arXiv subjects

James F. Steiner

Publications and source records attributed to James F. Steiner.

At least 19 recordsLinked to original sources

arXiv2026

Radio flares and X-ray hardening embedded in the long soft state of 4U 1543-475

We present a comprehensive multi-wavelength study of the black hole X-ray binary 4U 1543-475 during its 2021 outburst, focusing on radio flaring episodes that are commonly interpreted as signatures of episodic jet production and are embedded within states when the X-ray emission was dominated by an accretion disk component. The radio monitoring reveals at least two discrete flares that coincide with periods of enhanced Comptonized X-ray emission. Broadband spectral modelling shows a significant decrease in the reflection-to-disk flux ratio (by a factor of ~3-4) during these episodes, consistent with a temporary change in the geometry of the inner accretion flow, although the data do not allow the causal sequence to be firmly established. Optical photometry exhibits variability that broadly tracks the reflection fraction, consistent with changes in the illuminating component. The accompanying spectral hardening indicates that the radio flares were associated with short-lived excursions toward a "harder" state, departing from the soft state. X-ray timing analysis suggests that the radio flares may be associated with changes in the fractional rms variability; however, no consistent or unified pattern can be firmly established across different events. These results provide a multi-wavelength observational example of radio flaring activity in a black hole binary and highlight the complex interplay between accretion flow geometry, coronal emission, and jet-related phenomena.

astro-ph.HE↗

arXiv2026

Superorbital variability in the quiescent black hole X-ray transient A0620-00

Quiescent black hole low-mass X-ray binaries provide a key setting for probing accretion physics at low luminosities. A0620-00, the archetypal system in this class, has remained in X-ray quiescence for decades and exhibits complex optical variability, yet the long-term behaviour of its accretion flow remains poorly understood. Here, we report an analysis of long-term optical monitoring of A0620-00 from ZTF, LCO, and ATLAS. The full dataset spans nearly two decades, with the ZTF light curve providing the primary $\sim 2760$-day baseline for the period analysis. We identify a superorbital cycle with a period of $P = 261.9 \pm 9.4$ d and a peak-to-peak amplitude of $\sim 0.2$ mag. The signal is recovered independently across all three surveys, and red-noise simulations indicate that it is unlikely to arise from stochastic variability alone. Furthermore, the relative occurrence of the \textit{passive} and \textit{active} quiescent states displayed by the system seems to depend on the superorbital phase, with passive states concentrated near the cycle minimum and active states more common near maximum. We find that, among the possible interpretations, retrograde nodal precession of a hot inner accretion flow might be able to explain the observed long-term modulation. In this interpretation, the periodic signal may arise from cyclic reorientation of the inner flow, which modulates the photometric contribution from the innermost regions. The inferred modulation period would correspond to a characteristic dynamical radius of $\sim0.13a$ ($\sim10^4~R_{\rm g}$), where $a$ is the binary semi-major axis, broadly consistent with the expected transition between the outer thin disc and the inner hot accretion flow.

astro-ph.HE↗

arXiv2026

Black-Hole Spin Measurements from X-ray Reflection Spectroscopy: Quality Criteria and Community Recommendations

X-ray reflection spectroscopy provides one of the most powerful electromagnetic methods for measuring the dimensionless spin of accreting black holes. It has yielded spin constraints for stellar-mass black holes in X-ray binaries and supermassive black holes in active galactic nuclei, and is central to the science goals of current and future X-ray observatories. However, the technique is subject to observational and modeling systematics, including continuum-reflection degeneracy, limited spectral coverage, unresolved distant reflection or absorption, detector effects, source variability, accretion-state dependence, and assumptions inherent to reflection models. Motivated by discussions at the 2025 Wake Forest workshop *Recent Progress on Black Hole Spin Measurements Across the Electromagnetic and Gravitational Spectra*, we propose a practical framework for evaluating whether published reflection-based spin measurements should be considered robust, provisional, or not assessable from the available information. The framework is built on three principles: **detectability**, requiring an unambiguous relativistic reflection signal; **uniqueness**, requiring that the relativistic component be distinguishable from the continuum, distant reflection, absorption, and instrumental effects; and **robustness**, requiring that the inferred spin remain stable against reasonable changes in model assumptions, data selection, and accretion-state treatment. We translate these principles into assessment criteria, a quality-classification scheme, and a reporting checklist for future studies. Calibration of these criteria through dedicated simulations is outlined here and deferred to a companion paper. Our goal is to establish a reproducible path toward a community-maintained compilation of reliable black hole spin measurements for the high-throughput, high-resolution era of X-ray astronomy.

