Context. Hydrodynamical cosmological simulations based on the ΛCDM model predict that ∼40

We investigate correlations between prominent features in the relativistic X-ray reflection spectrum emitted by an accreting supermassive black hole (SMBH) binary with the underlying properties of the binary system. Model-independent measurements of the relativistic Fe Kα line (∼6.4 keV) and the Compton reflection hump (∼20-30 keV) are shown to be useful in constraining binary parameters. We compute 24,570 X-ray reflection spectra from two mini-disks attached to SMBHs at a fixed separation of 100 GM/c^2 on circular orbits, by varying its mass ratio, spin parameters, inclination, orbital phase and total mass accretion rate. We find that the location of the blue peak in the relativistic Fe Kα is a relatively robust diagnostic of the binary inclination, which could be obtained from a single-epoch X-ray spectrum. Given a few epochs of spectra, one may be able to determine the orbital phase of the binary and place constraints on its mass ratio by monitoring the Fe Kα centroid. Of all parameters, SMBH spin efects are most subtle and prone to measurement degeneracies. Some markers of high SMBH spin may nevertheless surface in the composite spectrum due to increased radiative efficiency. The approach developed here can be used to place preliminary constraints on binary parameters before a full parametrized X-ray spectral fitting method is available. It complements gravitational wave measurements by the Pulsar Timing Arrays (PTAs) and the Laser Interferometer Space Antenna by providing independent constraints on binary parameters that may be prone to degeneracy (inclination), or otherwise inaccessible (spin for PTAs).

We propose to add instruments to any potential future X-ray mission with focussing optics that is considered in NASA's ASTRA framework. Such an instrument is a necessity to study AGN wind outflows and feedback, find the missing baryons, study the intergalactic medium, and analyze abundances and chemical bonds in dust grains throughout the Milky Way. We conclude that those science goals can be achieved with a spectral resolving power > 3000 in the soft X-ray band (about 10-40 Ang) and an effective area a few times larger than current instruments. We describe a possible mission implementation for a soft X-ray grating spectrometer that can be folded in and out or be mounted permanently in the beam. Such an instrument can reach the requirements for a wide variety of host mission properties. A small UV imager and a UV spectrograph can be mounted on the same platform with independent optics. These added instruments vastly enhance the science capabilities of the host mission for a modest cost (100-200 million $) and with weight and power needs that can be easily accommodated in any major mission.

The 2023/2024 NICER monitoring campaign of the 7 Crab bright black hole X-ray binary Swift J1727.8-1613 covered the outburst in almost all accretion states. High-quality data are available in the high-Eddington-fraction hard-intermediate state, the hard-to-soft transition, the soft state, and the poorly studied back transition to the dim hard state, making it an ideal dataset for comparing the accretion flow at vastly different accretion rates. We apply disk continuum fitting techniques to investigate the evolution of the inner-disk radius throughout the outburst. Taking a temperature-dependent color-correction factor into account, we see an evolution of the disk inner radius by a factor of a few comparing the hard states to the thermal/soft state. We tentatively detect an onset of disk truncation in the soft-to-hard transition, right after the source leaves the soft state. After accounting for model systematics, we find the disk to be more truncated in the high-luminosity bright hard state compared to the low-luminosity dim hard state.

Quasi-periodic eruptions (QPEs) are a recently identified class of X-ray transient associated with tidal disruption events by supermassive black holes, and for which there are multiple possible explanations. In this paper, we present a simple model which requires the black hole be spinning, be misaligned with the accretion flow (both conditions of which are almost certainly met), and that the accretion rate is a few times the Eddington limit. We speculate that the resulting Lense-Thirring torques force the disc and entrained outflows to precess, leading to increased X-ray flux when the wind-cone is oriented at lower inclinations to the observer. We test the range of parameters for which this model could explain the period and brightness of the QPE events discovered thus far, and make qualitative comparisons between the observed X-ray spectra and light curves to those extracted from general relativistic radiation magnetohydrodynamic simulations. Overall, we find some areas of promising concordance, and identify challenges related to the details of current simulations.

