talk

Convener: Julien Malzac (IRAP (CNRS / Universite de Toulouse))

13:10 A black hole jet bent by stellar wind

Jets provide an important channel of kinetic feedback from accreting black holes into their environments. In the absence of instantaneous jet power measurements the extent of this feedback has proven difficult to quantify. Here we report the detection of stellar wind-induced bending of the jets in black hole X-ray binary Cygnus X-1, using 18 years of high-resolution radio imaging. By undertaking detailed physical modeling of the jet-wind interaction, we determine the current kinetic power of the jet at launch to be log$_{10}(L_{\rm jet}/{\rm erg\,s}^{-1}) = 37.3_{-0.2}^{+0.1}$, consistent with the long-term time-averaged power required to inflate the nebula around the system. We also measure the hard state jet speed of Cygnus X-1 to be 0.5c, and also rule out misalignment between the black hole spin and binary orbit thought to exist in the system. Our measurement of the instantaneous jet power robustly anchors the theoretically-derived scaling relation between radio luminosity and jet power across the black hole mass scale.

Speaker: Steve Prabu (Oxford University)

13:12 Are jets from stellar mass black holes as fast as those from supermassive black holes?

Jets from stellar mass black holes in X-ray binaries (XRBs) and supermassive black holes in blazars provide distinct opportunities to study the jets of black holes across two different mass regimes. They also represent samples with very different selection effects. Historically, the apparent speeds of XRB jets have been observed to be lower than those of blazars, leading to the assumption that this reflects the underlying distributions of Lorentz factors, i.e. stellar mass BHs produce slower jets. In this talk, I will present our detailed modelling of the parent population for large-scale BHXRB jets, which accounts for the selection effects present in the observed sample. Using nested sampling, we determined that the Lorentz factors of the parent population of BHXRBs are best described by a power law with a slope of $b=-2.63^{+0.87}_{-1.18}$ where $N(\Gamma) \propto \Gamma^{b}$ , the same model which has been historically applied to blazar jets. We can reject several other potential Lorentz factor distributions using Bayes factors. When comparing our findings to the parent distributions of Blazar jets documented in the literature, it is notable that we cannot rule out the possibility that both BHXRBs and Blazar jets share the same parent population Lorentz factor distribution. While other astrophysical considerations, beyond kinematics, may lead to constraints on the maximum Lorentz factor, we find no constraints on the upper limit, $\Gamma_{\text{max}}$. In other words, based on kinematics alone, jets from stellar mass black holes are broadly consistent with being just as relativistic as those from supermassive black holes.

Speaker: Clara Lilje (University of Oxford)

13:14 Estimating black hole jet power through astrochemistry: New ALMA observations of MAXIJ1348-630

Black Hole X-ray binaries (BHXBs) serve as powerful laboratories for studying relativistic jets and their impact on the surrounding medium. MAXI J1348-630 is a BHXB that was recently discovered in outburst. This source was shown to exhibit strong radio jets with a distinct deceleration pattern of motion, indicative of the presence of a jet-blown cavity in the interstellar medium (ISM). To investigate this further, we observed MAXI J1348-630 with ALMA to map the molecular line emission and search for evidence of a jet-ISM interaction in this region. In this talk, I will discuss the compelling evidence we found in favour of a jet-ISM interaction consistent with the previously proposed cavity structure surrounding this BHXB. These findings allow us to estimate the jet power and provide new insights into how BHXB jets inject energy into their environment, with broader implications for feedback mechanisms in galactic ecosystems.

Speaker: Pau Bosch-Cabot (University of Lethbridge)

13:16 Energy dissipation and particle acceleration in XRB jets: the potential role of the kink instability

Previous observational and simulation work has explored the role the kink instability (KI) could play in explaining the phenomenology of active galactic nuclei jets and gamma ray bursts. Internal modes of this instability have been shown to drive conversion of magnetic energy into thermal energy without disrupting the overall flow of the jet. We suggest that these modes may therefore be able to explain the particle acceleration region required for persistent compact jets in X-ray binaries (XRBs). In contrast, external modes can lead to disruption of the overall flow and rapid bursts of magnetic reconnection, which may relate to flaring and the release of discrete ejecta.

We calculate the linear growth rates of the m=1 KI for a range of jet models and parameters. In this talk, I will show that there exist areas of parameter space where a perturbation will grow. Small perturbations on length scales $10^{-3}$ of the jet base radius grow to significant fractions of the radius of the jet within the expected jet size along the propagation axis ($\sim10^{-5}$ parsecs). This growth exceeds the limits of the linear regime of this instability and therefore may lead to particle acceleration zones able to explain the observed compact jet’s power law spectra. In relation to flaring, we investigate the ability of the KI to grow and dissipate energy on the necessary timescales. Our results suggest that the KI could be an important mechanism for energy dissipation and particle acceleration in XRB jets.

Speaker: Emma Elley (University of Oxford)

13:18 Joint Radiative and Kinematic Modelling of X-ray Binary Ejecta: Energy Estimate and Reverse Shock Detection

Black hole X-ray binaries in outburst launch discrete, large-scale jet ejections which can propagate to parsec scales. The kinematics of these ejecta appear to be well described by relativistic blast wave models originally devised for gamma-ray burst afterglows. In kinematic-only modelling a crucial degeneracy prevents the ejecta energy and interstellar medium density from being accurately determined.

I will present the first joint Bayesian modelling of the lightcurves and kinematics of a large-scale jet ejection from the X-ray binary MAXI J1535-571. We find that the ejecta is launched perpendicular to the disc with an initial energy of $E_0 \approx 5 \times 10^{43} \, {\rm erg}$, and propagates into an underdense $n < 10^{-4} \, {\rm cm^{-3}}$ interstellar environment. We find that a long-lived reverse shock powers the bright, early ($t_{\rm obs} < 100$ days) ejecta emission. Further analysis suggests long lived reverse shocks are likely ubiquitous for outflows with moderate Lorentz factors, making them a unique laboratory for shock acceleration physics. This work lays the foundation for future parameter estimation studies using all available data of X-ray binary jet ejecta.