astro-ph.HE↗

arXiv2026

Neural-Spectral Discovery of Rotating Black Holes Beyond General Relativity

Finding rotating black hole solutions in higher-curvature theories of gravity is a problem of fundamental importance. Virtually every approach to reconcile gravity with quantum mechanics predicts corrections to the Einstein-Hilbert action, yet no systematic solution-generating method exists for the stationary sector. We close this gap with {\sc Akribeia}, a novel hybrid framework that pairs physics-informed neural networks with a pseudo-spectral refinement step, yielding certified neural-field rotating black hole solutions -- continuous, globally defined functions, parametric in the coupling constants -- whose residuals against the field equations are verified to extreme precision. We apply the method to theories quadratic and cubic in the curvature and construct, for the first time, families of rotating black holes featuring multiple non-vanishing angular momenta, parametric in the new coupling constants. After validating against previously known five-dimensional spacetimes, we present new solutions in scenarios leading to a highly non-linear/non-perturbative coupled system of ordinary differential equations. Our method can be systematically adapted to other setups involving partial differential equations as well.

gr-qc↗

arXiv2026

Spin Demographics of Active Supermassive Black Holes: Updated Estimates from X-ray reflection and Future opportunities

Understanding the growth of supermassive black holes (SMBHs) requires observational constraints on how their angular momentum (or spin) varies with mass, since the relative importance of coherent accretion, chaotic accretion, and mergers will be reflected in SMBH spin populations. Here we present an updated compilation of reflection-based SMBH spin measurements from the literature and assemble a set of ancillary quantities of interest for each SMBH (including redshift, Eddington ratio, and X-ray luminosity). No obvious apparent correlation between the Eddington-scaled accretion rate and the black hole spin is seen, noting that formal statistical tests are beyond the scope of this review. We discuss the limitations of using this heterogeneous mass--spin sample to test predictions of SMBH growth from semi-analytic models and cosmological simulations, emphasizing the need for a more uniform sample. We then highlight the encouraging prospects enabled by the next-generation NewAthena X-ray flagship observatory. Finally, we summarize how hierarchical Bayesian population inference applied to observed SMBH mass--spin populations will constitute a powerful framework for confirming tentative mass--spin trends in future samples.

astro-ph.HE↗

arXiv2026

Unraveling Chemical Enrichment in Extreme Emission-Line Galaxies: A Multi-Element Bayesian View of Bursty Star Formation and Galaxy Evolution in DESI

Extreme emission-line galaxies (EELGs) probe chemical enrichment in low-mass, bursty systems where star formation, feedback, and gas accretion are poorly constrained. Using DESI DR1, we select 23 nearby EELGs with detections of 19 ionic species (S/N $\geq$ 4), stellar masses $ M_* \geq 10^7 M_{\odot}$, and extreme H$α$ and [O III] 5007 equivalent widths (EW $\geq$ 500 Angstrom). We infer non-parametric star-formation histories and fit a Bayesian single-zone chemical-evolution model to O, N, Ne, S, and Ar, allowing time-dependent star-formation efficiency, outflow mass loading, and evolving inflow metallicity. We find short depletion timescales and large mass-loading factors, indicating rapid gas cycling in a burst-driven, non-equilibrium regime, with depletion times below Kennicutt-Schmidt expectations. Star-formation efficiency and outflows are well constrained, while inflow metallicity is weaker due to degeneracies with metal production. Abundance ratios isolate physical drivers: star-formation efficiency sets evolutionary tracks, outflows regulate metal retention and X/O normalization, and inflow metallicity sets baseline enrichment. N/O strongly constrains burst timing and gas flows, Ne/O remains nearly invariant, and S/O and Ar/O show intermediate sensitivity. These results demonstrate that multi-element abundances provide a direct probe of baryon-cycle processes in extreme low-mass starbursts.