Quasi-periodic eruptions (QPEs) are a recently identified class of X-ray transient associated with tidal disruption events by supermassive black holes, and for which there are multiple possible explanations. In this paper we present a simple model which requires the black hole be spinning, be misaligned with the accretion flow (both conditions of which are almost certainly met) and that the accretion rate is a few times the Eddington limit. We speculate that the resulting Lense-Thirring torques force the disc and entrained outflows to precess, leading to increased X-ray flux when the wind-cone is oriented at lower inclinations to the observer. We test the range of parameters for which this model could explain the period and brightness of the QPE events discovered thus far, and make qualitative comparisons between the observed X-ray spectra and lightcurves to those extracted from GR-RMHD simulations. Overall, we find some areas of promising concordance, and identify challenges related to the details of current simulations.

The reflection spectrum produced by a cold medium illuminated by X-ray photons is not isotropic and its shape depends on the emission angle. In the reflection spectrum of an accretion disk of a black hole, the value of the emission angle changes over the disk and, in general, is different from the value of the inclination angle of the disk because of the light bending in the strong gravitational field of the black hole. Current reflection models make some approximations, as calculating a reflection spectrum taking the correct emission angle at every point of the disk into account would be too time-consuming and make the model too slow to analyze observations. In a recent paper, we showed that these approximations are unsuitable to fit high-quality black hole spectra expected from the next generation of X-ray missions. Here, we present a reflection model with improved calculations of the emission angle that solves this problem.

We investigate the presence of supermassive black hole (SMBH) binary signatures and the feasibility of identifying them through X-ray reflection spectra. The X-ray emitting region is modeled as a set of two mini-disks bound to the individual SMBHs separated by 100 GM / c ^2 and the spectra calculated as a function of the mass, mass ratio, and total accretion rate of the binary. The X-ray reflection features are strongly influenced by the accretion-inversion phenomenon expected in SMBH binaries, which results in a wide range of ionization conditions in the two mini-disks. These are imprinted in the resulting composite spectra and the double-peaked and time-variable relativistic Fe K α line profiles. To test whether these features can be used as evidence for the presence of an SMBH binary, we fit mock 100 ks observations with a single active galactic nucleus (AGN) model. For a 10 ^9 M _⊙ binary targeted by pulsar timing arrays (PTAs), at z = 0.1, the single AGN model clearly fails to fit the data, while at z = 1, the fit is acceptable but unable to converge on the SMBH spin. For a 10 ^6 M _⊙ binary, a progenitor of a Laser Interferometer Space Antenna (LISA) source, spectral fitting is only possible at z = 0.1, with the outcomes similar to the PTA binary at z = 1. We also find that PTA binaries can be expected to show a distinct X-ray spectral variability in multiepoch observations, whereas for LISA precursors, orbital averaging results in the loss of spectral variability signatures.

Context. The reflection of X-rays at the inner accretion disk around black holes imprints relativistically broadened features in the observed spectrum. Aside from the black hole properties and the ionization and density of the accretion disk, these features also depend on the location and geometry of the primary source of X-rays, often referred to as the corona. Aims. We present a fast general relativistic model for spectral fitting of a radially extended, ring-like corona above the accretion disk. Methods. A common approach used to explain observed X-ray reflection spectra is the lamp post geometry, which assumes a point-like source on the rotational axis of the black hole. While it is typically able to explain the observations, this geometric model does not allow for any constraint to be placed on the radial size of the corona. We therefore extended the publicly available relativistic reflection model RELXILL by implementing a radially extended, ring-like primary source. Results. With the new RELXILL model allowing us to vary the position of the primary source in two dimensions, we present simulated line profiles and spectra and discuss the implications of carrying out a data fitting, in comparison to the lamp post model. We applied this extended RELXILL model to XMM-Newton and NuSTAR data of the radio-quiet Seyfert-2 active galactic nucleus (AGN) ESO 033-G002. The new model describes the data well and we are able to constrain the distance of the source to the black hole to be less than three gravitational radii, while the angular position of the source is poorly constrained. Conclusions. We show that a compact, radially extended corona close to the innermost stable circular orbit is able to explain the observed relativistic reflection as well as the lamp post corona does. This model has been made freely available to the community.

