The many tones of accretion - to the memory of Tomaso Belloni

7 Sept 2026, 11:00 → 11 Sept 2026, 14:00 UTC

Cefalù

Mariano Mendez (Kapteyn Astronomical Institute), Sara Elisa Motta (Istituto Nazionale di Astrofisica (INAF))

Abstract submission is now closed.
It will be possible to register untill July 1st 2026.

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The meeting "The many tones of accretion" is dedicated to the memory of Tomaso Belloni, whose pioneering contributions to X-ray timing and spectral states in compact accreting systems continue to shape the field.

This conference, to be held in Cefalù (Sicily, Italy) on 7–11 September 2026, will bring together the international high-energy astrophysics community to discuss recent advances in the understanding of accretion processes, variability and jet formation in X-ray binaries and related compact objects. 


Please note that who is not INAF staff in order to be able to register and submit an abstract has to first create an account at https://indico.ict.inaf.it/register/ and then log-in with the new credentials.
All INAF staff can register with their personal institutional credentials. 

Monday 7 September

Registration & coffee break

Opening remarks and memorial. Speaker: SOC chairs

Accretion Physics & Spectral States. Session 1.1: Accretion Foundation

1 New results on winds from accretion discs in binaries

I will review observations and theory of winds in accreting binaries.

Speaker: chris done (university of Durham)

2 Determination of accretion disk inner radius taking angular momentum exchange into account: transients, transitions and evolution

The determination of the inner radius of an accretion disk needs to take into account the rotation rates of the disk and the neutron star and the angular momentum exchange between the disk and the star. When angular momentum is taken into account, with conditions to accrete or expel matter, the inner radius is found to lie closer to the corotation radius, rather than the Alfven radius, especially in propeller sources, and transitional sources. The model explains the transitions between strong and weak propeller stages, with spin-down and accretion with spin-up, respectively; the associated X-ray luminosities and spin-down - spin-up rates observed in transitional millisecond pulsars and LMXB evolution.

Speaker: Ali Alpar (Sabanci University)

3 Accreting Millisecond X-ray Pulsars: Insights into Accretion and Spin Evolution

Low-mass X-ray binaries are interacting systems in which a compact object accretes matter from a low-mass companion star, releasing most of its energy in the X-ray band. When the compact object is a neutron star, these systems provide unique laboratories to investigate accretion physics under extreme conditions of gravity, density, and magnetic field strength.

Among them, accreting millisecond X-ray pulsars represent a particularly informative subclass. These systems host rapidly rotating, weakly magnetised neutron stars that channel the accretion flow onto their magnetic poles, producing coherent X-ray pulsations at millisecond periods. Their rich phenomenology includes transient outbursts, thermonuclear X-ray bursts, quasi-periodic oscillations, and strong variability across a wide range of timescales. The combination of fast timing and spectral diagnostics enables us to probe the interaction between the accretion disc and the neutron-star magnetosphere, study surface emission and thermonuclear burning, and trace the spin evolution of neutron stars.

In this talk, I will present an observational overview of accreting millisecond X-ray pulsars, focusing on recent advances obtained through coordinated timing and spectral studies of these systems. I will discuss how observations across different accretion states provide insight into the geometry and evolution of the accretion flow, the coupling between the disc and the magnetosphere, and the mechanisms driving variability on both short and long timescales. Particular attention will be given to the role of AMXPs as laboratories for investigating neutron-star properties and the physics of accretion in the strong-gravity regime.

Speaker: Andrea Sanna (Istituto Nazionale di Astrofisica (INAF))

Lunch break

Accretion Physics & Spectral States. Session 1.1: Accretion Foundation

4 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 (LEdd), 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 spectroscopy of a black hole outburst, revealing a puzzling behaviour during its soft-state decay. At lower luminosities, the optical and infrared spectra indicate a strongly irradiated outer disc, while near the outburst peak (~0.2LEdd), the spectrum is unexpectedly cooler and dominated by strong recombination lines of hydrogen and helium. This reversed evolution can be explained by self-shadowing effects, where a vertically extended inner accretion flow shields the outer disc from the high-energy radiation produced close to the black hole. The evolution of the X-ray spectra also fits naturally within this scenario, suggesting that the inner flow geometry changes as the outburst decays. This suggests that 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 could affect our understanding of black hole accretion flows and their observational properties, such as inferred black hole spins and X-ray polarisation measurements, as well as the physical conditions of the outer disc and its associated outflows.

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

5 Geometry of X-ray sources in accreting black-hole binaries

The structure of X-ray sources in the hard spectral state of accreting black hole binaries has been a subject of intense debate for many years. There are two main scenarios. In one, the accretion disk disappears below a certain radius and is replaced by a hot flow that emits hard X-rays. In the other, the disk continues to the innermost stable circular orbit, and the hard X-ray source is either an outflowing corona or a lamppost. Tomaso’s major involvement with this conundrum was serving as a Co-Leader (alongside the speaker) of the Team program titled “Sombreros and lampposts: The Geometry of Accretion onto Black Holes,” held at the International Space Science Institute in Bern from 2020 to 2022. I will show a short video clip of Tomaso presenting this program. Then, I will discuss recent results on the hard state of Cygnus X-1 from joint observations by XRISM, NICER, and NuSTAR, as well as from previous observations by IXPE.

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

Flash talks

Coffee break

Accretion Physics & Spectral States. Session 1.2: States & Transitions

6 States and transitions in X-ray binaries

X-ray binaries exhibit a rich phenomenology of accretion states and transitions, linking changes in the accretion flow to the production of jets, winds, and high-energy emission across a wide range of timescales. In this talk, I will review our current understanding of accretion states and state transitions in neutron star and black hole X-ray binaries, highlighting recent observational insights into accretion–outflow coupling, source diversity, and unusual behaviour that challenge classification schemes. I will conclude by discussing how current and upcoming facilities promise new opportunities to connect states and transitions to the physical processes that drive them.

Speaker: Nathalie Degenaar (University of Amsterdam)

7 Epic Fail! A Multi-Wavelength Summary of the Surprisingly Short 2024/25 Outburst from EXO 0748-676

After sixteen years in quiescence, EXO 0748-676 unexpectedly alerted the world to its return to outburst with a bright X-ray burst in June 2024, having been increasing in brightness since March 2024. However, in stark contrast to its previous outburst (1985 - 2008), EXO 0748-676 returned to quiescence in January 2025, after roughly 10 months in outburst. In this talk, I will present a multi-wavelength summary of this surprisingly short outburst, utilising data from thirteen detectors, spanning wavelengths from radio to X-ray. Unlike the 1985 outburst, both the rise and decay are fully captured at X-ray and optical wavelengths. The outburst light curve does not exhibit the hallmarks of a typical transient low-mass X-ray binary outburst, and the short outburst duration creates additional tension with EXO 0748-676’s previous ‘quasi-persistent’ classification. Our optical monitoring reveals a skewed, asymmetric optical phase curve that is unique to false widow binaries, and a cooling phase dominated by an inside-out recession of the ionised accretion disc. X-ray observations by XMM-Newton and NICER in June 2024 revealed that EXO 0748-676 was heavily absorbed/obscured at the beginning of the outburst, and exhibited X-ray spectra with a strongly dampened soft X-ray contribution. A spectral model that includes a clumpy, high-density absorber can describe the strong obscuration, consistent with the false widow sub-classification of EXO 0748-676. The X-ray colours show that EXO 0748-676 spent a large portion of the ~10-month outburst in a low-intensity, hard state. EXO 0748-676 marginally increased in brightness towards the end of the outburst, but never entered a soft spectral state. This behaviour is strongly indicative of a failed transition outburst or the onset of a transitional evolutionary phase.

Speaker: Amy Knight (Durham University, UK)

8 The Galactic population of accreting black holes

The Milky Way is thought to be populated with ~103-104 stellar-mass black holes (BHs) that are fed by gas accreted from companion stars in tight orbits. Most of these BHs are transient and exhibit episodic X-ray outbursts on decadal timescales - the smoking gun that led to their discovery. However, since their duty cycles are very low, the vast majority remain hidden in a dormant state, awaiting to be discovered.
Here I present the current census of such transient BH X-ray binaries, summarize their observed properties and discuss new avenues to increase the size of the known population.