astro-ph.GA↗

arXiv2026

Systematic assessment of disk truncation in the black hole X-ray binary Swift J1727.8-1613 using NICER

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 accretion states. High-quality data are available in the high-Eddington-fraction hard-intermediate state, hard-to-soft transition, the soft state, and the poorly studied back-transition to the dim hard state, making it an ideal dataset to compare 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 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.

astro-ph.HE↗

arXiv2026

Exploring the spin dependence on mass inclination and distance for the newly discovered black hole X-ray binary Swift J151857.0-572147

The black hole X-ray binary (BHXRB) Swift J151857.0-572147 was discovered during its first outburst in March 2024. We review the archive of NICER observations from this outburst, focusing on the soft states. We select spectra for which the disk to total flux ratio exceeds 0.8 and the coronal scattering fraction fsc is less than 25%, conditions under which the accretion disk is expected to extend to the innermost stable circular orbit (ISCO) and remains geometrically thin. Through a continuum fitting analysis, we explore the dependence of spin on mass, inclination, and distance. We constrain the spin within the parameter space typical of stellar-mass black holes (sBHs), assuming a mass around 10 solar masses, inclination angles between 20 and 60 degree, and distances between 4 and 16 kpc. For fiducial parameters: a mass of 10 solar masses, a distance of 10 kpc, and an inclination angle of 40 degree, a moderate spin of approximately 0.7 is obtained. However, precise determination of the spin will require accurate measurements of these parameters. Our analysis provides a framework to infer the spin and estimate its uncertainties once more precise measurements of mass, distance, and inclination become available. As we demonstrate, lower inclination angles, greater distances, or larger black hole masses result in higher spin values.

astro-ph.HE↗

arXiv2026

A clean broad iron line in GS 1354--64 as seen by XRISM

We present a spectroscopic analysis of XRISM and NuSTAR observations of the black hole X-ray binary GS~1354--64 during its 2026 outburst. A total number of 3.5 million photons are collected by the microcalorimeter Resolve on board XRISM, providing an unprecedented high-resolution view of the iron line profile. A clean broad iron line is found in the data, without significant narrow features. Modeling the broad iron line with relativistic reflection from the inner accretion disk suggests a rapidly spinning black hole (a>0.98) in the system. Measurements of the disk inclination angle from the reflection method are model-dependent. This work demonstrates the power of X-ray microcalorimeters in studying the inner accretion flow and constraining black hole parameters.

astro-ph.HE↗

arXiv2026

Discovery of Strong Energy-Dependent X-ray Polarization in the Intermediate State of GS 1354-64

We report the discovery of significant X-ray polarization from the dynamically confirmed black hole X-ray binary (BHXB) GS 1354-64 during its 2025-2026 outburst, obtained with the Imaging X-ray Polarimetry Explorer (IXPE). The observation, obtained shortly after a bright X-ray flare, captures the source in an intermediate state following a stalled (failed) state transition. We discover significant 2-8 keV polarization at the ~4% level with high statistical support--14-sigma significance from frequentist analysis and log Bayes Factor 283+/-1 in a Bayesian framework--measuring PD 4.0+/-0.2% and PA-1+/-2 degrees (90% credible interval). The PD exhibits a statistically significant increasing trend with energy--the strongest such increase yet observed by IXPE in a BHXB--going from 2.1+/-0.3% in the 2-3 keV band to 11+/-3% in the 6.5-7 keV band, while the PA appears stable across both energy and time to within statistical uncertainties. Timing analysis of the IXPE data reveals a ~5 Hz Type-C quasi-periodic oscillation. IXPE + NuSTAR spectropolarimetric modeling suggests that the data can be described by polarized thermal disk and Comptonized components with PAs differing by ~90 degrees, or by a dominant Comptonized polarized component whose effective PD increases across the IXPE bandpass--the inferred component-level polarization levels are therefore model-dependent. In either picture, GS 1354-64 retains a strong coronal component during the transitional period observed by IXPE. These results illustrate how X-ray polarimetry can provide a sensitive diagnostic of the accretion state and geometry in black hole X-ray binary accretion flows, exploring a liminal phase at the cusp of state transition.

astro-ph.HE↗

arXiv2026

A NICER view of the corona through time-dependent Comptonization of the quasi-periodic oscillations in nine black-hole X-ray binaries