X-ray observations are essential for understanding the multimessenger emission mechanisms of active galactic nuclei (AGN). Blazars, a subset of AGN whose X-ray emission predominantly originates from relativistic jets, have been proposed as promising high-energy neutrino sources. In this work, we study the candidate neutrino-emitting blazar 5BZB J0630-2406, which has been observed over multiple epochs with the XMM-Newton, NuSTAR, Neil Gehrels Swift-XRT, and eROSITA observatories. Analysis of the X-ray spectra in the 2.0–10.0 keV band shows significant variability, with high-flux states adhering to a power-law model indicative of jet emission. However, during low-flux states, the spectrum reveals an additional component in hard X-rays, indicating a transition from jet-dominated to multicomponent X-ray emission, possibly associated with hadronic processes. To investigate this spectral evolution, we tested various models and found it to be consistent with coronal emission or photoionized absorption processes typically observed in obscured AGN. The identification of the X-ray spectral variability in 5BZB J0630-2406, combined with its potential for neutrino production, opens new perspectives in multimessenger astrophysics of blazars, highlighting the synergies between the mechanisms of the jet and the nuclear environment.

IGR J17091−3624 is the only black hole X-ray binary candidate—aside from the well-studied black hole system GRS 1915+105—observed to exhibit a wide range of structured variability patterns in its light curves. In 2025, the source underwent a “failed” outburst: it brightened in the hard state but did not transition to the soft state before returning to quiescence within a few weeks. During this period, IGR J17091−3624 was observed by multiple ground- and space-based facilities. Here, we present results from six pointed NuSTAR observations obtained during the outburst. None of the NuSTAR light curves showed the exotic variability classes typical of the soft state in this source; however, we detected, for the first time, strong dips in the count rate during one epoch, with a total duration of ∼4 ks as seen by NuSTAR. Through spectral and timing analysis of all six epochs, we investigate the hard-state spectral evolution and the nature of the dips. A clear evolution of the coronal properties with luminosity is observed over all six epochs, with clear signatures of relativistic disk reflection that remain largely unchanged across the first five epochs. The first five epochs also show a strong and stable quasiperiodic oscillation feature in the power spectra. The dips observed in Epoch 5 are consistent with partial obscuration by ionized material with a column density N _H ≈ 2.0 × 10 ^23 cm ^−2 . We discuss possible origins for this material and place constraints on the orbital parameters and distance of the system.

The eROSITA instrument on board Spectrum-Roentgen-Gamma (SRG) has completed four scans of the X-ray sky, leading to the detection of almost one million X-ray sources in eRASS1 alone, including multiple new X-ray binary candidates. We report on analysis of the X-ray binary 1eRASS J085039.9−421151, using a ∼55 ks long NuSTAR observation, following its detection in each eROSITA scan. An analysis of the eROSITA and NuSTAR X-ray spectra in combination with X-shooter data of the optical counterpart provide evidence of an X-ray binary with a red supergiant (RSG) companion, confirming previous results. However, we did determine a cooler spectral type for M2–3, owing to the presence of TiO bands in the optical and near-infrared spectra. The X-ray spectrum is well-described by an absorbed power law with a high-energy cutoff typically applied for accreting high mass X-ray binaries. In addition, we detected a strong fluorescent neutral iron line with an equivalent width of ∼700 eV and an absorption edge, the latter indicating strong absorption by a partial covering component. It is unclear whether the partial absorber is ionised. There is no significant evidence of a cyclotron resonant scattering feature. We did not detect any pulsations in the NuSTAR light curves, possibly on account of a large spin period that has gone undetected due to insufficient statistics at low frequencies or potentially large absorption that causes pulsations to be smeared out. Even so, the low persistent luminosity, the spectral parameters observed (photon index, Γ < 1.0), and the minuscule likelihood of detection of RSG-black hole systems suggest that the compact object is a neutron star.

While the standard X-ray variability of black hole X-ray binaries (BHXBs) is stochastic and noisy, there are two known BHXBs that exhibit exotic `heartbeat'-like variability in their light curves: GRS 1915+105 and IGR J17091-3624. In 2022, IGR J17091-3624 went into outburst for the first time in the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the exotic variability and the corresponding spectral features with unprecedented detail. We find that as in typical BHXBs, the outburst began in the hard state, then the intermediate state, but then transitioned to an exotic soft state where we identify two types of heartbeat-like variability (Class V and a new Class X). The flux-energy spectra show a broad iron emission line due to relativistic reflection when there is no exotic variability, and absorption features from highly ionized iron when the source exhibits exotic variability. Whether absorption lines from highly ionized iron are detected in IGR J17091-3624 is not determined by the spectral state alone, but rather is determined by the presence of exotic variability; in a soft spectral state, absorption lines are only detected along with exotic variability. Our finding indicates that IGR J17091-3624 can be seen as a bridge between the most peculiar BHXB GRS 1915+105 and `normal' BHXBs because it alternates between the conventional and exotic behavior of BHXBs. We discuss the physical nature of the absorbing material and exotic variability in light of this new legacy dataset.

