arXiv subjects

J. Podgorný

Publications and source records attributed to J. Podgorný.

9 recordsLinked to original sources

arXiv2026

Inelastic scattering effects on polarization predictions for cold and warm atmospheres in the X-rays

X-ray polarization models of accreting compact objects often assume the so-called Chandrasekhar-Sobolev polarization prescription for elastic Thomson scattering in electron-scattering semi-infinite atmospheres. In this letter, we examine the effects of inelastic Compton scattering. We assume homogeneous slabs of cold and warm (up to 1 keV) electrons. We use a Monte Carlo approach. We discuss the polarization difference arising in the X-rays between the elastic and inelastic scattering assumptions for plane-parallel transmission and reflection, and the role it can play in the interpretation of X-ray polarimetric observations of accreting compact objects.

astro-ph.HE↗

arXiv2026

Disk reflection as the origin of the X-ray polarization of NGC 4151 with IXPE

We present an X-ray spectro-polarimetric study of the nearby type-1 active galactic nucleus NGC 4151 using two long IXPE observations obtained in 2022 and 2024, supported by simultaneous XMM-Newton and NuSTAR spectroscopy. IXPE measures a polarization degree of $\sim 6-7\%$ above 4 keV, with a polarization angle parallel to the radio jet, and a distinct low-energy component with a different angle, indicating at least two polarized components in the $2-8$ keV band. Previous work interpreted the hard X-ray polarization as evidence for a radially extended slab-like corona. Here we test an alternative scenario in which the observed polarization is produced predominantly by relativistic reflection from an accretion disk illuminated by a compact, lamp-post-like corona. Using recently developed models, we fit the IXPE Stokes spectra with a lamp-post plus distant-torus geometry, including partial-covering absorption and an additional soft polarized power-law component. We find that the data require a low coronal height ($h<9\,R_{\rm g}$ at $3σ$) and a relatively large torus opening angle ($>45^\circ$ at 3$σ$), while the disk reflection contributes $\sim 20\%$ of the 2-8 keV flux. The soft polarized component carries only $\sim 1-5\%$ of the flux but has a high polarization degree ($>10\%$) and a polarization angle around $20^\circ$. The same configuration provides acceptable fits to the $0.4-79$ keV XMM-Newton and NuSTAR spectra, demonstrating that disk reprocessing by a compact corona can simultaneously account for both the polarization and broadband spectral properties of NGC 4151.

astro-ph.HE↗

arXiv2026

X-ray polarization of reflected thermal emission

We compute rest-frame reflection spectra and the corresponding energy-dependent linear polarization degree and angle for (un)polarized single-temperature blackbody spectra impinging on a partially ionized constant-density optically thick slab. We use a combination of a Monte Carlo simulation that takes into account scattering, absorption, and spectral lines, with a non-LTE radiative transfer pre-computation of the ionization structure of the slab in photo-ionization equilibrium. We discuss the impact of the reflector's ionization and of the incident spectral shape on the obtained energy dependence of polarization. For isotropic incident intensity and for rather parallel than perpendicular unidirectional illumination with respect to the slab normal direction, despite the presence of highly polarized absorption features and low-polarized spectral lines, an underlying scattering-induced increase of polarization degree with energy in mid to hard X-rays naturally arises due to multiple Compton-scattering energy shifts. Such re-processing effect is particularly apparent in 2-8 keV for steep incident X-ray spectra reflecting from highly-ionized optically thick media. Integration of the resulting local reflection tables in specific large-scale reflection geometries occurring in X-ray binary systems, including relativistic effects, will be presented in a follow-up paper. Nonetheless, we anticipate that the obtained local energy-dependent features will imprint at large distances from the source to the observed X-ray polarization, and could contribute to the observed increase of total polarization degree with energy in 2-8 keV in some accreting systems by the IXPE mission.

astro-ph.HE↗

arXiv2026

Polarization properties of thermal accretion disk emission. I. Direct radiation

The X-ray polarimetric observing window re-opening is shedding new light on our current understanding of compact accreting sources. This is true, in particular, for stellar-mass black hole sources observed in the thermally-dominated state, for which the polarization signal is expected to depend on the accretion disk inclination and the black hole spin. Two main effects determine the polarization properties of the accretion disk emission: the absorption and scattering processes occurring before the radiation leaves the disk atmosphere, and the relativistic effects influencing its propagation towards the observer at infinity. In this work, we investigate these effects together considering only the contribution of direct radiation. We analyze how the ionization state of the disk atmosphere, approximated with a constant-density surface layer assumed to be either in collisional ionization equilibrium or photoionization equilibrium, can influence the spectro-polarimetric properties of the radiation at the emitting disk surface. Subsequently we study how these are modified by the propagation in a strong gravitational field.

astro-ph.HE↗

arXiv2025

Recovery of the X-ray polarisation of Swift J1727.8$-$1613 after the soft-to-hard spectral transition

We report on the detection of X-ray polarisation in the black-hole X-ray binary Swift J1727.8$-$1613 during its dim hard spectral state by the Imaging X-ray Polarimetry Explorer (IXPE). This is the first detection of X-ray polarisation at the transition from the soft to the hard state in an X-ray binary. We find an averaged 2$-$8 keV polarisation degree of (3.3 ${\pm}$ 0.4) % and a corresponding polarisation angle of 3° ${\pm}$ 4°, which matches the polarisation detected during the rising stage of the outburst, in September$-$October 2023, within 1$σ$ uncertainty. The observational campaign complements previous studies of this source and enables comparison of the X-ray polarisation properties of a single transient across the X-ray hardness-intensity diagram. The complete recovery of the X-ray polarisation properties, including the energy dependence, came after a dramatic drop in the X-ray polarisation during the soft state. The new IXPE observations in the dim hard state at the reverse transition indicate that the accretion properties, including the geometry of the corona, appear to be strikingly similar to the bright hard state during the outburst rise despite the X-ray luminosities differing by two orders of magnitude.