Speaker: Alex Cooper (University of Oxford)

13:20 The radio flare events in 4U 1543-47

Strong radio flare events were detected in black hole X-ray binary system 4U 1543-47 by MeerKAT. In this work, we investigated X-ray observation data during these periods. Our results suggest a four-step jet cycle: (1) the accretion disk is full, (2) the inner disk becomes unstable, (3) the jet is formed, (4) the disk refills.

Speaker: Zuobin Zhang (University of Oxford)

13:22 How Many Impostors are Among the IMXBs?

Recent endeavors to directly classify the donor star mass of extragalactic X-ray binaries (XRBs) through optical photometric modeling has uncovered an apparent overabundance of XRBs with donor stars with a mass between 3 and 8 solar masses. These so-called 'intermediate-mass XRBs' (IMXBs) make up as many as 30% of the XRBs in nearby spiral galaxies. A population of this size is in contradiction with current XRB evolutionary models, which indicate IMXBs should be extremely short-lived and, thus, rare. One possible explanation for these observations is that these IMXBs actually represent a population of low-mass XRBs that mimic the optical properties of intermediate-mass stars, as is the case with the Galactic low-mass XRB Cyg X-2. To test this hypothesis, I am conducting an analysis of XRBs in nearby elliptical galaxies, whose XRB populations are known to be comprised entirely of low-mass XRBs. Starting with M32 and M105, I will conduct an SED analysis of XRB donor stars using optical HST photometry in search of those with optical luminosities and spectral types that are incompatible with a standard low-mass star. The presence of such sources in elliptical galaxies would suggest that at least some of the IMXBs identified in spiral galaxies are actually over-luminous low-mass XRBs. On the other hand, the lack of this detection may not rule out the possibility that these enigmatic XRBs in spiral galaxies are low-mass, but it will allow us to put observational constraints on the time frame for which low-mass XRBs may host over-luminous donor stars.

Speaker: Qiana Hunt (University of Lethbridge)

13:30 → 16:30 Lunch

20:00 → 00:20 Social Dinner

Thursday 18 September

09:50 → 11:35 XRBs meet ULX and AGN

Convener: Dr Ciro Pinto (Istituto Nazionale di Astrofisica (INAF))

09:50 Super-Eddington accretion under the magnifier: the case of Ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are extremely bright, extragalactic, point-like X-ray sources with isotropic luminosities well in excess of $10^{39}$ erg s$^{-1}$. The vast majority of ULXs could be binary systems where a stellar mass black hole or a neutron star accretes matter at super-Eddington rates from the companion.
The number of confirmed ULXs is significantly increasing and now it counts more than a thousand of both persistent and transient sources. Regardless of the nature of the compact objects, ULXs show manifold properties at different energy bands and they currently represent a hot topic for the comprehension of physical processes related to supercritical accretion onto compact objects. Indeed, a number of pulsating ULXs were recently discovered, confirming the existence of neutron stars in such systems. This raises a strong challenge to the understanding of the accretion processes on compact objects.
In this talk, I will give an overview of the most recent discoveries on ULXs, from a multi-wavelength point of view that will comprise, amongst the others, the outflows from ULXs and the link with the nebulae/bubbles around many of them, the ULX main X-ray spectral properties and variability behaviours, and the features of the pulsating ULXs. Such observational results will be discussed within the context on the super-Eddington accretion processes observed in ULXs.

Speaker: Fabio Pintore (Istituto Nazionale di Astrofisica (INAF))

10:10 A mysterious source of fast outflows in a nearby dwarf galaxy: a ULX shrouded by a thick wind or envelope?

There is a mysterious point-like source in the starburst dwarf galaxy NGC 5408. It is the strongest compact radio source in that galaxy (5-GHz flux density of $\approx$2 mJy, corresponding to a luminosity of $\approx$3 $\times 10^{35}$ erg s$^{-1}$). In the optical, it is too bright and too red ($V \approx 19.5$, $R \approx 18.1$, corresponding to $M_V \approx -9$, $M_R \approx -10.4$) to be consistent with any individual star or HMXB or young star cluster. Its optical spectrum is completely dominated by two sets of strong emission lines: broad and narrow. Very broad Balmer and HeI lines imply a dense, powerful and highly variable outflow with speeds of up to $\pm$3,000 km s$^{-1}$. Photo-ionized, narrow higher-ionization lines (eg He II 4686) suggest an intrinsic X-ray luminosity of several times $10^{40}$ erg s$^{-1}$ (mostly blocked from our view). The amount of power required to explain X-ray and radio emission suggests the presence of a fast-accreting black hole, but the optical spectrum is very different from any previously known ULX. We speculate that it is a rare type of microquasar going through a short-lived evolutionary phase of its massive donor star. The optically thick outflow may come from a super-critical accretion disk or from common envelope ejection, with the large outflow photosphere responsible for the optical continuum (blackbody radius $\approx 1000 R_\odot$). The radio emission may trace a compact (very young?) shock-ionized bubble.

Speaker: Roberto Soria (Istituto Nazionale di Astrofisica (INAF))

10:25 An Observational Test of the ULX Nature of SS 433

The X-ray binary SS 433, a Galactic microquasar, has been hypothesized to be an ultraluminous X-ray source (ULX) like those observed in nearby galaxies. It is well known that the compact object emits oppositely directed semi-relativistic jets that carry a much larger kinetic power than observed radiatively in the X-ray band. Much of the radiative power may be obscured from our view, which would link SS 433 to ULXs. SS 433 has extended X-ray emission, like Sy 2s where the extended gas is photo-excited by the obscured nuclear X-ray emission. We carried out regular Chandra imaging of the arc-second scale extended emission over a period of several months for comparison to the continuum flux monitored with Swift. The extended emission appears to show structure that has not been previously observed.

Speaker: Herman Marshall (MIT Kavli Institute)

10:40 Magnetic field topology unveils a discrete jet in the exotic microquasar SS 433

The ejection of relativistic outflows is the most spectacular consequence of accretion onto compact objects, powered by the interplay of gravity, particles and magnetic fields. The microquasar SS 433, one of the most exotic binary systems in our Galaxy, shows powerful precessing jets. In these outflows, radio synchrotron emission unveils a complex magnetic field topology, seemingly parallel to the bulk velocity of propagation. Although the origin of this field remains unclear, it was suggested that it might be connected with the jet underlying morphology. In this contribution, we study this intriguing connection. By running cutting-edge numerical simulations that include for the first time the evolution of the source magnetic fields, we show that the observed field orientation is explained by the collision of discrete structures that propagate with different velocity on the sub-parsec scale. These prompt interactions also lead to the formation of elongated plasma bullets which are more stable, and thus more propitious to propagate to larger distances. This result represents a robust piece of evidence of the tight connection between the jet morphology and the topology of the magnetic field in the microquasar SS 433, leading to valuable insights into the broader picture of magnetic field evolution and non-thermal processes in relativistic jets.