Speaker: Jorge Casares (Instituto de Astrofisica de Canarias)

9 Optical spectroscopy of the transitional millisecond pulsar PSR J1023+0038

Transitional millisecond pulsars (tMSP) represent a dynamic category of celestial sources that establish a crucial connection between low-mass X-ray binaries and millisecond radio pulsars. These systems exhibit transitions from rotation-powered to accretion-powered states and vice versa, highlighting the tight evolutionary link expected by the so-called recycling scenario. In their active phase, these sources manifest two distinct emission modes named high and low, occasionally punctuated by sporadic flares.
In this talk, I will present high time-resolution optical spectroscopic observations of the prototypical tMSP J1023+0038 in its sub-luminous disc state. The dataset allows us to probe the system on minute timescales, revealing pronounced variability in the colour and in the properties of the main emission lines, including changes in line strength and profile. I will discuss how this short-timescale variability provides new insight into the structure and dynamics of the accretion flow and its interaction with the neutron star during its disc state.

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

Flash talks

Tuesday 8 September

Timing (Belloni Session). Session 2.1: QPOs

10 QPOs in X-ray binaries

Since the early X-ray timing studies, it has been known that both Black hole and Neutro stars X-ray binaries show narrow quasi-periodic oscillations between 0.01 and 100 Hz. Despite decades of studies, the physical origin of this intriguing phenomenon remains uncertain. In this talk I will review the main observational properties of these oscillation for both classes of systems, and describe the leading theories invoked to explain them. Finally, I will discuss the most promising paths forward — both for understanding the origin of QPOs with new data and for using them as tools to constrain the fundamental physical properties of accreting compact objects.

Speaker: Federico Vincentelli (Coventry University)

11 QPOs to track the Disk Restoration signatures after Type-I X-Ray Bursts from 4U 1636−536

Type-I thermonuclear bursts (TNBs) in neutron star low-mass X-ray binaries arise from unstable nuclear burning on the neutron star surface, whereas kHz quasi-periodic oscillations (QPOs) are believed to originate in the innermost regions of the accretion disk. The intense radiation released during a Type-I burst can perturb the inner accretion flow and thereby affect the kHz QPOs. Using 3–20 keV observations of 4U 1636−536 from AstroSat, we investigated the evolution of both upper and lower kHz QPOs around Type-I bursts. kHz QPOs were detected up to 200 s before burst onset but disappeared for approximately 100–200 s afterwards, reappearing only after ~200 s. This suppression is accompanied by a ~5–6% decrease in fractional rms amplitude, indicating a temporary disappearance of the QPO signal. The recovery timescale is comparable to the viscous timescale of the inner disk, suggesting that burst radiation temporarily disrupts the inner accretion flow, with the kHz QPOs re-establishing once the disk structure is restored.

Speaker: Suchismito Chattopadhyay (Inter-University Centre for Astronomy and Astrophysics)

12 Flip-Flops and QPO switching in GX 339–4 during the 2021 and 2024 NICER outbursts

GX 339–4 is a low-mass X-ray binary often used as an archetype, showing typical outburst behaviour. The source undergoes a full outburst every 2–3 years, passing through all known accretion states. We used NICER data to study the spectral-timing properties of the 2024 outburst of GX 339–4 and to compare them with the 2021 outburst. In particular we focused on rapid “flip-flop” transitions, i.e. step-like changes in X-ray flux that often come with changes
in the timing properties. We used Monte Carlo simulations to assess the statistical significance of candidate QPO peaks in the power spectra, testing for the short-timescale appearance and disappearance of Type-B QPO signatures. We used this statistically defined classification to compare variability across flip-flop levels, to extract energy spectra for intervals with and without QPO signatures, and to study relations between QPO peak frequency and other variability properties. We will present detailed
results and their implications.

Speaker: Mr Federico Ferretti (Università degli Studi Roma Tre, Universitat Politecnica de Catalunya)

13 A systematic method for determining neutron star parameters from kilohertz quasi-periodic oscillations

Confronting observational measurements with theoretical models of neutron star spacetimes presents several challenges. These are particularly pronounced due to the differences in the definitions of measurable physical quantities and the quantities commonly used to define the spacetime metric, with the dependence of these quantities on the NS equation of state playing a major role. Here, we make use of the universal relations between NS parameters that were previously found through empirical means. We demonstrate how these relations can be utilised to obtain neutron star parameters from observed kilohertz quasi-periodic oscillations, provided the NS spin frequency is known. We adopt two models as examples: one based on the orbital motion of accreted inhomogeneities and one based on the oscillations of inner accretion tori.

Speaker: Mrs Kateřina Klimovičová (Silesian university in Opava)

14 Spectral–Timing Insights into Millisecond Variability in 4U 1728–34 from AstroSat

We report simultaneous detection of twin kHz and $\sim 40$ Hz quasi-periodic oscillations (QPOs) in the time-resolved analysis of the AstroSat/LAXPC observation of the neutron star low mass X-ray binary, 4U 1728-34. We present the first robust detection of multiple sets of QPO triplets from a single observation whose frequencies are mutually correlated and consistent with their identification as the orbital, periastron and twice the nodal precessions frequencies. The observed relations, along with the known spin of the neutron star, put constraints on the mass and the ratio of moment of inertia to the mass of the neutron star to be $M^*_\odot = 1.92\pm 0.01$ and $I_{45}/M^*_\odot = 1.07\pm 0.01$ under the simplistic assumption that the metric is a Kerr one. We further investigate the correlation between the kHz QPOs and the spectral parameters of the source using all AstroSat observations from 2016--2019. We found that all eight epochs fall into two accretion regimes, namely AR1 and AR2, with accretion rates of $\sim$ $3 \times 10^{16} \, \text{g} \, \text{s}^{-1}$ and $7 \times 10^{16} \, \text{g} \, \text{s}^{-1}$, respectively. Notably, we find that for AR1, the lower kHz QPO frequecncy ($\nu_{\text{L}}$) is always $<$500 Hz, whereas for AR2 it is $\gtrsim$500 Hz. While the spectral index shows no significant correlation with $\nu_{\text{L}}$, the coronal parameters exhibit systematic evolution. For AR1, the coronal electron temperature (kT$_e$) and optical depth ($\tau$) are $\sim$10 and $\sim$5 keV, respectively. In contrast, for AR2, kT$_e$ decreases to $\sim$3 keV and $\tau$ increases to $\sim$12, showing correlations with $\nu_{\text{L}}$, with Spearman's rank correlation coefficients of $-0.78$, $0.71$, respectively. The transition of spectral parameters at $\nu_{\text{L}} \sim500$ Hz suggests the presence of a critical QPO frequency governed by the accretion state of the source, highlighting a strong coupling between millisecond timing phenomena and the accretion dynamics in 4U 1728–34.

Speaker: Kewal Anand (Indian Institute of Technology Kanpur, India)

Coffee break

Memorial talk

Timing (Belloni Session). Session 2.2: Methods (GHATS)

Timing (Belloni Session). Session 2.3: Methods (Stingray)

15 Timing analysis of X-ray binaries with Stingray

X-ray binaries are unique laboratories for studying the physics of accretion processes. They exhibit variability over a wide range of timescales, providing key information on the physics of compact objects and their surroundings.
Stingray is an open-source Python library designed to perform time series analyses and related tasks on astronomical light curves with a focus on X-ray astronomy. It comprises the most advanced Fourier analysis techniques, while also being able to simulate data and perform statistical modelling.
In this talk, I will briefly overview the main timing methods and I will show how Stingray can be employed to analyse X-ray binary observations, including the construction of light curves, power spectra, cross spectra, and other variability diagnostics. I will give practical examples on real observational data of X-ray binaries illustrating the workflow from data preparation to the extraction of scientifically relevant timing products.

