arXiv subjects

Andrzej A. Zdziarski

Publications and source records attributed to Andrzej A. Zdziarski.

At least 19 recordsLinked to original sources

arXiv2026

Are jet speeds governed by accretion modes?

Relativistic jets are observed in both stellar-mass black hole X-ray binaries (BHXRBs) and active galactic nuclei (AGNs), yet their bulk Lorentz factors differ systematically---those in black hole X-ray binaries are typically below $\sim2$, whereas AGN jets can reach $\sim50$. The origin of this discrepancy remains unclear. Searching the literature, we compile a sample of 333 AGNs with well-measured jet component motions, consisting of 270 quasars, 47 BL Lac objects, 10 FRI, and 6 FRII galaxies. We find that quasars/FRIIs exhibit minimal bulk Lorentz factors ranging from $\sim$1.0 to 41.5, with a mean of 11.5 (median 9.5). In contrast, BL Lac objects/FRIs show $Γ_{\rm jet}\sim$1.0--21.9, averaging 4.2 (median 1.5). These values, particularly the median, closely resemble those of BHXRBs, implying a strong correlation between jet speed and accretion mode. The Lorentz factor of a magnetically driven jet is mainly determined by the ratio of the magnetic pressure to rest mass energy density at the jet base. In BL Lacs/FRIs/BHXRBs, the field is maintained by the advection-dominated accretion flow (ADAF), and the gas at the ADAF surface is magnetically driven into the jets. In quasars/FRIIs, the field is maintained by the disc, while the jet base is connected to the corona. Our model calculations show that the disc field is always much stronger than that of the ADAF, and therefore leads to a larger $Γ_{\rm jet}$, which can explain the systematic difference in $Γ_{\rm jet}$ between these two types of sources.

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arXiv2026

Electron-positron cascade in magnetospheres of supermassive Kerr black holes and the origin of relativistic AGN jets

The avalanche mechanism of plasma production in active galactic nuclei (AGNs) is detailed, and constraints on system parameters required for efficient electron-positron pair cascades are explored. Whether an AGN falls within this favorable parameter range may explain the observed radio-loud versus radio-quiet dichotomy. However, this study shows that cascades produce orders of magnitude fewer pairs than are needed to explain the synchrotron emission observed in luminous jets. This suggests the existence of either an alternative lepton source, namely pair production of photons from the hot accretion flows around AGN central black holes, or matter loading of the jets from the surrounding medium, or, most likely, both. The case of the radio galaxy 3C 120 is considered in detail.

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arXiv2026

Evolution of accretion disc-corona in the TDE Candidate AT 2019avd

X-ray observations of the tidal disruption event (TDE) candidate AT 2019avd show drastic variabilities in flux and spectral shape over hundreds of days, providing clues on the accretion disc-corona evolution. We utilize a disc-corona model, in which a fraction of the gravitational energy released in the disc is transported into the hot corona above/below. Some soft photons emitted from the disc are upscattered to X-ray photons by the hot electrons in the optically thin corona. By fitting the NICER observations of AT 2019avd during epochs when the spectra exhibit significant hardening, we derive the evolution of the mass accretion rate, $\dot{m}$, and the coronal energy fraction, $f$. Our results show that $f$ decreases with increasing $\dot{m}$, which is qualitatively consistent with that observed in active galactic nuclei (AGNs), while the slope of this source, $f\propto \dot{m}^{-0.30}$, is much shallower than that of AGNs. We also find that the non-thermal X-ray spectrum in this source is significantly softer than those typically seen in AGNs and black-hole X-ray binaries. We argue that these quantitative differences can be a powerful diagnostic of the underlying magnetic turbulence, which may imply a stronger magnetic field within the TDE accretion disc than that in typical AGNs. It is also found that the evolution of the fitted neutral hydrogen column density follows a similar pattern to that of the accretion rate evolution, which may reflect the accumulation of absorbing material originating from the inflowing streams of stellar debris and/or other related sources.