Speaker: Jose Lopez Miralles (Aurora Technology for ESA, ESAC/ESA, XMM-Newton SOC)

10:55 Understanding the link between XRBs and AGN

There has been a large effort in trying to investigate the similarities between supermassive black holes (SMBH) in active galaxies and stellar black holes (SBH) in X-ray binary systems (XRB). Both exhibit changes in accretion states, but on different timescales. Changes in the accretion state of Changing-Look Active Galactic Nuclei (CLAGN) are particularly interesting, as they occur on shorter timescales (months-years) and are accompanied by changes in luminosity and Eddington ratio. These CLAGN can be used to compare accretion state changes between AGN and XRB. Similarly, Tidal Disruption Events in SMBH are allowing to investigate the onset of the coronal and jet activity. A comparison between these systems may provide new clues in our understanding of the accretion and ejection phenomenon around black holes in their fundamental nature.

Speaker: Dr Francesca Panessa (Istituto Nazionale di Astrofisica (INAF))

11:15 Changing-state AGN and X-ray binaries

AGN are known to show flux variability over all observable timescales and across the entire electromagnetic spectrum. Over the past decade, a growing number of sources have been observed to show dramatic flux and spectral changes, both in the X-rays and in the optical/UV. Such events, commonly described as “changing-look AGN”, can be divided into two well-defined classes. Changing-obscuration objects show strong variability of the line-of-sight column density, mostly associated with clouds or outflows eclipsing the central engine of the AGN. Changing-state AGN are instead objects in which the optical/UV continuum emission and broad emission lines appear or disappear, and are typically triggered by strong changes in the accretion rate of the supermassive black hole. In my talk, I will review our current understanding of Changing-State AGN, and then focus on similarities between these objects and X-ray binaries.

Speaker: Prof. Claudio Ricci (University of Geneva)

11:35 → 12:05 Coffee Break

12:05 → 13:05 Accretion-ejection (observation, theory, simulations)

Convener: Dave Russell (New York University Abu Dhabi)

12:05 The accretion-ejection connection in X-ray binaries: an observational perspective

Jets are a ubiquitous phenomenon in all classes of accreting stellar-mass compact objects; white dwarfs, neutron stars, and black holes. From the observations, it is clear that jet production appears to be fundamentally related to the process of accretion, however, the precise nature of the coupling remains to be determined. In this talk, I will review what recent (and not so recent) observing campaigns have revealed about the connection between the accretion inflow and jet outflows during an outburst. I will also discuss what the immediate future holds for accretion-ejection studies of X-ray binaries.

Speaker: Thomas David Russell (Istituto Nazionale di Astrofisica (INAF))

12:35 Out with a bang: Simulating the impact of thermonuclear explosions on accretion disks and jets

Matter accreting onto the surface of a neutron star can ignite in a thermonuclear explosion visible as a ~1-min long bright flash of X-ray emission. Such thermonuclear X-ray bursts radiate a total energy of ~E39 erg and can recur on a timescale of hours. Every time an explosion goes off, a strong radiation field suddenly floods the direct environment of the neutron star and observations have revealed a detectable response in both the accretion flow (disk, corona) and jet. This opens up the exciting opportunity to use thermonuclear explosions as repeating, dynamical probes to study the physics of accretion inflows and outflows. Recent observational discoveries align well with steady advances in numerical modeling, which now make it possible to simulate the impact of an X-ray explosion to the accretion disk, corona and jet. This talk will show initial results of GRMHD simulations and provide an outlook for future research, including how we can connect simulations to observations.

Speaker: Nathalie Degenaar (University of Amsterdam)

12:50 The accretion ejection interplay in GX 340+0: Insights from joint X-ray and radio campaign

Accreting low-mass X-ray binaries (LMXBs) provide a unique testbed to investigate the complex interplay between accretion flows and relativistic jets. In both neutron star and black hole systems LMXBs, X-ray and radio emissions are observed to be correlated, though neutron star systems exhibit significantly lower radio fluxes at comparable X-ray luminosities. Among neutron star LMXBs, Z-sources are particularly notable for their rapid X-ray variability, often accompanied by correlated changes in radio emission. To explore the accretion geometry and jet properties of the enigmatic Z-source GX 340+0, we conducted a multi-wavelength observational campaign using various X-ray and radio observatories. In this talk, I will present how polarimetric measurements with IXPE, along with spectro-timing studies using AstroSat and NICER, provide critical insights into the structure of the Comptonizing medium and accretion flow components. Additionally, simultaneous radio observations allow us to track the evolution of the radio spectrum, identify spectral breaks, and examine how the jet properties evolve along the Z-track. These results offer new constraints on the coupling between accretion processes and jet formation in neutron star X-ray binaries.

Speaker: Dr Yash Dilip Bhargava (Istituto Nazionale di Astrofisica (INAF))

13:05 → 13:21 Accretion-ejection (observation, theory, simulations): Flash Talk

Convener: Dave Russell (New York University Abu Dhabi)

13:05 Doublet Type-I X-ray Bursts from EXO 0748–676 Observed with SVOM/ECLAIRs and MXT: Comparison with 2024 AstroSat and NuSTAR Observations