Accretion is a universal astrophysical process that plays a key role in cosmic history, from the epoch of reionization to galaxy and stellar formation and evolution. Accreting stellar-mass black holes in X-ray binaries are one of the best laboratories to study the accretion process and probe strong gravity—and most importantly, to measure the angular momentum, or spin, of black holes, and its role as a powering mechanism for relativistic astrophysical phenomena. Comprehensive characterization of the disk-corona system of accreting black holes, and their co-evolution, is fundamental to measurements of black hole spin. Here, we use simulated data to demonstrate how key unanswered questions in the study of accreting stellar-mass black holes will be addressed by the High Energy X-ray Probe (HEX-P). HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging and broad spectral coverage (0.2–80 keV) with a sensitivity superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. We illustrate the capability of HEX-P to: 1) measure the evolving structures of black hole binary accretion flows down to low (≲ 0.1%) Eddington-scaled luminosities via detailed X-ray reflection spectroscopy; 2) provide unprecedented spectral observations of the coronal plasma, probing its elusive geometry and energetics; 3) perform detailed broadband studies of stellar mass black holes in nearby galaxies, thus expanding the repertoire of sources we can use to study accretion physics and determine the fundamental nature of black holes; and 4) act as a complementary observatory to a range of future ground and space-based astronomical observatories, thus providing key spectral measurements of the multi-component emission from the inner accretion flows of black hole X-ray binaries.

It has been recently discovered that a few super-Eddington sources undergoing black hole super-Eddington accretion exhibit X-ray reflection signatures. In such new systems, one expects the coronal X-ray emissions to be mainly reflected by optically thick super-Eddington winds instead of thin disks. In this paper, we conduct a series of general-relativistic ray-tracing and Monte Carlo radiative transfer simulations to model the X-ray reflection signatures, especially the characteristic Fe K alpha line, produced from super-Eddington accretion flows around nonspinning black holes. In particular, we allow the photons emitted by a lamppost corona to be reflected multiple times in a cone-like funnel surrounded by fast winds. We find that the Fe K alpha line profile most sensitively depends on the wind kinematics, while its exact shape also depends on the funnel open angle and corona height. Furthermore, very interestingly, we find that the Fe K alpha line can have a prominent double-peak profile in certain parameter spaces, even with a face-on orientation. Moreover, we compare the Fe K alpha line profiles produced from super-Eddington and thin disks and show that such lines can provide important insights into the understanding of black hole systems undergoing super-Eddington accretion.

The eROSITA telescope array aboard the Spektrum Roentgen Gamma (SRG) satellite began surveying the sky in December 2019, with the aim of producing all-sky X-ray source lists and sky maps of an unprecedented depth. Here we present catalogues of both point-like and extended sources using the data acquired in the first six months of survey operations (eRASS1; completed June 2020) over the half sky whose proprietary data rights lie with the German eROSITA Consortium. We describe the observation process, the data analysis pipelines, and the characteristics of the X-ray sources. With nearly 930 000 entries detected in the most sensitive 0.2-2.3 keV energy range, the eRASS1 main catalogue presented here increases the number of known X-ray sources in the published literature by more than 60%, and provides a comprehensive inventory of all classes of X-ray celestial objects, covering a wide range of physical processes. A smaller catalogue of 5466 sources detected in the less sensitive but harder 2.3-5 keV band is the result of the first true imaging survey of the entire sky above 2 keV. We present methods to identify and flag potential spurious sources in the catalogues, which we applied for this work, and we tested and validated the astrometric accuracy via cross-comparison with other X-ray and multi-wavelength catalogues. We show that the number counts of X-ray sources in eRASS1 are consistent with those derived over narrower fields by past X-ray surveys of a similar depth, and we explore the number counts variation as a function of the location in the sky. Adopting a uniform all-sky flux limit (at 50% completeness) of F0.5-2 keV > 5 x 10(-14) erg s(-1) cm(-2), we estimate that the eROSITA all-sky survey resolves into individual sources about 20% of the cosmic X-ray background in the 1-2 keV range. The catalogues presented here form part of the first data release (DR1) of the SRG/eROSITA all-sky survey. Beyond the X-ray catalogues, DR1 contains all detected and calibrated event files, source products (light curves and spectra), and all-sky maps. Illustrative examples of these are provided.