We present a systematic study of the evolution of the corona geometry in nine black hole X-ray binaries (BHXRBs) using archival data from NICER. We identify 171 observations exhibiting quasi-periodic oscillations (QPOs) across various spectral states and model the time-averaged energy spectra of the source, as well as the energy-dependent rms and phase-lag spectra of the QPO, with the time-dependent Comptonization model vKompthdk. This allows us to simultaneously constrain the corona size and feedback fraction during outbursts. By using the power color hue diagnostics, we identify different spectral states, and observe that the QPO frequency increases from $\sim$0.1 Hz to $\sim$10 Hz in the low-hard and hard-intermediate states (LHS and HIMS), and remains approximately constant at 4--5~Hz in the soft-intermediate state (SIMS). The corona size shows significant evolution: the corona is large ($\sim10^4$--$10^5$ km) in the LHS, contracts rapidly to $\sim10^3$ km in the HIMS, and exhibits a flare-like expansion near the HIMS-to-SIMS transition. In the SIMS and high-soft state (HSS), the corona becomes compact and stable (4000--8000~km). The feedback fraction of the corona photons increases during the periods in which the corona contracts and decreases during the periods in which the corona expands, indicating a change of the disk-corona coupling. Our results are consistent with previous QPO-based studies using vKompthdk on some individual sources. This work, however, provides the first view of the coronal evolution across outbursts for a diverse BHXRB sample, offering critical insights into coronal behavior as a function of the spectral state of the source.

astro-ph.HE↗

arXiv2026

Evidence of mutually exclusive outflow forms from a black hole X-ray binary

Accretion onto black holes often leads to the launch of outflows that significantly influence their surrounding environments. The two primary forms of these outflows are X-ray disk winds-hot, ionized gases ejected from the accretion disk-and relativistic jets, which are collimated streams of particles often expelled along the rotational axis of the black hole. While previous studies have revealed a general association between spectral states and different types of outflows, the physical mechanisms governing wind and jet formation remain debated. Here, using coordinated NICER and MeerKAT observations of the recurrent black hole X-ray binary 4U 1630-472, we identify a clear anti-correlation between X-ray disk winds and jets: during three recent outbursts, only one type of outflow is detected at a time. Notably, this apparent exclusivity occurs even as the overall accretion luminosity remains within the range expected for a standard thin disk, characteristic of the canonical soft state. These results suggest a competition between outflow channels that may depend on how the accretion energy is partitioned between the disk and the corona. Our findings provide new observational constraints on jet and wind formation in X-ray binaries and offer a fresh perspective on the interplay between different modes of accretion-driven feedback.

astro-ph.HE↗

arXiv2026

Decoding Cygnus X-2: The Critical Role of Reflection in IXPE Data

We present a spectro-polarimetric re-analysis of the first IXPE observation of Cygnus X-2 which we determine to be mainly in the normal branch, from quasi-simultaneous observations with NuSTAR, NICER, and INTEGRAL. We measure the hard X-ray polarization angle and find it to be consistent with the previously measured position angle of the radio jet. Leveraging NuSTAR's detection of both the relativistic Fe K emission line and the Compton hump, we constrain the flux contribution of the reflected emission from the inner accretion disk to be 10% of the total X-ray flux in the IXPE energy band. Unlike previous studies that modeled only the Fe K emission line, we fit the full-band reflection spectrum using a fully relativistic disk model. There is strong degeneracy between the Comptonized and reflection components. Given that the Comptonized component is not expected to be highly polarized, a polarization degree of approximately 20% for the reflection component could explain the X-ray polarization data from IXPE. We also discuss the disk inclination angle inferred from our spectro-polarimetric modeling, as well as other possible explanations for the data.

astro-ph.HE↗

arXiv2026

Variational Autoencoder with Normalizing flow for X-ray spectral fitting

Black hole X-ray binaries (BHBs) can be studied with spectral fitting to provide physical constraints on accretion in extreme gravitational environments. Traditional methods of spectral fitting such as Markov Chain Monte Carlo (MCMC) face limitations due to computational times. We introduce a probabilistic model, utilizing a variational autoencoder with a normalizing flow, trained to adopt a physical latent space. This neural network produces predictions for spectral-model parameters as well as their full probability distributions. Our implementations result in a significant improvement in spectral reconstructions over a previous deterministic model while performing three orders of magnitude faster than traditional methods.