Neutron star X-ray binaries often exhibit Type-I X-ray bursts, during which the X-ray luminosity can rise up to 100 times the persistent flux. These bursts are triggered by unstable thermonuclear burning of accreted hydrogen and/or helium on the neutron star’s surface. In certain instances, two bursts are observed in close succession, typically separated by an interval of 10–20 minutes. The second burst is generally attributed to the ignition of residual fuel not consumed during the initial burst. Such events are referred to as doublet bursts. Consequently, the luminosity and spectral features of these bursts are often correlated, offering insights into the complex nuclear processes occurring on the neutron star surface. EXO 078–676 is a known X-ray burster and was observed to exhibit a Type-I burst with SVOM-ECLAIRs on 1 February 2025 in the 5–20 keV band. This was followed by a second burst approximately 17 minutes later, detected with SVOM/MXT. The integrated 4–20 keV flux decreased from (7.4$\pm$0.6)$\times$10$^{-9}$ erg s$^{−1}$ cm$^{−2}$ to (4.5$\pm$0.1)$\times$10$^{−9}$ erg s$^{−1}$ cm$^{−2}$ between the first and second bursts. We compare the profiles of these two bursts and perform time-resolved spectral analysis during both the persistent and burst phases to investigate the underlying mechanism. Additionally, we compare the SVOM observations with Type-I bursts detected for the source by AstroSat and NuSTAR in July 2024, with a particular focus on a detailed study of the photospheric radius expansion (PRE) phase

Speaker: Dr Divya Rawat (Observatoire astronomique de Strasbourg,)

13:07 Bubbles of ionized gas around Ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) are accreting stellar compact objects whose X-ray luminosities exceed the Eddington limit. In order to get more insights on the nature of their accreting compact object (black hole or neutron star) and the geometries of super-Eddington accretion flows, it is useful to look at the influence of ULXs on their environment. I will present how VLT/MUSE 3-D spectroscopy can help studying bubbles of ionized gas formed around some ULXs. I will discuss the nature of those bubbles: ionization by X-ray radiation and/or shocks which would indicate the presence of winds.

Speaker: Anastasia Kilina (IRAP)

13:09 XMM–Newton, NICER and NuSTAR observations of the ultraluminous source NGC 4190 ULX-1

We report the results obtained from the analysis of XMM–Newton observations together with simultaneous NICER and NuSTAR observations of the ultraluminous X-ray source NGC 4190 ULX-1. Our goal is to constrain the structure of the accretion disk and the geometrical properties of the source, performing temporal and spectral analyses in the 0.4−30 keV energy range. The temporal analysis shows no flaring activity in the light curves. No pulsation is detected by any instrument. The source exhibits a characteristic ULX spectrum, which can be fitted with two thermal blackbody components plus a Comptonization tail at high energies. The luminosity-temperature relation of each thermal spectral component is consistent with the $L \propto T^{2}$ relation expected from an advection-dominated supercritical disk. We interpret these results as a super-Eddington accreting BH seen almost face-on where a dense wind ejected from the disk obscures the central source, and a hot electron plasma is evacuated through the funnel formed above the hole. Geometric beaming is responsible for the ULX soft emission, whereas the hard tail is the result of Comptonization of soft photons by the electrons ejected through the funnel. We were able to model the data assuming a black hole of 10 $M_{\odot}$ and an accretion rate of $\sim 10 \;\dot{M}_{\rm Edd}$. The matter escaping from the funnel could form a synchrotron emitting jet potentially detectable with future high resolution radio observations. Detection of Balmer and He lines may provide more information about the wind, in particular its velocity.

Speaker: Dr Jorge Ariel Combi (Instituto Argentino de Radioastronomía (IAR))

13:11 When ULXs Blink: A Detailed Look at Short-Lived Super-Eddington Accretion in M51

Ultraluminous X-ray sources (ULXs) are thought to be powered by super-Eddington accretion onto stellar-mass compact objects and represent the high-luminosity end of accreting binary systems. Here, we report the discovery of a new transient ULX candidate (M51 ULX) in the face-on spiral galaxy M51, identified using archival Swift, XMM-Newton, and Chandra data. The source underwent a short-lived outburst in 2018, reaching a peak X-ray luminosity of ~7.8 $\times$ 10$^{38}$ erg s$^{-1}$ and showing flux variability by a factor of ~200. The X-ray spectrum remained consistently soft, with no significant spectral evolution across the outburst. No pulsations were detected, with pulsed fraction upper limits of ~30\%. Optical and UV data from Swift and XMM-Newton revealed no counterparts, arguing against a background AGN or foreground object. We performed spectral modeling using both phenomenological and physical disk models, and applied Bayesian parameter estimation with BXA to characterize the emission properties and test for a multi-component structure. The results are consistent with a stellar-mass black hole or neutron star accreting at or above the Eddington limit. M51 ULX adds to a growing class of transient ULXs in which short outbursts challenge standard models of disk stability and accretion state transitions. In this talk, I will present our analysis of M51 ULX* and explore how short-lived ULX transients are key to understanding the behavior and accretion processes of microquasars, linking accretion physics across a wide luminosity range and helping to unify our understanding of how co›mpact objects accrete under extreme conditions.

Speaker: Dr Anastasiya Yilmaz (INAF-IAPS)

13:13 Microquasars to QSOs with CCI: big leap or continuous spectrum?

We have used The Chandra Source Catalog to extract light curves of point sources from nearby galaxies, ULXs, and 25 nearby (z lt 0.2) QSOs. We have Chandra grating observations of 25 Galactic XRBs adjusted to ACIS count rates at a distance of 1 Mpc. A three-dimensional color-color-RelInt diagram shows that the distribution of ULXs is well matched in both color and luminosity with point sources from early-type and starburst galaxies. QSOs match the colors of ULXs but are several orders of magnitude higher in luminosity. The extra-galactic point sources also extend to much higher luminosities than XRBs within our Galaxy. Very few ULXs line up with Galactic XRBs. The ULX M82 X-1 is the only ULX associated with a black hole compact object for which we have CSC2 lightcurves. None of the pulsing ULX candidates (M82 X-2, NGC5907 ULX-1, NGC7793 P13, and NGC4599 ULX) line up in colors with Galactic pulsars although we note that each of these sources had only 2-4 significant detections in CSC2. If location in a color-color plane is related to the type of compact object in an XRB, then a significant fraction of extragalactic point sources and ULXs do not line up with any type of Galactic XRB.  The fact that the distribution of ULXs line up so well with the distribution of extragalactic point sources both in the color-color plane and in luminosity suggests a population of objects that have no XRB counterpart in the Galaxy or the Magellanic Clouds.

