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...
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...
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...
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...
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...
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,...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
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...
Resolving spatially the complex emission geometry of the micro-quasar SS433 requires interferometric observations to be understood. SS433 is likely composed of a black hole accreting from a massive star. Multi-wavelength observations show an unresolved accretion disk, a more extended circumbinary outflow of variable geometry and the presence of cold dust.
The systems present two precessing...
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...
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...
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...
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...
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...
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,...
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,...
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...
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...
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...
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...
Currently a sample of 5 microquasars are reported by LHASSO to have ultrahigh-energies (UHE, above 100TeV) emissions which remains a puzzle for the mechanism underneath. Among this sample, evidences of jet-dominated X-ray emissions were revealed from MAXI J1820+070 during an outburst well observed by Insight-HXMT. Here we report similar properties found in Swift J1727.8–1613 from the joint...
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...
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...
Polarized X-ray emission from weakly magnetized neutron stars provides key insights into the geometry of the innermost regions of the accretion flow. The results of the spectro-polarimetric analysis of GX 340+0 in both HB and NB, as well as of GX 349+2 in the NB, will be presented. Notably, the soft spectral component exhibits variation between the hard and soft states, while the Comptonized...
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....
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...
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...