Speaker: Dr Eleonora Veronica Lai (INAF - OAC)

Lunch break

Timing (Belloni Session). Session 2.1: QPOs

16 Type-B QPOs in the Hard-Intermediate State of Black Hole X-ray Binaries

Black hole X-ray binaries are key laboratories for studying the behavior of matter in strong gravitational fields. During outbursts, these systems evolve through different spectral-timing states, whose distinct properties suggest changes in the geometry of the accretion flow. Previous studies have shown that the transition from the hard-intermediate state to the soft-intermediate state is typically associated with three major observational changes: the switch of Type-C quasi-periodic oscillations by Type-B QPOs, a sharp decrease in the fractional rms amplitude of the broadband noise, and a transition from a steady radio jet to discrete jet ejections.

Taking advantage of a joint-fitting approach to the power and cross spectra, weak variability components that are hidden or difficult to identify in the power spectrum alone can be detected. Using this method, we detect Type-B QPOs in the hard-intermediate state, suggesting that Type-B QPOs are not restricted to the soft-intermediate state. In this talk, I will present the detection of Type-B QPOs in the hard-intermediate states of two black hole X-ray binary systems, Swift J1727.8−1613 and MAXI J1820+070, and discuss their implications for the physical origin of Type-B QPOs and the geometry of the accretion flow.

Speaker: Pei Jin (Kapteyn Astronomical Institute)

Timing (Belloni Session). Session: Variability & Lags

17 The accretion flow in black hole X-ray binaries through the “lens” of X-ray spectral-timing

Black hole X-ray binaries (BHXRBs) provide an exceptional laboratory for studying the physics of accretion under strong gravity. In particular, energy-dependent variability and X-ray time lags carry key information about the geometry, dynamics, and evolution of the accretion flow. In this talk, I will review the current state-of-the-art in spectral-timing studies of BHXRBs, focusing on how lags and energy-dependent variability can provide information on the coupling between the accretion disc and the Comptonization region. Observational results will also be discussed in the context of modern spectral-timing models, highlighting how the combination of spectral, timing (and polarimetric) observables is opening new avenues for probing the structure and evolution of accretion flows around black holes.

Speaker: Barbara De Marco (Universitat Politècnica de Catalunya)

Coffee break

Timing (Belloni Session). Session: Variability & Lags

18 Evidence for the different timing behaviours of radio-loud and radio-quiet XRBs

Accreting stellar-mass black holes are known to populate two distinct tracks in the radio:X-ray luminosity plane, commonly referred to as the radio-loud and radio-quiet branches. Despite nearly four decades of observational and theoretical efforts, the physical origin of this dichotomy remains poorly understood. We have carried out a systematic study of the X-ray timing properties of a large sample of black hole low-mass X-ray binaries using archival NICER observations. Our analysis reveals that radio-loud and radio-quiet systems exhibit markedly different power spectral density (PSD). In particular, radio-loud sources tend to display 'softer' power spectra, with a larger fraction of their variability power concentrated at low frequencies. These results suggest that the radio-loud/radio-quiet dichotomy may be linked not only to the average properties of the accretion flow but also to its variability structure. We discuss the implications of this finding for the accretion–ejection connection and for understanding the physical mechanisms that regulate jet production in accreting black holes.

Speaker: Zuobin Zhang (University of Oxford)

19 Evolution in concert: Spectral-timing correlation of variability components in Cyg X-1

The black hole X-ray binary Cygnus X-1 exhibits high levels of variability spanning many decades in frequency across all of its spectral states. This broad band variability power is comprised of several distinct components evolving in concert as the source changes states. The amplitudes, frequencies, and energy dependent lags of these components reveal information about the physical accretion processes which give rise to them. To reveal these components and understand their evolution, we model 14 years of Rossi X-ray Timing Explorer observations of Cygnus X-1. We consider the appearance and disappearance of these components, their contribution to the observed time lags, and assess their degree of independence. As opposed to most previous studies, we consider lag properties, and their dependence upon spectral properties, decomposed into distinct variability components. In doing so, we get a fuller picture of the spectral-timing correlations in this source across multiple failed and successful spectral state transitions.

Speaker: Seth Larner (Washington University, St. Louis MO, USA)

20 Neutron star mass and equation of state determined from precession models of quasi-periodic oscillations

Quasi-periodic oscillations (QPOs) in the X-ray variability of accreting neutron stars provide key insights into accretion in strong gravity and the equation of state of dense matter (EoS). We discuss the possible precession origin of twin-peak QPOs, focusing particularly on two specific models. The first is the relativistic precession model, which links QPO frequencies to the frequencies of general relativistic test particle motion near the innermost stable circular orbit. The second model makes the same QPO frequency assignment but considers the precession of a critically thick fluid accretion flow rather than particle precession. Despite being based on different physical assumptions, both models share the same set of governing equations, which are expressed in opposite limits. Consequently, they have an equal number of free parameters, which facilitates comparison with observations. The fluid precession model clearly fits the data better than the geodesic model and implies lower NS masses. Using the CompOSE library, we demonstrate that, in contrast to the geodesic model, the fluid model is well compatible with a wide range of over 130 NS EoS. However, this range narrows if low-frequency QPOs are interpreted as vertical precession of the flow.

Speaker: Gabriel Török (Silesian University in Opava)

Memorial talk

21 Tomaso, dips, Cygnus X-1

Tomaso and I were the first post-doc and PhD student, respectively, of Michiel van der Klis when he became professor at the API in Amsterdam. Afterwards, in time at various meetings we interacted and even combined our research interests, especially related to (absorption) dips in the light curves of black-hole X-ray binaries.
In a paper in 1996, Tomaso with colleagues reported on an intermediate state seen in Cyg X-1. They concluded that "Perhaps also the state transitions of Cyg X-1 observed in the 1970s were not to a high state but to this intermediate state" and "In earlier observations Cyg X-1 may have been encountered in this intermediate state as well, and it is possible that the earlier reports of state transitions also involved such an intermediate state." From a new study of the fast-time variability in observations from the 1970s, I confirm that at various times Cyg X-1 indeed transited from the soft to hard state and vice versa.

Speaker: Erik Kuulkers (ESA/ESTEC)

Flash talks

Wednesday 9 September

Multi-wavelength, Jets & Winds. Session 2.4: Spectral Modelling

22 A gravitationally redshifted absorption line in the NICER spectrum of 4U 1820–30

We report the detection of a mysterious absorption feature at approximately 3.8 keV in the Neutron star Interior Composition Explorer (NICER) spectrum of the ultracompact low-mass X-ray binary 4U 1820–30. We interpret this feature as a gravitationally redshifted iron absorption line produced in the vicinity of the neutron-star surface. This scenario is supported by the temporal proximity of the NICER observation to the detection, by the MAXI X-ray monitor, of a carbon superburst from the same source, suggesting that the line may be associated with the extreme physical conditions triggered by this rare and energetic thermonuclear event.
We model the feature with a photoionized absorption component and measure a redshift of (1+z \simeq 1.72). If the line originates close to the neutron-star surface, this redshift directly constrains the stellar compactness, yielding (R/M = 4.46 \pm 0.13) km (M_{\odot}^{-1}), corresponding to (R/M = 3.02 \pm 0.09) in dimensionless units. Such a high redshift provides a direct probe of the strong-gravity regime around the neutron star and offers valuable constraints on its mass-radius relation.
This unique feature highlights the diagnostic power of high-quality X-ray spectroscopy in probing the equation of state of ultra-dense matter. Future observations of superbursts and detailed radiative-transfer modelling will be crucial to confirm the origin of the line and to establish gravitationally redshifted absorption features as robust tools for neutron-star compactness measurements.