Speaker: Saeqa Vrtilek (Center for Astrophysics | Harvard & Smithsonian)

13:15 Tracing Accretion State Transitions in AGN: Insights from Photoionisation Simulations and IR Nebular Diagnostics

Accretion states, which are universally observed in stellar-mass black holes in X-ray binaries, are also anticipated in active galactic nuclei (AGN). The luminosity-excitation diagram (LED), based on IR nebular emission-line ratios, successfully identifies distinct accretion regimes in AGN —from jet/corona-dominated emission in low-luminosity AGN (LLAGN) to disc-dominated emission in Seyfert galaxies and quasars. To characterise the evolution of the ionising continuum across these states, we present new photoionisation simulations using theoretical continua from the JED-SAD model. Our simulations successfully reproduce the observed distribution of AGN along the LED sequence across a diverse sample of 320 sources, establishing a robust link between nebular line excitation signatures and the underlying accretion physics. A multi-diagnostic analysis, incorporating faint transitions from high-ionisation species recently observed by JWST, indicates that disc-dominated nuclei require a gradual softening of their ionising continuum bluewards of the UV bump, consistent with the contribution from a soft X-ray excess component. Conversely, LLAGN exhibit ionisation signatures indicative of a harder, jet/corona-dominated continuum. These findings demonstrate that IR nebular lines uniquely probe the extreme UV/X-ray regime, otherwise inaccessible due to hydrogen absorption, offering crucial insights into the accretion physics of supermassive black holes.

Speaker: Dr Juan Antonio Fernandez Ontiveros (Centro de Estudios de Física del Cosmos de Aragón (CEFCA))

13:17 Revisiting the LS I+61303 environments

We present a reanalysis of archive radio interferometric data for the gamma-ray binary LS I +61303. Our aim is to expand our previous searches for associated extended emission at the few arc-second/arc-minute level. The existence of such large scale features is expected no matter the physical scenario that is really at work in this system: pulsar wind interaction or microquasar. The main motivation to revisit the LS I +61303 extended emission issue is the present availability of highly sensitive archive data from the Expanded Very Large Array (EVLA). These observations were originally conducted for different purposes, but with a much better sensitivity than the historic VLA with narrow bandwidth. Our preliminary analysis, after combining different EVLA data sets into a single map, confirms that extended emission is clearly detected in the target vicinity. However, proof of a physical connection remains to be obtained.

Speaker: Pedro Luis Luque Escamilla (Universidad de Jaén)

13:19 Evidence for enhanced mass transfer in the disc preceding the transition to the soft state in MAXI J1820+070

We investigate the 2018 main outburst and the subsequent mini-outbursts of the black hole low-mass X-ray binary MAXI J1820+070 using optical/ultraviolet data from the Las Cumbres Observatory (LCO), the American Association of Variable Star Observers (AAVSO), and SWIFT/UVOT, as well as X-ray data from Insight-HXMT and SWIFT/XRT. Given the high-cadence observations, we identify a broad dip-like feature in both the optical and X-ray light curves preceding the transition to the soft state, with the X-ray dip lagging the optical dip by approximately 10 days. We propose that the dip is caused by a brief decrease followed by an increase in the mass accretion rate as it propagates through the disc, ultimately triggering the transition to the soft state. This might be a potential tool to predict impending hard-to-soft state transitions, although such a dip has not yet been observed in many sources. Additionally, we find that optical colour (g’-i’) becomes bluer and less variable before the transition to the intermediate state, preceding a dramatic change in the hardness ratio. This appears to be an unusual case, differing from the typical scenario where the optical colour changes usually along with the transition to the soft state. Finally, we explore the implications of the complex evolution of optical/X-ray correlation during both main outbursts and mini-outbursts. In particular, we find a loop-like evolutionary track before the transition to the soft state, which is linked to the optical/X-ray dips in the light curves.

Speaker: Pengcheng Yang (Kapteyn Astronomical Institute, University of Groningen)

13:21 → 16:00 Lunch

16:00 → 17:15 Accretion-ejection (observation, theory, simulations)

Convener: Sara Elisa Motta (Istituto Nazionale di Astrofisica (INAF))

16:00 Simulations of black hole accretion and jets

In my talk, I will review the recent progress in simulating the accretion and outflows around black holes using general relativistic magnetohydrodynamic simulations. In particular, I will focus on the recent push to extend the scale separation accessible to the simulations and connecting the black hole (event horizon) to the much larger feeding (galaxy) scales.

Speaker: Sasha Tchekhovskoy

16:30 Magnetized Accretion-Ejection Structures, or how to go from JEDs to MADs.

Jet emitting disks (JEDs) provide the theoretical framework of mathematically exact steady-state accretion and ejection solutions. A large-scale vertical magnetic field threads a turbulent, strongly magnetized, accretion disk driving laminar, bipolar, super-Alfvénic jets. In previous treatments of JEDs, turbulence has been considered to provide only anomalous transport coefficients, namely magnetic diffusivities and viscosity. However, 3D numerical MHD experiments show that turbulent magnetic pressure also sets in.

It will be firstly shown how this turbulent pressure deeply affects the overall accretion-ejection behavior. The disk becomes puffier and less electrically conductive, causing radial and toroidal electric currents to flow at the disk surface. This is in close agreement with numerical experiments, such as the inadequately called magnetically arrested disks (MADs).

Then, I will report 2D simulations done with the MHD code PLUTO. While the are key to reproduce systems on both, astrophysically relevant, spatial and temporal scales, turbulence in 2D simulations must be introduced with prescriptions and their accuracy remain an open issue. It will be shown that analytical JED solutions reproduce perfectly 2D numerical simulations. This cross-validation opens up the route for educating 2D simulations with local turbulence prescriptions derived from costly 3D simulations.

It will be finally argued that hybrid disk configurations are unavoidable. A JED/MAD innermost region, where the disk magnetization is near unity, settles in, leaving an outer zone with a much smaller disk magnetization. This outer region is best described as a wind emitting disk (WED), since it undergoes a massive mass loss as slow winds.