cs.LG↗

arXiv2025

The First X-Ray Polarimetry of an Eclipsing Low-Mass X-Ray Binary: Serendipitous IXPE Observation of AX J1745.6-2901

We present the first X-ray polarimetric measurement of the neutron star low-mass X-ray binary system AX J1745.6-2901 conducted by the Imaging X-ray Polarimetry Explorer (IXPE) satellite. This transient source, located within $ \sim $1.5' of the Galactic center, was observed serendipitously during a MAXI J1744-294 observation with a duration of 150 ks. The complex nature of the region in which AX J1745.6-2901 is located poses a challenge for studying its polarization. By performing a detailed analysis of the contamination from MAXI J1744-294 and the Galactic center diffuse emission, we find the source polarization degree PD = 14.7$\% \pm$ 4.0$\%$ and polarization angle PA = 122$^\circ \pm 8^\circ$. The phase-resolved analysis shows increase in polarization during the eclipse phase, with PD = 34.2$\% \pm$ 8.7$\%$, suggesting that the polarization-inducing mechanisms are of scattering nature, probably originating from disk winds.

astro-ph.HE↗

arXiv2025

Super-Eddington Accretion Geometry: a Remarkable Stability of the Hidden Ultraluminous X-Ray Source Cygnus X-3

We report on the average and orbital phase-resolved polarization of Cyg X-3 in the hard state during the 2023 Imaging X-ray Polarimetry Explorer (IXPE) observational campaign. We find the polarization degree of $ 21.2 \pm 0.4 \% $ and polarization angle of $ 92.2 \pm 0.5^\circ $, well compatible with the first hard-state IXPE observation in 2022. As the observed polarization depends on both the accretion geometry and the X-ray emission mechanism, which we attribute to reflection from the optically thick envelope surrounding the central source, our result indicates that both are very stable on year-long timescale. We discuss time- and energy-dependent polarization properties and their implications for the geometry and stability of the accretion funnel.

astro-ph.HE↗

arXiv2025

Possible Coronal Geometry in the Hard and Soft State of Black Hole X-ray Binaries from MONK Simulations

Understanding the coronal geometry in different states of black hole X-ray binaries is important for more accurate modeling of the system. However, it is difficult to distinguish different geometries by fitting the observed Comptonization spectra. In this work, we use the Monte Carlo ray-tracing code MONK to simulate the spectra for three simple corona toy models widely proposed in observational studies: sandwich, spherical, and lamppost, varying their optical depth and size (height). By fitting the simulated NuSTAR observations with the simplcut*kerrbb model, we infer the possible parameter space for the hard state and soft state of different coronal geometries. The influence of the disk inclination angle, black hole spin and coronal temperature is discussed. We find that in the lamppost model, if we exclude the case of a very extended corona, the disk emission is always dominant, making the lamppost geometry incompatible with the hard state. While the sandwich and spherical models can produce similar spectra in both the hard and soft states, the simulated IXPE polarimetric spectra show the potential to break this degeneracy. Geometrical effects arising from the limited size of the corona become evident in lower-spin black holes and affect the spectral fitting, where the larger ISCO reduces the corona coverage of the inner disk.

astro-ph.HE↗

arXiv2025

Modeling X-ray photon pile-up with a normalizing flow

The dynamic range of imaging detectors flown on-board X-ray observatories often only covers a limited flux range of extrasolar X-ray sources. The analysis of bright X-ray sources is complicated by so-called pile-up, which results from high incident photon flux. This nonlinear effect distorts the measured spectrum, resulting in biases in the inferred physical parameters, and can even lead to a complete signal loss in extreme cases. Piled-up data are commonly discarded due to resulting intractability of the likelihood. As a result, a large number of archival observations remain underexplored. We present a machine learning solution to this problem, using a simulation-based inference framework that allows us to estimate posterior distributions of physical source parameters from piled-up eROSITA data. We show that a normalizing flow produces better-constrained posterior densities than traditional mitigation techniques, as more data can be leveraged. We consider model- and calibration-dependent uncertainties and the applicability of such an algorithm to real data in the eROSITA archive.

astro-ph.HE↗