Speaker: rosario iaria (DiFC - UNIPA)

23 X-ray reverberation from a ring-like corona

X-ray reverberation lags probe the light travel time due to relativistic curvature effects in the reflected emission close to black holes, and can be used to measure properties of the accreting system, including the mass of the central compact object. Existing models for X-ray reverberation lags and relativistically blurred reflection spectra are, however, limited by their treatment of coronal geometry. New public time-averaged models have recently been developed and used to fit the reflection spectrum of an accretion disc illuminated by a ring-like corona. They show it is practically impossible to distinguish a lamppost from a class of geometrically extended coronae. The spectral-timing analog of these ring-like geometries, particularly for the X-ray reverberation lag, have been suspected to break this degeneracy, but are computationally prohibited by existing modelling methods. I will present a new public flavour of the Reltrans model that includes detailed calculations for X-ray reverberation from a ring-like corona in strong gravity, making use of novel computational techniques, and discuss general properties of how our simulations differ from a lamppost corona.

Speaker: Fergus Baker (Durham University)

Diversity, Equity & Accessibility. The Glass Ceiling Through the Lens of Science: Gender Inequality in Astronomy, at INAF and Beyond. Speaker: Sara Bonito

The Glass Ceiling Through the Lens of Science: Gender Inequality in Astronomy, at INAF and Beyond. Speaker: Sara Bonito

Coffee break

Multi-wavelength, Jets & Winds. Session 3.1: Accretion/Ejection

24 Accretion - ejection coupling in X-ray binaries

I will review the current state of our understanding of accretion and ejection in black hole X-ray binaries. While I will keep the talk current and topical I will also reflect on how working with Tomaso Belloni brought a much fuller understanding of accretion to someone trained in radio astronomy, and how looking beyond one's silo is almost always worth it.

Speaker: Rob Fender (University of Oxford)

25 Modelling the kinematics of large-scale jets from black hole X-ray binaries

Black hole X-ray binaries (BH XRBs) launch powerful outflows in the form of radio-emitting discrete jet ejecta, which are observed to be produced during outburst phases, at the transition between different accretion states. Once spatially resolved, these components are observed to propagate up to parsec scales far from the BH, often displaying apparent superluminal motion. However, little is known about the powering mechanism, the formation and composition of these jets, and studying them is important for understanding both their physics and the feedback that BH XRBs have on the surrounding environment.

In this talk, I will discuss what is possible to achieve by modelling the kinematics of these discrete jet components with numerical relativistic blast-wave models derived from gamma-ray burst (GRB) afterglows. I will present results associated to a sample of 7 sources analyzed in recent years, with a particular focus of new results on the decelerating jets from 4U 1543-47 and Swift J1727.8-1613. I will show how this approach allows us to obtain important constraints on the initial Lorentz factor and on the ejection time, which is important in the search for X-ray signatures associated with jet production.

Under several assumptions on the ejection duration, on the jet opening angle and on the available accretion power, we are able to derive stringent constraints on the maximum jet kinetic energy for each source, as well as placing upper limits on the density of the ISM through which the jets are propagating. Overall, our results highlight the potential of applying models derived from GRBs to the physics of jets from BH XRBs and support the emerging picture of these sources as preferentially embedded in low-density environments, which is a scenario that is starting to be confirmed both from simulations and mm/sub-mm observations.

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

26 Load and behold: simulating the launching and mass loading of discrete ejecta in X-ray binaries

Discrete ejecta from X-ray binaries are crucial for understanding jet physics from event horizon to parsec scales: launched during accretion state transitions, they probe the link between the accretion flow and jet launching, and then propagate up to $10^11$ gravitational radii, efficiently distributing mass and energy through their interaction with the ISM. Understanding how massive these jets are, where that mass comes from, and whether it is carried primarily by pairs or baryons is essential for constraining the launching mechanism and the relationship between the jet and its environment. New constraints on the ejecta masses have emerged thanks to detailed analyses of XRB radio-flaring episodes (XKAT team), which, combined with kinematic and hydrodynamic modelling of resolved ejecta, have provided new opportunities to trace the mass, energy, and structure of transient jets across a range of times and radii. These results raise a central question: do discrete ejecta obtain their mass direct from the accretion flow, by sweeping up a pre-existing hard-state jet, or through interaction with winds and the external medium? I will address this question using observation-informed relativistic hydrodynamic simulations designed to test the hypothesis that a steady hard-state jet can be swept up by an increasingly fast and dense transition flow, naturally producing the discrete ejecta and flaring episodes we observe. Using state-of-the-art observations as anchor points, these simulations will track how much mass can be swept up or entrained from a pre-existing jet or external medium, and how much must be sourced from the accretion flow itself. This will aim to place constraints on the jet mass, composition and energetics while exploring this formation mechanism as a proof of concept. More broadly, the project aims to unify observations and theory across scales, using radio data and hydrodynamic modelling to build a holistic picture of how X-ray binary jets are launched, mass-loaded, and coupled to their environments.

Speaker: Katherine Savard (University of Oxford)

27 The peculiar outburst of the black hole binary candidate 4U 1630-47: unveiling the radio polarimetry with MeerKAT

X-ray binaries (XRBs) hosting a stellar-mass black hole (BH) or neutron star produce strong emission through accretion of matter from a companion star onto the compact object. This process can also lead to the formation of fast, collimated outflows, called jets. Most BH-XRBs are transient systems, producing steady compact jets during quiescence and hard X-ray states, and discrete ejecta during transitions to softer, brighter states. Radio polarimetric observations of these jets provide a valuable probe of their magnetic field structure and geometry. Although synchrotron emission is theoretically expected to reach linear polarisation fractions up to ~70% at radio frequencies, polarised emission has only been detected in a limited number of XRBs, typically at levels below 20%. This discrepancy is likely due to a combination of instrumental and physical depolarisation effects.
4U1630–47 is a recurrent BH-XRB transient that undergoes outbursts every ~600 days. During its most recent outburst in April 2025, the source reached an exceptionally bright radio state, with a peak flux density of 26 mJy at 1.3 GHz: the highest radio flux measured from this system to date. In this talk I will present the results of our multi-wavelength campaign of 4U1630–47 in both X-rays and radio using the Swift satellite and the MeerKAT interferometer.
We found that the unusual radio behaviour is likely linked to the peculiar X-ray evolution of the outburst, during which the source remained in an atypical hard state and never transitioned to the soft state. Radio polarimetric results show an intrinsic polarisation degree of ~20% and Rotation Measure RM ~1600 rad/m2, among the highest ever observed from an XRB. Moreover, the detection of temporal variability in the RM suggests that the observed depolarisation is likely driven by internal effects within the jet plasma itself, highlighting the potential of radio polarimetry as a powerful tool to probe the composition and physical properties of XRB jets.

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

28 Coupling accretion flow and jets in black hole X-ray binaries: insights from the multi-wavelength emission of Cygnus X-1

We investigate the radio to MeV spectral energy distribution (SED) of the proto typical stellar-mass black hole, Cygnus X-1.

We model the SED by combining the jet Internal Shocks Emission Model (ISHEM) with the JED-SAD model (Jet Emitting Disk – Standard Accretion Disk) which links accretion and ejection.During the fitting procedure the jet kinetic power and average Lorentz factor of the ISHEM jet are tied those predicted by the JED-SAD model. This coupling provides a self-consistent fit across the electromagnetic spectrum, confirming the physical connection between the disk and jet. The ISHEM jet reproduces the flat radio to IR spectrum as driven by internal shocks, while the JED-SAD hybrid accretion flow model accounts for the X-ray components of the spectrum.

Yet, the so called 'MeV excess' component —a long-standing puzzle— remains unaccounted for. Polarimetric observations with INTEGRAL have suggested a synchrotron origin in a highly ordered magnetic field for the MeV component. Although jet synchrotron emission may appear as a natural explanation, we find that the jet model struggles to account for the observed MeV flux levels.

As an alternative, we suggest a magnetospheric synchrotron scenario, where leptons accelerated near the horizon of the black hole emit synchrotron radiation in an extreme magnetic field (~10^8 G). This component, integrated with ISHEM and JED-SAD, successfully reproduces the broad-band spectrum of Cygnus X-1, offering a plausible explanation for the MeV emission.

The upcoming launch of the Compton Spectrometer and Imager (COSI) should clarify the nature of the MeV excess.