Speaker: Prof. Jonathan FERREIRA (Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France)

16:45 Events leading up to the ejection of the corona in a microquasar

Superluminal ejections were first reported from microquasars almost thirty years ago. Their apparent superluminal motion is due to discrete, relativistic, collimated ejections of plasma launched from the vicinity of the black hole, during hard-to-soft state transitions. It is suspected, but not definitively proven, that these jets are fed from particles in the highly energetic corona, rather than the inner accretion disc material. The ejections are associated with abrupt changes in the fast X-ray variations in the corona. The physical cause of the discrete ejections, and the suppression of the corona and the steady jet, have remained contentious topics of debate. Here, we show that the vertical extent of the corona, and the base of the steady jet, both shrink together and then grow to their largest size contemporaneously, in the Galactic black hole, MAXI J1535-571. This suggests corona particles indeed feed the steady jet, and we find that the corona is largest at exactly the time of the discrete, superluminal ejection. A likely explanation is that when the corona shrinks, it feeds less particles into the steady jet, reducing its power. This results in the inflowing particles remaining in the corona, causing it to grow in size, and eventually escape as the superluminal ejection. This explanation of the cause of ejection of the corona, and the quenching of the steady jet, has implications for the disc-jet connection in both X-ray binaries and active Galactic nuclei, and could explain how quasars reach their bright, disc dominated state.

Speaker: Dave Russell (New York University Abu Dhabi)

17:00 Probing the jet response to neutron star X-ray bursts through the millimetre wavelength window

Astrophysical systems housing compact objects produce high-energy transient events that can be leveraged to study the processes of accretion and jet ejection. In particular, thermonuclear fusion on the surface of accreting neutron stars produces bursts of X-ray radiation that not only strongly impact the accretion structures in these systems, but as a recent novel experiment has shown, can also invoke a response in the outflowing jet. In this talk, I will discuss first results of the next iteration of this jet burst experiment. By extending our work into the millimetre bands, we can begin to take true advantage of this new methodology, place accurate constraints on jet speeds, characterize how the jet burst properties correlate with system properties, and ultimately understand the dominant mechanisms powering jets.

Speaker: Alex Tetarenko (University of Lethbridge)

17:15 → 17:45 Coffee Break

17:45 → 18:45 Accretion-ejection (observation, theory, simulations)

Convener: Alexandra Tetarenko (University of Lethbridge)

17:45 Jets and Luminosity Extremes: Insights from Swift J1727.8-1613’s 2023-2024 Outburst

Swift J1727.8-1613 is a recently discovered black hole low-mass X-ray binary (BH LMXB) that began its first recorded outburst in the autumn of 2023. The source rapidly became one of the brightest X-ray sources in the sky — reaching an X-ray flux nearly an order of magnitude greater than that of the Crab Nebula — and prompted a comprehensive multi-wavelength observing campaign. The radio emission from Swift J1727.8-1613 — and thus its relativistic jets — demonstrated a rich phenomenology, including multiple bright (>100 mJy) radio flares, spatially resolved ballistic ejections, and complex polarimetric evolutions.

In this talk, I will present key findings and interpretations from our multi-facility radio monitoring campaign, which tracked the evolution of the jets throughout this explosive outburst. I will also compare our radio (i.e., jet-centric) perspective to results at other wavelengths, particularly focusing on the remarkable X-ray luminosity at the start of the outburst. Perhaps most strikingly, at its current distance estimate, Swift J1727.8-1613 appears to be the brightest BH LMXB ever detected in the hard state, suggesting that it may harbour the most massive black hole yet identified in such a system.

Speaker: Andrew Hughes (University of Alberta)

18:00 New techniques for studying the real-time evolution of relativistic jets

For decades, high angular resolution VLBI observations of X-ray binary jets have been key to understanding the physics of jet launching. By precisely tracking the motions of transient jets, and thus inferring their ejection date, we can learn about the causal connection between changes in the inner accretion flow and the launching of jets. These efforts are often hindered by the difficulty of analysing observations of jets exhibiting rapid intra-observational motion and variability. We have devised a new modelling approach to study the real-time motion and variability of transient jets within a single observation. We have used this approach to study the jets launched by Swift J1727.8-1613 during its 2023 outburst. Our campaign consisted of 15 observations across North American, European and Australian VLBI arrays throughout the entirety of the outburst, including the reverse transition. In every observation, we detected either an extended self-absorbed ‘compact’ jet, fast-moving transient jet knots, or both. Using our modelling technique, we precisely measure the intra-observational motion, flux density evolution, and expansion of multiple transient jet knots. Some of these are among the most precise measurements of transient jet evolution in an X-ray binary and were constrained from only a single observation. With these new techniques, we were able to show that the ejection of transient jets occurred contemporaneously with drastic changes in the X-ray properties of the accretion inflow. We also studied the properties and evolution of the relativistic jets throughout the various phases of the outburst, including the reverse transition.

Speaker: Callan Wood (ICRAR)

18:15 No need for speed: Placing upper limits on Lorentz factors of large-scale XRB jets informed by hydrodynamic simulations

Measuring the Lorentz factors of jets from black holes is essential for understanding the jet launching mechanisms and overall energy budget of the black holes themselves. However, this is a notoriously difficult task due to special relativistic effects which are exacerbated for off-axis sources, namely X-ray binary (XRB) jets. In fact, from kinematics alone we find that, remarkably, we can only place lower limits on their bulk Lorentz factor and thus, we cannot constrain the Lorentz factor distribution of jets from stellar-mass black holes as different from AGN.
In this talk, we present novel methods for estimating upper-limits of XRB jet Lorentz factors from observational data, as well as perspectives on the dynamics of these systems from hydrodynamic simulations. We show how off-axis jets are often de-boosted, and therefore high Lorentz factors imply intrinsically brighter sources, which under basic assumptions implies higher internal energy in the ejecta. This allows us to place two independent upper limits on the Lorentz factors through constraining the allowed values of internal energy – the internal energy is likely lower than the kinetic energy, and the total energy is also likely lower than the maximum jet power integrated over the associated flare period. The upper limits on internal energy also limit the maximum Lorentz factor given an observed flux and known inclination. We apply these limits to several known XRBs.
In conjunction with Lilje et al., this work develops our understanding of the key differences (or lack thereof) between black holes across the mass range.