astro-ph.HE↗

arXiv2025

Shape and ionization of equatorial matter near compact objects from X-ray polarization reflection signatures

Motivated by the success of the IXPE mission, we elucidate what can be inferred about 3D matter structures forming about the equatorial plane of accreting compact objects from 0.1-100 keV linear polarization induced by non-relativistic large-scale reflection. We construct a model of an optically thick elevated axially symmetric reflecting medium with arbitrary ionization profile, representing the known diverse scattering environments: from thick winds and super-Eddington funnel structures formed around black holes and neutron stars, to Compton-thick dusty tori of active galactic nuclei and their broad line regions. We assume a central X-ray power-law source with an isotropic, cosine, and slab-corona emission distribution, including intrinsic polarization. The reprocessing is based on constant-density local reflection tables produced with a Monte Carlo method combined with detailed non-LTE radiative transfer, although we also show examples with classical (semi-)analytical reflection prescriptions. We conclude that varying ionization has a similarly strong impact on observed polarization as the observer's inclination and the skew and opening angle of the reflector's inner walls, altogether producing up to tens of % of reflected polarization both parallelly or perpendicularly to the projected axis, depending on the parameter values combination. After testing 3 different ad-hoc shapes of the reflector: a cone, an elliptical torus, and a bowl, we conclude that while in some configurations, their altered curvature produces more than 30% absolute difference in observed total polarization, in others, the adopted shape has a marginal impact. Lastly, we discuss the change of the observed polarization due to relaxing the optically thick assumption on equatorial winds and accreted matter, providing a continuous range of energy-dependent examples between the optically thick and thin scenarios.

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arXiv2024

Simple numerical X-ray polarization models of reflecting axially symmetric structures around accreting compact objects

We present a series of numerical models suitable for X-ray polarimetry of accreting systems. Firstly, we provide a spectropolarimetric routine that integrates reflection from inner optically thick walls of a geometrical torus of arbitrary size viewed under general inclination. In the studied example, the equatorial torus is illuminated by a central isotropic source of X-ray power-law emission, representing a hot corona. Nearly neutral reprocessing inside the walls is precomputed by Monte Carlo code STOKES that incorporates both line and continuum processes, including multiple scatterings and absorption. We created a new XSPEC model, called xsstokes, which in this version enables efficient X-ray polarimetric fitting of the torus parameters, observer's inclination and primary emission properties, interpolating for arbitrary state of primary polarization. Comparison of the results to a Monte Carlo simulation allowing partial transparency shows that the no-transparency condition may induce different polarization by tens of %. Allowing partial transparency leads to lower/higher polarization fraction, if the resulting polarization orientation is perpendicular/parallel to the rotation axis. We provide another version of xsstokes that is suitable for approximating nearly neutral reflection from a distant optically thick disc of small geometrical thickness. It assumes local illumination averaged for a selected range of incident angles, representing a toy model of a diffuse corona of various physical extent. Assessing both xsstokes variants, we conclude that the resulting polarization can be tens of % and perpendicularly/parallelly oriented towards the rotation axis, if the reflecting medium is rather vertically/equatorially distributed with respect to a compact central source.

astro-ph.HE↗

arXiv2024

IXPE observation confirms a high spin in the accreting black hole 4U 1957+115

We present the results of the first X-ray polarimetric observation of the low-mass X-ray binary 4U 1957+115, performed with the Imaging X-ray Polarimetry Explorer in May 2023. The binary system has been in a high-soft spectral state since its discovery and is thought to host a black hole. The $\sim$571 ks observation reveals a linear polarisation degree of $1.9\% \pm 0.6\%$ and a polarisation angle of $-41^\circ.8 \pm 7^\circ.9$ in the 2-8 keV energy range. Spectral modelling is consistent with the dominant contribution coming from the standard accretion disc, while polarimetric data suggest a significant role of returning radiation: photons that are bent by strong gravity effects and forced to return to the disc surface, where they can be reflected before eventually reaching the observer. In this setting, we find that models with a black hole spin lower than 0.96 and an inclination lower than $50^\circ$ are disfavoured.

astro-ph.HE↗

arXiv2023

X-ray polarization from parsec-scale components of active galactic nuclei: observational prospects

We present a broad analysis of X-ray polarimetric observational prospects for radio-quiet active galactic nuclei (AGN), focusing on the role of parsec-scale components. We provide a revision of self-consistent type-1 and type-2 generic AGN radiative transfer models that were obtained with a Monte Carlo code STOKES, evaluating the effects of absorption and scattering. Our model consists of a central disc-corona emission obtained with the KYNSTOKES code in the lamp-post geometry, an equatorial wedge-shaped dusty torus and two symmetric conical polar outflows. We argue that the information on the mutual orientation, shape, relative size and composition of such components, usually obtained from spectroscopy or polarimetry in other wavelengths, is essential for the X-ray polarization analysis of the obscured type-2 AGNs. We provide general detectability prospects for AGNs with 2-8 keV polarimeters on board of the currently flying IXPE satellite and the forthcoming eXTP mission. Finally, we assess the role of contemporary X-ray polarimetry in our understandings of the unified AGN model after the first year and a half of IXPE operation.

astro-ph.HE↗