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

Lunch break

Multi-wavelength, Jets & Winds. Session 3.2: Winds (XRISM)

29 Accretion disc winds in X-ray binaries: a multiwavelength view

X-ray binaries offer a unique opportunity to study 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. These winds appear in both high-ionisation phases, typically traced by X-ray absorption lines, and low-ionisation phases, revealed through optical and infrared transitions, showing that substantial amounts of both hot and cold gas are expelled from these systems. In this talk, I will highlight key observational advances of this rapidly evolving field, with emphasis on the likely multiphase nature of the phenomenon and its possible impact on the dynamics of the accretion disc.

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

30 Ubiquitous broad absorptions in low-mass X-ray binaries: their impact on accretion-disc wind studies

Optical spectroscopy has become a powerful probe of accretion and ejection in outbursting low-mass X-ray binaries. Over the past decade, outflow signatures once confined to X-rays have been routinely identified at optical, near-infrared, and ultraviolet wavelengths, establishing a new framework for investigating accretion-disc winds and their connection to the accretion flow. Canonical optical wind tracers include P-Cygni profiles, broad emission wings, and flat-top line morphologies. However, interpreting these features remains challenging because they are superposed on complex line profiles produced within the accretion disc itself.

We focus on a prominent but long-neglected spectroscopic component: broad absorptions (BAs). These features can engulf the broad, sometimes double-peaked emission profiles arising from the outer regions of a Keplerian disc. Furthermore, they extend to velocities comparable to those of wind signatures, mimicking the blue-shifted absorption component of P-Cygni profiles or suppressing broad emission wings. A systematic characterisation of BAs is therefore essential for robust wind identification.

We present the first population study of BAs, complemented by a time-resolved spectroscopic database of six low-mass X-ray binaries in outburst. We show that BAs are ubiquitous, appearing in most black-hole systems despite limited outburst coverage. Their presence is independent of inclination and compact-object type, but favoured in short-period systems (P_orb < 11 h). They predominantly affect the Balmer series, strengthen toward shorter wavelengths, and persist across all X-ray states. BA profiles are well described by Gaussians with σ_abs = 1400 +- 500 km s⁻¹ and centroid velocities near the systemic velocity, pointing to an accretion-disc origin. Crucially, we also find that the absorption depth is anti-correlated with luminosity, while line ratios remain constant across the sample. Together, these properties support an origin in a stable, optically thick disc layer below a hotter, chromosphere-like emission region.

By establishing their observational fingerprints, we provide new diagnostics to distinguish intrinsic disc absorption from genuine outflows, improving studies of the accretion–ejection connection in accreting compact objects.

Speaker: Daniel Mata Sanchez (IAC)

31 Unveiling the disk atmosphere properties of UCXB 4U1916-05

Ultra-Compact X-ray Binaries (UCXBs) are a subclass of neutron star low-mass X-ray binaries (NS-LMXBs) characterized by very short orbital periods ($P<80$ min) and compact system sizes, making them ideal systems for probing the properties of accretion disks, in particular the structure and kinematics of winds and outflows. The detection of wind features in X-ray spectra is not always straightforward, but the advent of XRISM has significantly expanded our capabilities in this regard.
We present the first results from XRISM observations of the UCXB 4U 1916-05, a high-inclination system with an orbital period of $P = 50\ \rm min$. The light curve shows periodic dips caused by obscuration from a vertically extended, ionized absorber in the outer accretion disk. Spectral analysis reveals, for the first time, the presence of four distinct absorbing components in the atmosphere of the system. These include a remarkably high redshifted $ Fe{XXVI}$ absorption line at $zv \sim 1500\ \rm km/s$
, alongside a blueshifted feature at $zv\sim 70\ \rm km/s $ associated with a lower ionization state. We find that the source exhibits a complex behaviour both as a function of time and orbital phase, with the dipping intervals playing a key role in modulating the blueshifted component.
These results provide new insights into the structure and dynamics of ionized absorbers in high-inclination UCXBs, and suggest a physical connection between the dipping activity and the kinematics of the accretion disk wind.

Speaker: Sara Peluso (ESO)

Coffee break

Multi-wavelength, Jets & Winds. Session 3.3: MW Correlation

32 Multi-wavelength correlations in X-ray binaries and beyond

X-ray binaries radiate brightly from radio to X-ray due to the accretion and ejection of matter in the system. Complex, correlated flux variations probe links between emitting components, on short as well as long timescales. Here I review the progression of multi-wavelength studies of X-ray binaries, focusing on long-term (day+ timescales) correlated behaviour between radio, optical/infrared and X-ray emission. Power law correlations exist between these different wavelength regimes, especially in the hard X-ray state, with interesting exceptions. Broadly speaking, the slopes and normalizations of these correlations can be explained with a combination of disc, corona and jet contributions, and in most cases are consistent with analytical model predictions. As well as being used to probe the accretion - ejection paradigm in these systems, both the radio/X-ray and optical/X-ray correlations can be used to constrain the nature of the compact object for new X-ray transients. Recently, dependencies of the correlation slopes on the choice of waveband have added an extra dimension to these studies. Correlations tend to break down in the soft state, due to quenching of the jet and evolution of the corona. Black hole systems are systematically brighter at radio, and optical/IR frequencies, at a given X-ray luminosity compared to neutron star X-ray binaries. We demonstrate that using these correlations in conjunction with spectral energy distributions and colour-magnitude diagrams can successfully isolate the different emitting components in order to measure accurate outer disc temperatures and jet synchrotron spectral indices. Some correlations have been confirmed to also exist on short timescales from fast timing data. Lags between components, on both long and short timescales, can produce scatter in global correlations. Finally, while it is well known that the radio/X-ray correlation extends, with a mass term, to supermassive black holes in AGN (the Fundamental Plane of black hole activity), we present a new optical/X-ray correlation that also extends to AGN. This power law correlation, with a slope consistent with theoretical expectations from X-ray binaries, is between the optical emission due to reprocessing on the disc surface, and the Bolometric X-ray luminosity, with a term for the disc size included. These results imply that the process of X-ray reprocessing is universal for all accretion disks, from stellar-mass black holes to supermassive black holes.

Speaker: Dave Russell (New York University Abu Dhabi)

33 The Homogeneous MeerKAT and Swift/XRT Radio:X-ray Plane

During the hard and quiescent spectral states in X-ray binaries, a non-linear correlation is observed between radio and X-ray luminosities, providing a valuable tool to probe the connection between accretion and jet production. This relation was originally thought to define a single ‘standard’ correlation spanning several orders of magnitude in X-ray luminosity, and was extended to active galactic nuclei by including a mass term. However, subsequent studies have revealed numerous outliers that challenge the universality of the correlation. Most previous investigations of the radio:X-ray plane have combined observations from multiple radio and X-ray facilities, introducing significant systematic uncertainties associated with flux conversions between observing frequencies and differing instrumental systematics. These limitations reduce the diagnostic power of the plane and hinder efforts to identify the physical drivers of its observed diverse correlations. In contrast, we have compiled a homogeneous dataset from the five-year ThunderKAT and SwiftKAT programmes, comprising quasi-simultaneous MeerKAT and Swift/XRT observations of outbursting X-ray binaries. These data have been made publicly available through an interactive website. A key finding is the frequent detection of unresolved radio emission during the soft state, likely dominated by previously launched jet ejecta. Using these data, we have also constructed the largest homogeneous radio:X-ray plane for X-ray binaries to date. I will discuss the results of this study and the implications for our understanding of accretion flows and jet launching.

Speaker: Justine Crook-Mansour (University of OXford)

Multi-wavelength, Jets & Winds. Session 3.4: MW Variability

34 Multiwavelength Variability from X-ray Binaries

Fast multiwavelength variability is a key tool for studying the coupling between accretion flows and relativistic jets in X-ray binaries, providing unique insights into the physical processes governing jet launching and energy dissipation in the innermost regions around compact objects. In this talk, I will review the current observational and interpretative landscape, highlighting recent advances and future prospects for high-time-resolution multiwavelength studies.