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arXiv2026

The disk luminosity deficit as a tracer of receding disk during Soft-to-Hard transitions in Black Hole X-ray Binaries

Tracing the evolution of the thin accretion disk during the soft-to-hard state transition in black hole X-ray binaries (BHXRBs) remains difficult because conventional spectral estimates of the disk inner radius become highly model-dependent once the thermal component weakens. We present evidence that the thin disk recedes during this transition, obtained from a systematic study of RXTE/PCA observations of 26 BHXRBs. In 24 outbursts, the disk luminosity decays exponentially in the soft state, then drops significantly below the extrapolated baseline. This thermal luminosity deficit is considered a signature of reduced accretion efficiency, caused by the outward receding of the optically thick disk. Under this framework, we found that the estimated characteristic truncation radius increases rapidly as the systems evolve through the soft-to-hard transition. This interpretation is supported by timing analysis: in observations with well-constrained power density spectra, the characteristic frequencies of broadband noise and low-frequency QPOs generally decrease as the inferred truncation radius increases, consistent with the expansion of a hot inner flow. The onset and rapidity of recession vary substantially across different sources and outbursts. Our results demonstrate that luminosity deficits provide a practical empirical tracer of thin disk receding during soft-to-hard transitions, when direct spectral radius measurements become unreliable.

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arXiv2026

Model dependence of XRISM black-hole spin constraints in Cyg X-1

We study the persistent black hole X-ray binary Cyg X-1, recently observed by XRISM Resolve and simultaneously by NICER and NuSTAR in its hard spectral state. We confirm the result of Draghis et al. that fits of the Resolve data alone with the simplest available relativistic reflection model, relxill, yield a black hole spin parameter close to the maximum, $a_* = 0.99$. However, fitting with an improved, Comptonization-based model, relxillCp, yields a low $a_*=0.0^{+0.17}$. A similarly low range is obtained with another Comptonization-based model, reflkerrD. Then, fits to the combined data require two Comptonization models but are consistent with any spin value. We conclude that the spin value of Cyg X-1 is strongly model-dependent. However, low spin values are consistent with the constraints from gravitational waves. All of the models constrain the inner disk radius to be <10 gravitational radii, which is consistent with a recent finding of the weakness of thermal reverberation in Cyg X-1. The suggested source geometry is that of an outflowing disk corona, which was also proposed to explain the X-ray polarization observed from this source.

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arXiv2026

Dynamic disk-corona coupling during the state transition of Swift J1727.8-1613

State transitions during outbursts of black hole X-ray binaries exhibit complex, rapidly evolving disk-corona coupling. Understanding this dynamic phase is essential for deciphering accretion physics and the mechanisms that drive outbursts, yet it remains poorly understood because of the scarcity of high-quality, high-cadence observations. Here, we present an analysis of observations from the Hard X-ray Modulation Telescope (HXMT) during the 2023 outburst of the newly discovered low-mass black hole X-ray binary Swift J1727.8-1613. Follow-up, high-cadence monitoring reveals pronounced variability in disk emission, attributable to fluctuations in the accretion rate. These disk fluctuations exhibit damped amplitudes and shortened flare periods. This evolving disk emission modulates the supply of soft seed photons to the corona, producing a dynamically changing positive correlation between the photon index $Γ$ and the Comptonization luminosity $L_{\rm Comp}$. As the transition proceeds, the correlation shifts toward higher $Γ$ and a narrower range of $L_{\rm Comp}$. We further suggest that the damped disk variability arises from fluctuations generated at large disk radii and propagating inward, possibly linked to the thermal-viscous disk instability.