Speaker: Katherine Savard (University of Oxford)

18:30 Mini-HAWKs, a pilot survey to unveil dormant black holes

Mini-HAWKs is a 50 sqr deg pathfinder of the Galactic Plane down to r≃21 that employs 3 custom Hα filters, optimized to select targets with very broad Hα emission lines. These are the hallmark of strong gravitational fields as they are typically formed in accretion discs around black holes (BHs). Mini-HAWKs will prove a novel photometric strategy that, when extended to the entire northern Galactic Plane, will eventually lead to the discovery of ≃150 new dormant BH transients i.e. a ten-fold increase over the known population. That will allow constraining the number density, orbital period distribution, kinematics and, ultimately, the BH mass spectrum. It will also uncover and characterize large numbers of other Hα emitting objects in the Galaxy to unprecedented depths, such as hundreds of cataclysmic variables (mostly eclipsing) and countless numbers of Be stars, Symbiotics, T Tauri, planetary nebulae etc. Furtthermore, because of its observing layout, mini-HAWKs will furnish ≃14h light curves of every object which will be instrumental to identify variable objects and measure periods from, for example, pulsating stars, eclipsing binaries and ellipsoidal modulations. Mini-HAWKs has been selected as one of the 4 Legacy Surveys of the JAST80 telescope at the Observatorio de Javalambre in Teruel and has been granted with 400 h of observing time. The survey started in Dec 2024 and will be deployed over the next 2-3 years.

Speaker: Jorge Casares (Instituto de Astrofisica de Canarias)

19:00 → 20:30 Public Outreach Event (Italian language)

Friday 19 September

09:00 → 10:30 Very High Energy Emission from MQ

Convener: Fabio Pintore (Istituto Nazionale di Astrofisica (INAF))

09:00 Observations of very-high-energy gama-ray emission from microquasar jets

The origin of the cosmic-ray spectrum measured at Earth remains one of the biggest mysteries in high-energy astrophysics. Microquasar jets have long been proposed as sites for particle acceleration to TeV and PeV energies, and experimental evidence supporting such claims is mounting. In my talk I will review the recent observational results and their implications. I will also briefly discuss what to expect from the class as a whole and from observations with future and current facilities

Speaker: Laura Olivera-Nieto (Max Planck Institut fur Kernphysik)

09:30 LHAASO observations of UHE gamma-rays from Microquasars

The Large High Altitude Air Shower Observatory (LHAASO) is a mega facility designed to investigate cosmic rays and gamma rays at very-high (VHE; E > 0.1 TeV) and ultra-high energies (UHE; E > 0.1 PeV). In its initial years of operation, LHAASO has detected more than 100 gamma-ray sources, with approximately half constituting a newly emerging population of UHE gamma-ray emitters. Among these are extended gamma-ray structures spatially associated with powerful microquasars, pointing to the presence of nearby PeVatrons, astrophysical accelerators capable of producing particles at peta-electronvolt energies. In this talk, I will present the microquasars detected by LHAASO and describe their spectral, spatial, and temporal characteristics. I will also discuss the implications of these findings for understanding the origin of UHE gamma rays and their progenitors—PeV electrons and protons likely accelerated within or around the microquasar environments.

Speaker: Jieshuang Wang (Max Planck Institute for Plasma Physics)

10:00 Microquasars in the TeV regime with ASTRI Mini-Array

The recent discovery of ultra-high-energy gamma-ray emission from a set of five Galactic microquasars by the LHAASO collaboration has significantly strengthened the case for this class of sources as potential accelerators of cosmic rays up to the knee region of the spectrum. This breakthrough opens a completely new window for studying the mechanisms of particle acceleration in binary systems with an accreting black hole. The next generation of Imaging Atmospheric Cherenkov Telescopes (IACTs) will play a crucial role in providing detailed spectro-morphological characterisations in the GeV-to-TeV energy band of these systems, which show relativistic outflows (jets) interacting with the environment, potentially revealing the sites of particle acceleration. In this contribution, we will discuss the prospects for observing bright microquasars in the TeV band with the ASTRI Mini-Array. It is an array of nine Cherenkov telescopes currently in the deployment phase at the Teide Observatory on the Canary Island of Tenerife, which will provide the best angular resolution (~0.06 deg) among current IACTs in the critical 1-200 TeV energy band. This capability, together with its good sensitivity and large field of view, will allow ASTRI Mini-Array to significantly advance our understanding of these extreme astrophysical environments.

Speaker: Dr Antonino D'Ai' (Istituto Nazionale di Astrofisica (INAF))

10:15 Revising the TeV emission of microquasars; the case of Cygnus X-1

Cosmic rays (CRs) remain one of the most mysterious phenomena in modern physics, with their dominant sources and acceleration mechanisms still unresolved. While shock waves of supernova remnants (SNRs) have traditionally been considered the primary origin of Galactic CRs up to the “knee,” this view is increasingly challenged by the lack of consistent TeV counterparts and the discovery of non-SNR Galactic PeVatrons. These developments highlight the need to explore alternative CR accelerators. Black-hole X-ray binaries (BHXBs), the small-scale analogues of active galactic nuclei (AGN), are potential contributors to the Galactic CR spectrum. Recent quasi-stable ~100TeV detection of Cygnus X-1 and other BHXBs by LHAASO suggest CR escape from jets as a plausible mechanism for explaining such emissions. These detections open a new window for revisiting the role of microquasars in the TeV regime. Using a multi-zone, lepto-hadronic jet model, we leverage broadband multiwavelength data to probe BHXB jets. Specifically, we analyze the contribution of the jets to the radio-to-γ-ray spectrum of Cygnus X-1. In this work, we adopt a leptonic jet at launching that is mass-loaded with baryons further out due to the wind of the accretion disc. We investigate how varying proton acceleration assumptions impact jet properties, CR escape efficiency, and the observed spectra. We further examine how such a mass-loading jet model and escape can explain the GeV-to-TeV regime that strongly depends on the multiwavelength emission. Ultimately, BHXBs emerge as compelling candidates for understanding the Galactic CR landscape alongside traditional SNRs.

Speaker: Dimitrios Kantzas (LAPTh/CNRS)

10:30 → 11:00 Coffee Break

11:00 → 12:00 Very High Energy Emission from MQ

Convener: Maria Diaz Trigo (ESO)

11:00 Long-term campaign on Cygnus X-3 with the MAGIC telescopes.