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

35 The peculiar variability of the Be/NS system 1A 0538-66

The source 1A 0538-66, located in the Large Magellanic Cloud, contains the fastest-spinning neutron star in an accreting X-ray binary and shows a combination of extreme properties unmatched in other NS/Be systems.
It is characterized by extraordinary multiwavelength variability, including sporadic episodes of seconds-long X-ray flares with a dynamic range >1000 and peak luminosities exceeding 10$^{38}$ erg/s, which are correlated with bright, fast optical flares. The fast pulsations (69 ms), discovered in the 80's during a super-Eddington outburst and never reobserved for almost 40 yrs, were recently detected at a much lower luminosity level, challenging our understanding of accretion physics. In 2025, 1A 0538-66 displayed a new outburst reaching super-Eddington luminosities typical of ultraluminous X-ray sources.
I will present the results of recent observations of this binary system and discuss how they constrain the physical mechanisms, such as magnetic field geometry and accretion flow dynamics, responsible for its unique variability.

Speaker: Dr Sandro Mereghetti (Istituto Nazionale di Astrofisica (INAF))

36 Probing black hole jets through rapid multi-wavelength timing techniques

Black hole X-ray binaries are accreting objects that launch powerful relativistic jets. These systems evolve through bright outburst phases on timescales of days to months, allowing us to probe jet and accretion phenomena in real-time. As X-ray binaries produce highly variable emission, time-series observations can be a powerful tool to study these systems. However, as these systems emit across the electromagnetic spectrum, with the in-flowing matter dominating shorter wavelengths and the outflowing jet dominating longer wavelengths, we need a suite of rapid-timing capable facilities to take full advantage of time-domain techniques. With methodologies for rapid-timing observations successfully developed across the electromagnetic spectrum, we now have access to a completely new data dimension, rapid variability properties, to understand and measure fundamental jet properties (e.g., size scales, geometry, jet speeds, jet power, and the sequence of events leading to jet launching). In this talk, I will discuss new rapid mm-timing data of the black hole X-ray binary Cygnus X-1, obtained using a combination of the MUSTANG2 instrument on the Green Bank Telescope and new algorithms designed to extract time-series measurements. Further, I will discuss several exciting new directions where multi-wavelength timing studies are headed.

Speaker: Dr Alex Tetarenko (University of Lethbridge)

37 Multi-band analysis of the pulsed profiles of the accreting millisecond pulsar SAX J1808.4-3658

SAX J1808.4-3658 is the prototype of accreting millisecond X-ray pulsars and the first of this class to show optical and UV pulsations during an accretion outburst. These pulsations challenge standard rotation- or accretion-powered scenarios and provide a unique window into the emission mechanisms of pulsating neutron stars.
I will present a multi-band timing study based on XMM-Newton, HST/STIS and TNG/SiFAP2 observations obtained during the 2022 outburst. Using XMM-Newton data collected during the final flaring stage of the outburst, we find pulse phase and amplitude shifts, including a phase jump of approximately 0.4 cycles, tightly correlated with the X-ray flux. We interpret such a correlation in terms of hot-spot drift on the neutron star surface, driven by an increase in the inner-disk radius when the mass accretion rate decreases.
Simultaneous HST observations confirmed the presence of significant UV pulsations, with a luminosity exceeding the predictions of standard emission models. Analyzing optical observations, we find an anti-correlation between the X-ray flux and the amplitude of the optical pulsations, with the latter increasing as the X-ray flux decreases.
Finally, I will present optical observations obtained during the quiescent state, probing the pulse emission mechanism over a large range of mass accretion rate.

Speaker: Caterina Ballocco (Istituto Nazionale di Astrofisica (INAF))

Conference Dinner

Thursday 10 September

Beyond Stellar-Mass BHs & New Windows. Session 4.1: AGN

38 AGN high energy emission: highlights from selected space missions.

Deep JWST surveys have revolutionized our knowledge of the formation and early evolution of Active Galactic Nuclei at the cosmic dawn.
Webb uncovered a large population of compact Broad Lined AGN with moderate luminosities, log LBOL ~ 42-45, high redshifts, z > 4, and black holes’ inferred masses 106-8 MSUN. A subclass exhibits red, V-shaped UV-optical continua and is commonly classified as Little Red Dots (LRD). Among the most remarkable properties of the JWST-discovered AGN, their extreme X-ray weakness is the most puzzling. High obscuration by dust-free gas (to account for the broad lines) or intrinsically weak X-rays, as in super-Eddington accretion models, were two possible explanations.
Another open and widely debated issue concerns their large abundance at early epochs, the evolution of the faint end of the luminosity function, and the implications for BH growth and the setup of the BH host galaxy relations.
I will review the most recent results in light of current models, highlighting their weaknesses and the consequences for the early evolution of supermassive black holes.

In the second part of the talk, if time allows, I would like to review the most recent results obtained from the calorimeter onboard the Japanese mission XRISM. High-resolution (5-7 eV) X—ray spectroscopy at the energies of the iron lines complex (6—7 keV) has radically changed our understanding of AGN physics in the innermost regions of the accretion disk close to the central Black Hole.

Speaker: Andrea Comastri (Istituto Nazionale di Astrofisica (INAF))

39 All they know is what they’re not: the accretion of neutrino-emitting blazars

Blazars are a class of AGN characterised by relativistic jets pointed towards our LoS. Traditionally, they are classified into two families: BL Lacs (no emission lines suggesting an inefficient accretion flow and faint bluer broad band SEDs) and FSRQs (strong emission lines, radiatively efficient accretion and brighter red SEDs). However, recent studies have highlighted that blazars identified as potential neutrino emitters show unusual spectral properties which don't allow a standard classification. I will present the analysis of a spectroscopic sample of neutrino-emitter blazars. Many of these sources exhibit peculiar spectral features that have led to their interpretation as masquerading BL Lacs, i.e. blazars whose powerful jet emission hides the spectral signatures of a radiatively efficient accretion disk. To better understand their nature, we use the broad emission lines as tracer of the ionising disc luminosity, and thus constrain their accretion regime, most likely transitional between standard and inefficient. We exploit the interplay between spectroscopic analysis and broad band photometric data, comparing the results obtained from these two independent methods. This approach allows us to investigate whether these sources are intrinsically efficient accretors whose emission lines are diluted by the jet, or they represent a distinct population. Understanding their accretion properties and emission mechanisms will also provide important clues about the connection between blazars and high-energy neutrino emission, lately linked also to non-jetted AGN

Speaker: Ms Gaia Delucchi (INAF Brera Astronomical Observatory)

40 Circumbinary accretion onto Supermassive Black hole Binaries with the presence of a Dark Matter Spike

Since the Laser Interferometer Space Antenna (LISA) project was announced, interest in binary systems of supermassive black holes (SMBHBs) has grown, as it is expected that these systems will be observable with this instrument. In particular, efforts have been made to produce realistic simulations of SMBHBs surrounded by a circumbinary disk (CD), since this specific scenario could be the source of electromagnetic signals caused by dynamic interactions between the binary system and the disk. Furthermore, accretion disks have also been proposed as a means of facilitating the merger of SMBHBs in situations where gravitational radiation or dynamic friction are not strong enough.

Circumbinary accretion and the morphology of the CD have been studied using a wide range of codes and physical setups. However, most previous studies have been conducted using 2D viscous hydrodynamic simulations, which allow for the exploration of a wide range of parameters. Although numerous simulations of SMBHBs have been conducted at various scales, there is not a lot of work looking at the impact a DM halo may have on the circumbinary accretion and morphology. Dark matter (DM) profiles in the central regions at subparsec scales is still widely debated, and some researchers have proposed the DM density to be significantly enhanced at these scales. This sharp morphological feature is called a DM spike.