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arXiv2026

The counterjet dominates the production of PeV photons from Cyg X-3

We study the physical mechanisms underlying the production of orbitally-modulated PeV photons from Cyg X-3, recently discovered by the LHAASO collaboration. Our key findings are as follows. Helium nuclei are accelerated in a compact and strongly magnetized region within the jet, but they then quickly advect downstream to regions with a weaker field, allowing them to diffuse out of the jet, where they produce pions in hadronic collisions with both the stellar photons and the stellar wind of the Wolf-Rayet donor. The optical depths across the binary are $\lesssim$1 for both types of interactions, implying that their rates are proportional to the column densities along the particle paths. Given the low viewing angle of Cyg X-3 ($i\approx26^\circ$--$28^\circ$), most of the observed photons are produced by the relativistic hadrons accelerated in the counterjet (for which the column densities toward the observer are much longer than for the jet). This also explains the peak of the phase-folded PeV photon flux to be on the opposite side of the superior conjunction than that for the (also orbitally-modulated) GeV photons, which are produced by collisions of relativistic electrons with stellar photons in the optically thick regime. This then implies that the GeV emission is produced in the approaching jet.

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arXiv2026

Covariance spectrum of MAXI J1820+070: On the nature of the Comptonizing flow

We present an analysis of the covariance spectrum of the black hole X-ray binary MAXI J1820+070 during its hard state. For the first time, we extend coherence and covariance studies into the hard X-ray band up to 150 keV. We detect a clear drop in coherence above 30 keV on both short- and long-timescales relative to the 2-10 keV reference band. To investigate the origin of the coherent variability, we simultaneously fit the short- and long-timescale covariances and the time-averaged spectra with a Comptonization model. Surprisingly, the electron temperature associated with long-timescale variability is significantly higher than that on short timescales. Moreover, the temperature on long timescales remains relatively constant throughout the hard state, whereas the short-timescale temperature evolves with X-ray luminosity. We attribute the drop in coherence to multiple sources of seed photons, i.e., the blackbody and synchrotron photons. The independence between these two photon fields leads to the drop in coherence. To explain the lower electron temperature on short timescales, we propose a two-Comptonization framework in which short-timescale variability arises from a vertically extended central region, while long-timescale variability originates at larger radii. The elevated geometry of the inner region leads to illumination primarily by cooler outer-disk photons, yielding a lower electron temperature. In this case, the evolution of the height of the elevated region could explain the evolution of the electron temperature associated with the coherent variability throughout the hard state.

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arXiv2026

The strong Fe K line and spin of the black-hole X-ray binary MAXI J1631-479

We study the transient black hole binary MAXI J1631--479 in its soft spectral state observed simultaneously by the NICER and NuSTAR instruments. Its puzzling feature is the presence of a strong and broad Fe K line, while the continuum consists of a strong disk blackbody and a very weak power-law tail. The irradiation of the disk by a power-law spectrum fitting the tail is much too weak to account for the strong line. Two solutions were proposed in the past. One invoked an intrinsic Fe K disk emission, and the other invoked disk irradiation by the returning blackbody emission. We instead find that the strong line is naturally explained by the irradiation of the disk by the spectrum from Comptonization of the disk blackbody by coronal relativistic electrons. The shape of the irradiating spectrum at $\lesssim$10 keV reflects that of the disk blackbody; it is strongly curved and has a higher flux than that of a fit with a power-law irradiation. That flux accounts for the line. While this result is independent of the physical model used for the disk intrinsic emission, the value of the fitted spin strongly depends on it. When using a Kerr disk model for a thin disk with a color correction, the fitted spin corresponds to a retrograde disk, unlikely for a Roche-lobe overflow binary. Then, a model accounting for both the disk finite thickness and radiative transfer yields a spin of $a_*\approx0.8$--0.9, which underlines the strong model-dependence of X-ray spin measurements.