Cygnus X-3 is a microquasar consisting of a compact object of unknown nature and a Wolf-Rayet star, which orbit each other with a period of 4.8 hours. The compact object launches powerful jets that are an excellent site for particle acceleration up to relativistic energies. The presence of these relativistic particles, combined with the proximity to the star and its high luminosity, create a very favorable scenario for inverse Compton scattering of stellar photons by the jet electrons, resulting in gamma-ray emission. Cygnus X-3 has been detected from radio to gamma rays above 100 MeV, although it has never been confirmed as a very-high-energy (VHE; above 100 GeV) gamma-ray emitter. Studies of microquasars in gamma rays have recently become a hot topic after the LHAASO detection of four microquasars above 100 TeV, establishing these sources as potential contributors to the Galactic cosmic-ray spectrum at energies above the PeV.
The MAGIC telescopes have observed Cygnus X-3 in the VHE band since they became operational. In this contribution, we present a long-term analysis of 130 h collected by MAGIC between 2013 and 2024. This represents the largest available dataset (in both exposure and time coverage) at VHE to date, resulting in the strongest VHE upper limits of the source between 100 GeV and a few TeV. Temporal and spectral constraints of Cygnus X-3 during this period will be interpreted within the multi-wavelength context, providing meaningful constraints on the source properties based on its (lack of) emission in gamma rays at different energies.

Speaker: Edgar Molina (IAC)

11:15 Large-scale radio bubbles around the PeVatron microquasar V4641 Sgr

Black holes in microquasars can launch powerful relativistic jets, that can travel up to several parsecs from the compact object and interact with the interstellar medium. Recently, the detection of large-scale ultra-high-energy gamma-ray emission around black hole transient V4641 Sgr and other BH-jet systems suggested that jets from microquasars may play an important role in the production of galactic cosmic rays. V4641 Sgr is known for its superluminal radio jet discovered in 1999, but no radio counterpart of a large-scale jet has been observed yet. Here, we report on the discovery of a large-scale (∼ 35 pc), ’bow-tie’ shaped, diffuse, radio structure around V4641 Sgr, detected in deep radio observations conducted with MeerKAT. After discussing the association of the structure with V4641 Sgr, we show that a combined leptonic (radio and X-rays) and hadronic (gamma-rays) can explain the observed broad-band SED. We suggest that the ’bow-tie’ structure arose from the long-term action of large-scale jets from V4641 Sgr. If the emission mechanism is of synchrotron origin, the radio/X-ray extended structure implies acceleration of electrons up to ∼ 130 TeV as far as tens of parsecs from the black hole.

Speaker: Noa Grollimund (CEA Saclay / Université Paris Cité)

11:30 Search for neutrino emission from Microquasars with KM3NeT/ORCA detector

Microquasars are galactic binary systems that exhibit relativistic jets
whose composition remains uncertain. If protons are present in situ,
neutrino production becomes possible. The relative proximity of
these systems and the evidence of their high-energy emissions
documented in the literature make neutrinos good candidates for
addressing the question of jet composition and identifying
microquasars as galactic cosmic ray accelerators.

KM3NeT/ORCA is a neutrino telescope capable of detecting such
neutrinos in the GeV to multi-TeV energy range. Located at the
bottom of the Mediterranean Sea near La Seyne-sur-Mer, France, it
began data collection in early 2020 and will continue throughout its
construction until it reaches full detection capability.

This study uses the full KM3NeT/ORCA dataset to look for neutrino
emission from microquasars. The analysis focuses on outburst
periods to reduce the atmospheric background, leveraging
wavelength observations, and employs machine-learning
techniques to filter out likely atmospheric events. Finally, the results
are compared with theoretical expected fluxes and other neutrino
experiments' upper limits.

Speaker: Francesco Magnani (Aix-Marseille Université, CPPM)

11:45 Exploring the transient sky with SVOM

Astrophysical transients, such as gamma-ray bursts, and X-ray binaries, represent some of the most extreme and short-lived phenomena in the Universe. To study these events, it is crucial to have missions that can rapidly detect and monitor them across multiple wavelengths.

Launched in 2024, the SVOM (Space-based Multi-band Astronomical Variable Objects Monitor) is a mission resulting from a collaboration between France and China. The satellite is equipped with several onboard instruments capable of rapidly detecting and monitoring new transient sources, sending alerts to ground stations in real-time. Additionally, a network of ground-based telescopes is in place to provide multi-wavelength follow-up observations of these transient sources. Although SVOM is particularly focused on the detection and monitoring of gamma-ray bursts, the first year of data collection since the launch has shown that it can also provide significant contributions to the monitoring of other sources, including X-ray binary systems.

In this talk, I will present an overview of the SVOM mission, emphasizing its unique capabilities for real-time detection and follow-up of transient events. I will also share the mission's first scientific results, highlighting its success in monitoring X-ray binary systems. These early results highlight SVOM's potential to advance our understanding of accretion processes around compact objects.

Speaker: Dr Divya Rawat (Observatoire astronomique de Strasbourg,)

12:00 → 12:30 Formation, Evolution and Merging

12:00 Formation, evolution, and merging: the MQ point of view

Microquasars (MQs) -- binary systems which host compact objects accretion from stellar companions -- offer a unique window into the physics of accretion, jet launching, and the life cycles of massive stars. In this talk, I will review the formation and evolutionary pathways leading to MQs primarily from the perspective of binary stellar evolution. Using insights from detailed simulations and population synthesis studies, I will discuss how the physics of mass transfer, common envelope evolution, and core-collapse supernovae impact the observable properties of MQs and other connected populations such as X-ray binaries and gravitational-wave sources. Finally, I will explore how current and upcoming multiwavelength and multi-messenger observations can constrain these formation channels and the physics of binary evolution.

Speaker: Dr Kyle Akira Rocha (Northwestern University)

12:30 → 12:50 Concluding Remarks

Speakers: Dr Gregoire Marcel (University of Turku), Matteo Lucchini (University of Amsterdam)

12:50 → 13:00 Acknowledgements

Speakers: Melania Del Santo (Istituto Nazionale di Astrofisica (INAF)), Dr pierre-olivier petrucci (Institute of Planetary science and Astrophysics of Grenoble)