The black hole X-ray binary H1743-322 lies in a region of the Galaxy with high extinction, and therefore it has not been possible to make a dynamical mass measurement. In this paper, we make use of a recent model which uses the X-ray reflection spectrum to constrain the ratio of the black hole mass to the source distance. By folding in a reported distance measurement, we are able to estimate the mass of the black hole to be 12 +/- 2 M-circle dot (1 sigma credible interval). We are then able to revise a previous disc continuum fitting estimate of black hole spin a & lowast; (previously relying on a population mass distribution) using our new mass constraint, finding a(& lowast; )= 0.47 +/- 0.10. This work is a proof of principle demonstration of the method, showing it can be used to find the mass of black holes in X-ray binaries.

X-ray reflection spectroscopy is a powerful tool to study the strong gravity region of black holes. The next generation of astrophysical X-ray missions promises to provide unprecedented high-quality data, which could permit us to get very precise measurements of the properties of the accretion flow and of the spacetime geometry in the strong gravity region around these objects. In this work, we test the accuracy of the relativistic calculations of the reflection model relxill and of its extension to non-Kerr spacetimes relxill_nk in view of the next generation of X-ray missions. We simulate simultaneous observations with Athena/X-IFU and LAD of bright Galactic black holes with a precise and accurate ray-tracing code and we fit the simulated data with the latest versions of relline and relline_nk. While we always recover the correct input parameters, we find residuals in the fits when the emission from the inner part of the accretion disk is higher. Such residuals disappear if we increase the number of interpolation points on the disk in the integral of the transfer function. We also simulate full reflection spectra and find that the emission angle from the accretion disk should be treated properly in this case.

'Bare' active galactic nuclei (AGN) are a subclass of Type 1 AGN that show little or no intrinsic absorption. They offer an unobscured view of the central regions of the AGN and therefore serve as ideal targets to study the relativistic reflection features originating from the innermost regions of the accretion disc. We present a detailed broad-band spectral analysis (0.3-70 keV) of one of the most luminous bare AGN in the local Universe, RBS 1124 (z=0.208) using a new, co-ordinated high signal-to-noise observation obtained by XMM-Newton and NuSTAR. The source exhibits a power-law continuum with Gamma similar to 1.8 along with a soft excess below 2 keV, a weak neutral iron line and curvature at high energies (similar to 30 keV). The broad-band spectrum, including the soft excess and the high-energy continuum, is well fit by the relativistic reflection model when the accretion disc is allowed to have densities of log(ne/cm(-3)) greater than or similar to 19.2. Our analysis therefore suggests that when high-density effects are considered, relativistic reflection remains a viable explanation for the soft excess.

Context. The soft excess, a surplus of X-ray photons above 2 keV with respect to a power law, is a feature of debated physical origin found in the X-ray spectra of many type-1 active galactic nuclei (AGN). The eROSITA instrument aboard the Spectrum-Roentgen-Gamma (SRG) mission will provide an all-sky census of AGN suitable for spectral analysis. Aims. The primary goal of this work is to test a variety of models for the soft X-ray emission of AGN (thermal emission, non-thermal emission, ionised absorption, or neutral partial covering absorption) to help identify the physical origin of the soft X-ray spectral complexity. Differences between these models are examined in the context of this sample to understand the physical properties. Methods. We used Bayesian X-ray analysis to fit a sample of 200 AGN from the eFEDS hard X-ray--selected sample with a variety of phenomenological and physically motivated models. Model selection was performed using the Bayes factor to compare the applicability of each model. Results. We find that 29 sources have evidence for a soft excess at a confidence level >97.5%, all of which are better modelled by an additional soft power law than by thermal blackbody emission. We find 23 of these sources prefer a warm corona model, while six sources prefer relativistic blurred reflection. Additionally many sources show evidence for complex absorption, with 29 preferring a warm absorber and 25 a partial covering absorber. Sources with a soft excess show a significantly higher Eddington ratio than those with warm absorbers. We discuss the implication of these results for the physical processes in the central regions of AGN. Conclusions. Spectral fitting with Bayesian statistics is ideal for the identification of complex absorption and soft excesses in the X-ray spectra of AGN and can allow one to distinguish between different physical interpretations. (Abridged)