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arXiv2026

Evolution of the Inner Accretion Flow in Swift J1727.8$-$1613 across Intermediate States: Insights from Broadband Spectral and Timing Analysis

We present a comprehensive broadband spectral and variability study of the newly detected black hole X-ray binary Swift~J1727.8--1613 in the intermediate states during its 2023 outburst, using multi-mission observations from NICER, NuSTAR, AstroSat, and Insight-HXMT. Spectral data up to 78 keV in the hard-intermediate state (HIMS) require models with two Comptonizing regions. In contrast, models with a single Comptonizing region adequately describe the soft-intermediate states (SIMS), implying a significant evolution in the disk-corona geometry between the states. The hard X-ray tail above $100$ keV in the HIMS, detected with both AstroSat/CZTI and Insight-HXMT/HE, indicates that the electron population in the corona is not purely thermal but rather hybrid, with a power-law distribution above the thermal cutoff. While both the reflection modeling and disk continuum fitting favor a truncated disk geometry in the HIMS, the disk in the SIMS moves substantially closer to the innermost stable circular orbit, accompanied by a significant rise in disk temperature. This interpretation is further supported by the increase in the QPO frequency from $\sim1.3$ to $\sim6.6$ Hz. From joint modeling of the disk continuum and reflection component and assuming the distance of 3.4 kpc, we estimate a black hole mass of $10.3^{+5.5}_{-2.5}~M_\odot$, spin of $0.79^{+0.07}_{-0.15}$, and the disk inclination angle of $\sim37\degr$--$53\degr$, which match well with the previously reported spectro-polarimetric measurements. We find a weakly variable or stable disk and a highly variable Comptonized component.

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arXiv2026

Spins of Black Holes in X-ray Binaries and the Tension with the Gravitational Wave Measurements

We review current challenges in understanding the values and origin of the spins of black holes in binaries. Thanks to recent advances in astrophysical instrumentation, the spins can now be measured using both gravitational waves emitted by merging black holes and electromagnetic radiation from accreting X-ray binaries containing black holes. A key finding of the gravitational-wave observatories is that premerger black holes in binaries have low spin values, with an average dimensionless spin parameter of $a_*\sim$0.1--0.2, with 90\% having $a_*\lesssim 0.6$. This implies that the natal spins of black holes are generally low, and the angular momentum transport in massive stars is efficient. On the other hand, most of the published spins in X-ray binaries are very high. In particular, this is the case for binaries with high-mass donors (potential progenitors of mergers), where their published spins range from 0.8 to 1.0. At the same time, their short lifetimes prevent significant spin-up by accretion. Those with low-mass donors could be spun-up to $a_*\gtrsim 0.7$ by accretion only if the donor initial masses were more than several solar masses, which remains unproven. However, the existing methods of spin measurements suffer from significant systematic errors. The method relying on relativistic X-ray line broadening is based on the separation of the observed spectra into incident and reflected ones, which is highly uncertain. The method relying on spectral fitting of accretion disk continua uses models that predict the disk to be highly unstable, while stability is observed. Improved stable models predict lower spins. The published spin measurements in X-ray binaries are uncertain. The spins of the binaries with high-mass donors may be low, while those with low-mass donors have a broader spin distribution, ranging from low to high.

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arXiv2025

Accretion Geometry of the New Galactic Black Hole Candidate AT2019wey in the Hard State

We perform broadband spectral and timing studies of the Galactic low-mass black hole candidate AT2019wey using quasi-simultaneous NICER, Swift, and NuSTAR observations obtained in 2022. The long-term MAXI light curve, along with the hardness-intensity diagram (HID), indicates that the source remained in the hard state and did not switch to the soft state. Spectral modeling using two different model combinations reveals that the broadband spectrum is best described by two distinct Comptonizing regions, associated reflection components, and thermal emission from the disk. The harder Comptonizing region dominates ($\gtrsim80\%$) the total flux and is primarily responsible for the observed reflection features from the distant part of the disk. We find that the accretion disk is truncated at a radius of $\sim16-56~r_{\rm{g}}$, while the luminosity is $\sim1.9\%$ of the Eddington limit, assuming a black hole mass of $10 ~ M_\odot$ and distance of 8 kpc. Our spectral results also show consistency in the estimated inner disk radius obtained through two independent methods: modeling the disk continuum and the reflection spectrum. The variability studies imply the presence of intrinsic disk variability, likely originating from an instability in the disk. We also detect hard time lags at low frequencies, possibly arising from the inward propagation of mass accretion rate fluctuations from the outer to the inner regions of the accretion disk. Moreover, an observed deviation of the lag-energy spectrum from the log-linear trend at $\lesssim 0.7$ keV is most likely attributed to thermal reverberation, arising from the reprocessing of hard coronal photons in the accretion disk.