In this work, we will present the impact of a DM spike potential of different masses and profiles on a system of SMBHBs with a CD. We find that the DM spike can significantly alter the accretion and the formation of an eccentric cavity on the binary system, and we analyze whether the Rossby-Wave Instability can explain the impact of the DM spike on the formation of the cavity. These simulations were carried out using a modified version of RAMSES, a hydrodynamic code with adaptive mesh refinement.

Speaker: Edwin Santiago (Instituto Nacional de Astrofísica, Óptica y Electrónica)

41 Variability Signatures of a Fluctuating AGN Warm Corona

Active Galactic Nuclei (AGN) serve as cosmic laboratories for probing extreme physics around supermassive black holes (SMBHs). The X-ray spectra of AGN exhibit an excess in flux above the base power-law continuum at energies below 2 keV, known as the soft excess. In addition, reverberation studies find low-frequency positive lags between the soft (0.3–1 keV) and hard (4–7.5 keV) X-ray bands. A promising model for the soft excess is Comptonization of disc photons by a warm corona located near the surface of the inner accretion disc, but its variability properties are unknown. Here, we present the variability signatures of a fluctuating warm corona by solving the time-dependent radiative transfer equation and examine how its properties shape the observed lag spectrum and power spectral density across the soft and hard X-ray bands. Our results lay the groundwork for interpreting current and future X-ray timing observations of AGN in the context of a dynamic, fluctuating warm corona.

Speaker: Vishal Sudhakar (Georgia Institute of Technology)

Coffee break

Flash talks

Beyond Stellar-Mass BHs & New Windows. Session 4.2: ULXs

42 The enchanted kingdom of ultra-luminous X-ray sources

Most stars in galaxies reside in binary systems, many of which are interacting and powered by matter transfer onto a compact object. The brightest and most extreme among these systems are ultraluminous X-ray sources (ULXs). Reaching luminosities well above $10^{41}$ erg/s, ULXs routinely breach the classical Eddington limit of stellar-mass objects, making them the "enchanted kingdom" of high-energy astrophysics. The discovery of coherent pulsations, powerful winds, and ultra-soft spectra suggests that the majority of these sources are powered by neutron stars and stellar-mass black holes undergoing supercritical accretion. Their extreme nature is mirrored in relativistic winds — detected via blueshifted X-ray absorption lines up to 0.3$c$ — which carry enough kinetic power to inflate 100-pc superbubbles and self-regulate the accretion process. Yet, many secrets of this kingdom remain unsolved, from the physics of spectral transitions to the exact population demographics and wind structures. In this talk, I will provide a comprehensive overview of the ULX landscape, highlight recent breakthroughs, and discuss how upcoming flagship missions — most notably NewAthena — will revolutionise our understanding of these cosmic behemoths.

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

43 A MeerKAT detected ULX radio bubble in NGC 1313

Ultraluminous X-ray sources (ULXs) are a class of extreme X-ray binaries, most of them powered by super-Eddington accretion onto stellar mass black holes and neutron stars. Extended optical or radio emission (and a single case in X-rays) has been observed around some ULXs, with a bubble-like shape. These huge bubbles have diameters > 100 pc and contain an energy 10-100 times larger than the energy content of supernova remnants. ULX bubbles are thought to be shock-ionized, formed by the interaction of ULX outflows (winds or jets) with the surrounding interstellar medium. The study of ULX bubbles is crucial to understand the feedback of these extreme accretors on the surrounding environment.
In this talk, I will focus on the results obtained from the analysis of a MeerKAT dataset of the ULX bubble around NGC 1313 X2, which is the first radio detection of this bubble, previously observed only in the optical. At least two different emitting regions can be identified in the radio map, one brighter and one weaker, with a compact and a steep spectral shape, respectively. The bubble dimension as seen in the radio is consistent with the Halpha emission from MUSE data: I will show a comparison of the morphology at radio and optical wavelengths. Finally, I will discuss the results from the study of this bubble in a larger context, including a comparison with Galactic X-ray binaries, first of all the extreme SS433 and its W50 nebula.

Speaker: Chiara Salvaggio (Istituto Nazionale di Astrofisica (INAF))

44 What can we learn from the variability of M82 ULXs?

The M82 galaxy harbors two of the most interesting ultraluminous X-ray sources (ULXs) -- off nuclear point sources radiating above the Eddington limit for a typical stellar remnant black hole.
Some ULXs might be sub-Eddington intermediate-mass black holes, but the bulk of this class is now widely believed to be powered by super-Eddington accretion onto compact objects.
In a handful of cases, this was proven by the detection of pulsations, which unequivocably identify the accretor as a neutron star. In a few other cases, the luminosity is so high that the leading interpretation remains sub-Eddington X-ray binaries powered by IMBH.
M82 contains the first discovered pulsating ULX and one of the leading IMBH candidates.
In this talk, I will review the information these remarkable sources have given us on extreme accretion over the years, and present some recent results exploiting time domain information to investigate these sources.

Speaker: Dr Matteo Bachetti (Istituto Nazionale di Astrofisica (INAF))

45 A Ring is not enough: Testing the origin of extreme ULX populations in Ring Galaxies

Ring galaxies (RiGs) represent a particularly favorable environment for the formation of Ultra Luminous X-ray sources (ULXs). These systems are produced by a cosmic crash between a disk galaxy and a smaller companion, generating an expanding density wave that compresses the interstellar medium and triggers intense star formation along a ring. The archetypal Cartwheel galaxy and several other well known RiGs host unusually large populations of ULXs, suggesting that the physical conditions created by these collisions may enhance the production of the most luminous X-ray binaries.

Previous studies of a small sample of X-ray bright RiGs revealed an X-ray luminosity function flatter than that observed in more heterogeneous galaxy samples, indicating either a larger number of ULXs or the presence of a brighter population of binaries. However, these galaxies were selected among the most spectacular and actively star-forming systems, raising the question of whether they are representative of the RiGs population as a whole

To address this issue, we have assembled the first complete sample of nearby ring galaxies (up to D <= 70 Mpc) from the Arp&Madore sample, and obtained a combination of archival and new observations. Our program includes Chandra X-ray observations to identify and characterize ULX populations, complemented by optical imaging and spectroscopy from SOAR to investigate the stellar populations, star formation history, and metallicity of the rings. Recent results indicate that the rings host very young stellar populations and lower metallicities than the central regions of their parent galaxies, conditions that are thought to favor ULX formation.

We present the complete sample, describe the multiwavelength dataset, and report preliminary results on the ULX content of these systems. We compare the properties of the complete sample with those of the previously studied X-ray bright ring galaxies and with other star-forming galaxies, testing whether the presence of a ring alone is a tell-tale sign of an extreme ULX populations or whether additional factors are required.

Speaker: Anna Wolter (Istituto Nazionale di Astrofisica (INAF))

Lunch break

Special session. Vera C. Rubin Observatory and the Legacy Survey of Space and Time: the start of the decade-long survey. Speaker: Sara Bonito

Vera C. Rubin Observatory and the Legacy Survey of Space and Time: the start of the decade-long survey. Speaker: Sara Bonito

Beyond Stellar-Mass BHs & New Windows. Session 4.4: Polarimetry

46 X-ray polarimetric view of X-ray binaries

I will review the first results of X-ray polarimetry in accreting X-ray binary systems, focusing on the puzzles and challenges to the models of accretion they posed, along with potential ways towards the solutions.