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arXiv2025

Variability of X-ray polarization of Cyg X-1

We present the results of a three-year X-ray, optical, and radio polarimetric monitoring campaign of the prototypical black hole X-ray binary Cyg X-1, conducted from 2022 to 2024. The X-ray polarization of Cyg X-1 was measured 13 times with the Imaging X-ray Polarimetry Explorer (IXPE), covering both hard and soft spectral states. The X-ray polarization degree (PD) in the hard state was found to be $\approx4.0\%$, roughly twice as high as in the soft state, where it was around $2.2\%$. In both states, a statistically significant increase of PD with the energy was found. Moreover, a linear relation between PD and spectral hardness suggests a gradual and continuous evolution of the polarization properties, rather than an abrupt change of polarization production mechanism between states. The polarization angle (PA) was independent of the spectral state and showed no trend with the photon energy. The X-ray PA is well aligned with the orientation of the radio jet, as well as the optical and radio PAs. We find significant orbital changes of PA in the hard state, which we attribute to scattering of X-ray emission at intrabinary structure. No significant superorbital variability in PD or PA was found at the period $P_{\rm{so}}$ = 294 d. We also find no correlation between the X-ray and optical polarization; if any, there is a long-term anti-correlation between the X-ray PD and the radio PD.

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arXiv2025

A Novel Method of Modeling Extended Emission of Compact Jets: Application to Swift J1727.8-1613

Flat radio spectra of compact jets launched by both supermassive and stellar-mass black holes are explained by an interplay of self-absorbed synchrotron emission up to some distance along the jet and optically thin synchrotron at larger distances (Blandford & Konigl 1979). Their spatial structure is usually studied using core shifts, in which the position of the peak (core) of the emission depends on the frequency. Here, we propose a novel method to fit the spatial dependence of the flux density at a given frequency of the jet and counterjet (when observed) using the theoretical spatial dependencies, which we provide as simple analytical formulae. We apply our method to the spatial structure of the jets in the luminous hard spectral state of the black hole X-ray binary Swift J1727.8-1613. It was the most resolved continuous jet from an X-ray binary ever observed. We find that the observed approaching jet is significantly intrinsically stronger than the receding one, which we attribute to an increase in the emission of both jets with time (observationally confirmed), together with the light travel effect, causing the receding jet to be observed at an earlier epoch than the approaching one. The jets are relatively slow, with the velocity $\sim(0.3$-$0.4)c$. Our findings imply that the magnetic field strength increased with time. Also, the magnetic flux is much lower than in jets launched by `Magnetically Arrested Disks'. Our method is general, and we propose that it be applied to jets launched by stellar-mass and supermassive black holes.

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arXiv2025

Is the Spin of the Black Hole in GX 339-4 Negative?

We have studied the accreting black hole binary GX 339--4 using two highly accurate broad-band X-ray data sets in very soft spectral states from simultaneous NICER and NuSTAR observations. Joint fitting of both data sets with relativistic models of the disk, its Comptonization and reflection allows us to relatively accurately determine the black-hole mass and spin, and the distance and inclination. However, we find the measured values strongly depend on the used disk model. With widely used Kerr disk models treating departures from local blackbody spectra using color corrections, we find relatively low black-hole masses and strongly negative spins (i.e., retrograde accretion). Then, models employing radiative transfer calculations of the disk atmosphere predict moderately positive spins and high masses. When adding a warm corona above the disk (as proposed before for both AGNs and accreting binaries), we find the spin is weakly constrained, but consistent with zero. In all cases, the fitted inclination is low, $\approx 30$--$34^\circ$. For the spin axis aligned with the binary axis, the mass function for this source implies large values of the mass, consistent only with those obtained with either disk-atmosphere models or the presence of a warm corona. We also test different disk models for an assumed set of mass, distance and inclination. We find that different models yield values of the spin parameter differing up to $\sim$0.3. Our results confirm previously found strong model dependencies of the measured black-hole spin, now by comparing different disk models and for a low-mass X-ray binary.

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arXiv2024

Constraining the physical properties of large-scale jets from black hole X-ray binaries and their impact on the local environment with blast-wave dynamical models

Relativistic discrete ejecta launched by black hole X-ray binaries (BH XRBs) can be observed to propagate up to parsec-scales from the central object. Observing the final deceleration phase of these jets is crucial to estimate their physical parameters and to reconstruct their full trajectory, with implications for the jet powering mechanism, composition and formation. In this paper we present the results of the modelling of the motion of the ejecta from three BH XRBs: MAXI J1820+070, MAXI J1535$-$571 and XTE J1752$-$223, for which high-resolution radio and X-ray observations of jets propagating up to $\sim$15 arcsec ($\sim$0.6 pc at 3 kpc) from the core have been published in the recent years. For each jet, we modeled its entire motion with a dynamical blast-wave model, inferring robust values for the jet Lorentz factor, inclination angle and ejection time. Under several assumptions associated to the ejection duration, the jet opening angle and the available accretion power, we are able to derive stringent constraints on the maximum jet kinetic energy for each source (between $10^{43}$ and $10^{44}$ erg, including also H1743$-$322), as well as placing interesting upper limits on the density of the ISM through which the jets are propagating (from $n_{\rm ISM} \lesssim 0.4$ cm$^{-3}$ down to $n_{\rm ISM} \lesssim 10^{-4}$ cm$^{-3}$). Overall, our results highlight the potential of applying models derived from gamma-ray bursts to the physics of jets from BH XRBs and support the emerging picture of these sources as preferentially embedded in low-density environments.

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arXiv2024

A physical model for radio and X-ray correlation in black hole X-ray binaries

A tight correlation between the radio and X-ray emission in the hard state of black hole X-ray binaries (BHXRBs) indicates an intrinsic disc-jet connection in stellar black hole accretion systems, though the detailed physics processes at work are still quite unclear. A hot accretion flow is suggested to match the outer cold thin disc at a certain radius in the hard state, which may vary with the accretion rate. In this work, we assume that the magnetic field generated in the outer thin disc is advected inwards by the inner hot accretion flow, which is substantially enhanced near the BH. Such a strong field threading the horizon of a spinning BH is responsible for launching relativistic jets in BHXRBs via the Blandford-Znajek mechanism. Thus, both the jet power and the X-ray emission increase with the mass accretion rate, and we find that our model calculations are able to reproduce the observed radio/X-ray correlation quantitatively. For some individual BHXRBs, the slopes of the radio/X-ray correlations become steeper when the sources are brighter. Our model calculations show that this feature is due to the transition of the outer disc with gas pressure dominated to radiation pressure dominated, which leads to different accretion rate dependence of the field strength in the outer disc.

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arXiv2024

Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE

The accretion of matter by compact objects can be inhibited by radiation pressure if the luminosity exceeds the critical value, known as the Eddington limit. Discovery of ultraluminous X-ray sources has shown that accretion can proceed even when the apparent luminosity significantly exceeds this limit. High apparent luminosity might be produced thanks to geometric beaming of the radiation by an outflow. The outflow half-opening angle, which determines the amplification due to beaming, has never been robustly constrained. Using the Imaging X-ray Polarimetry Explorer, we made the measurement of X-ray polarization in the Galactic X-ray binary Cyg X-3. We find high, over 20%, nearly energy-independent linear polarization, orthogonal to the direction of the radio ejections. These properties unambiguously indicate the presence of a collimating outflow in the X-ray binary Cyg~X-3 and constrain its half-opening angle, <15 degrees. Thus, the source can be used as a laboratory for studying the super-critical accretion regime. This finding underscores the importance of X-ray polarimetry in advancing our understanding of accreting sources.

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