Speaker: Alexandra Veledina (University of Turku)

Coffee break

Beyond Stellar-Mass BHs & New Windows. Session 4.4: Polarimetry

47 Polarimetry Across the Spectrum: Probing Accretion and Outflows in Black Hole X-ray Binaries

Low-mass X-ray binaries are perfect laboratories for studying accretion physics and its coupling with jets and outflows. The advent of X-ray polarimetry with IXPE has opened a new window onto the geometry of the innermost accretion regions, yet a complete physical picture demands a broader, multi-wavelength perspective.
I will show how optical polarimetry provides an important complementary view, tracing emission from the accretion disc, jets, and disc winds on scales inaccessible to X-ray instruments alone. I will present case studies illustrating the diagnostic power of this approach. In GX 339−4, simultaneous IXPE and VLT/FORS2 observations reveal X-ray and optical polarisation angles both aligned with the resolved jet axis, pointing to a corona horizontally extended in the disc plane. In Swift J1727.8−1613, dense optical monitoring uncovers a polarisation change coincident with discrete radio ejections, with an intrinsic angle offset from the jet, interpreted as spin–orbit misalignment and scattering in a disc wind. In IGR J17091−3624, an exceptionally high X-ray and optical polarisation degree with consistent angle favour disc wind scattering. Finally, preliminary results from GS 1354-64 reveal optical polarisation angle variations across the outburst which reflect changes in the dominant emission component or disc geometry.
These results show that combining optical and X-ray polarimetry with broadband spectral modelling allows us to simultaneously constrain accretion geometry, jet and wind launching conditions, and their complex interplay.

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

48 X-ray polarization from hybrid electrons in coronae of accreting black holes

The launch of the Imaging X-ray Polarimetry Explorer (IXPE) marked a major advance in X-ray astronomy by enabling measurements of X-ray polarization from compact astrophysical objects. In accreting black holes, polarization provides a powerful tool for probing the geometry of the emitting region and constraining the black hole spin.

IXPE measured a polarization degree of about 2% in Cygnus X-1 during its soft state, when the emission is dominated by thermal radiation from the accretion disc. However, the observed polarization angle was found to be aligned with the jet direction, showing no evidence of rotation with energy. This result is in tension with classical models of polarization produced in electron-scattering-dominated atmospheres of accretion discs around Kerr black holes. It has recently been proposed that the combined effects of returning radiation, which is polarized parallel to the disc axis, and coronal radiation reflected from the disc may explain the observations, provided that the dimensionless black hole spin is close to unity.

These models, however, assume that the coronal emission is produced by thermal Comptonization, an assumption that is inconsistent with the observed spectrum, particularly the power-law-like tail extending to MeV energies in Cygnus X-1. The data are instead better described by Comptonization of disc photons in a corona containing a hybrid thermal/non-thermal electron population, a natural consequence of particle acceleration by magnetic reconnection or shocks followed by cooling and thermalization through Coulomb collisions and synchrotron self-absorption.

Here I will describe the polarimetric properties of such models and show that Comptonization by the low-temperature component of the electron distribution produces polarization consistent with the IXPE measurements. This model also alleviates the need to invoke a rapidly spinning black hole in Cygnus X-1, a scenario that remains in tension with several observational constraints and theoretical expectations.

Speaker: Juri Poutanen (University of Turku)

49 A SERENDIPITOUS IXPE VIEW OF THE ECLIPSING NEUTRON STAR BINARY AX J1745.6-2901

We present the first X-ray polarimetric measurement of the eclipsing neutron star low-mass X-ray binary system AX J1745.6-2901 obtained with the Imaging X-ray Polarimetry Explorer (IXPE). This transient source lies within ∼1.5′ of the Galactic center and was observed serendipitously during a 150 ks IXPE pointing at MAXI J1744-294. The crowded and highly absorbed Galactic center environment makes polarization measurements particularly challenging, requiring a careful assessment of contamination from the primary target and the surrounding diffuse emission.

We detect a polarization degree of PD = 14.7% ± 4.0% and a polarization angle of PA = 122◦ ± 8◦. Phase-resolved analysis reveals a significant increase in polarization during the eclipse and dip phase, reaching PD = 34.2% ± 8.7%. The enhanced polarization when the central X-ray source is obscured indicates that scattering processes dominate the polarized emission, likely originating in an equatorial disk wind or extended reprocessing structure. These results provide new constraints on the geometry of high-inclination accreting neutron star systems and demonstrate the diagnostic power of X-ray polarimetry for probing accretion physics in complex Galactic center environments.

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

50 The first IXPE view of the eclipsing ADC source 4U 1822-37

4U 1822-371 is the prototype accretion-disc corona neutron-star LMXB. In this geometry, the direct emission from the innermost regions is obscured by the outer disc rim, and the observed X-ray radiation is dominated by scattering and reprocessing. We present the first X-ray polarimetric detection of 4U 1822--371 and the first simultaneous broadband spectro-polarimetric analysis of the source, based on a coordinated campaign with IXPE, XMM-Newton, NuSTAR, and Swift.
The spectral analysis supports the standard ADC scenario: only a fraction of about 1/25 of the intrinsic radiation reaches the observer, consistent with scattering in an extended, optically thin corona with $\tau_{\rm C}\simeq0.05$.
In the 2-8 keV band, IXPE detects a highly significant polarisation signal, with ${\rm PD}=7.9\%\pm0.6\%$ and ${\rm PA}=-24^\circ\pm2^\circ$. The polarisation degree increases with energy, while the polarisation angle remains approximately constant. The orbital-phase-resolved analysis provides a further geometrical constraint: the PD decreases during eclipse, reaching ${\rm PD}=5.5\%\pm1.7\%$, while the PA remains consistent with the phase-averaged value. To our knowledge, 4U 1822--371 is the only high-inclination neutron-star LMXB observed so far with IXPE to show a decrease, rather than an increase, of the PD during eclipse/dipping intervals. This suggests that the eclipse selects a less polarimetrically efficient portion of the extended scattering structure.
These results provide direct polarimetric evidence that 4U 1822--371 is observed in an extreme high-inclination, scattering-dominated regime, where the extended accretion-disc corona shapes both the apparent X-ray luminosity and the observed polarisation properties.

Speaker: Alessio Anitra (Istituto Nazionale di Astrofisica (INAF))

Beyond Stellar-Mass BHs & New Windows. Session 4.3: CVs

51 Optical variability properties of accretion white dwarfs: Tomaso’s legacy beyond X-ray binaries

Accreting white dwarfs (AWDs) are exceptionally bright and relatively abundant in our Galaxy, making them invaluable laboratories for studying both accretion physics and binary evolution. In this talk, I will focus on cataclysmic variables (a subset of AWDs) and discuss how ground- and space-based observations have provided unprecedented detail on their broad-band and quasi-periodic variability as well as transient behaviour over the past decade. I will review the many striking phenomenological similarities between the variability properties of AWDs to accreting stellar-mass black holes and neutron star X-ray binaries, most of which has been directly inspired and motivated by the long-lasting legacy pioneered by Tomaso’s foundational work on X-ray binaries.

Speaker: Dr Simone Scaringi (Durham University)

Memorial talk

Friday 11 September

Future Missions & Legacy. Session 5.1: X-ray Missions

52 X-ray binaries: Future and perspectives

The past decades have yielded a huge amount of information on the physics of X-ray binaries. The combination of X-ray variability measurements and spectroscopy, which has recently been supplemented with exciting X-ray polarization data, and multi-wavelength observations has allowed us to constrain the accretion flow physics and the phenomenological behavior of these sources. We have also been able to study the interaction with their donor stars, understand their progenitors, and have mapped out the populations of compact objects in our Galaxy and in the Universe. However, many open questions remain.

With the advent of new observational capabilities in the next 20 years in the electromagnetic spectrum, such as SKA, E-ELT, NewAthena, and CTAO, together with gravitational wave observatories such as ET, new avenues to further our understanding of X-ray binaries will become available.

In this talk I will discuss the next steps in X-ray binary research with these exciting facilities.

Speaker: Joern Wilms (Remeis-Observatory & ECAP, FAU Erlangen-Nuernberg)

53 Advances in X-ray Missions

The current and planned scenario of the observational capabilities available to the X-ray scientific
community in the foreeseable future is far from ideal, with ambitious candidate X-ray missions recently not
selected for further developments both in Europe and the US. Yet, new ideas and developments are in place
from Europe to China to India and beyond, trying to design a better observational future. In this talk I
will review the current status and the perspectives for future X-ray missions, expected to improve our
understanding of the many tones of accretion.

Speaker: Prof. Marco Feroci (INAF Brera Astronomical Observatory)

Coffee break

Closing remarks

Future Missions & Legacy. Session 5.2: Best Talk prize · Conclusions