X-ray polarization of reflected thermal emission
Abstract
X-ray thermal emission is inherent in neutron-star and black-hole X-ray binary systems. Within these systems, it may reflect from optically thick matter, which will create characteristic observable X-ray spectro-polarimetric features. We compute rest-frame reflection spectra and the corresponding energy-dependent linear polarization degree and angle for (un)polarized single-temperature black-body 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. 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 many accreting systems by the IXPE mission.
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BibTeX
@article{podgorny2026xray,
title = {{X-ray polarization of reflected thermal emission}},
author = {J Podgorný and M Dovciak and R Goosmann and F Marin and L Marra and G Matt and A Rózanska and R Taverna and M Gupta},
journal = {Astronomy \& Astrophysics - A\&A},
year = {2026},
month = sep,
doi = {10.1051/0004-6361/202556686},
eprint = {2507.23687},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05201451v1},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - X-ray polarization of reflected thermal emission
AU - J Podgorný
AU - M Dovciak
AU - R Goosmann
AU - F Marin
AU - L Marra
AU - G Matt
AU - A Rózanska
AU - R Taverna
AU - M Gupta
PY - 2026
DA - 2026/09/23
JO - Astronomy & Astrophysics - A&A
DO - 10.1051/0004-6361/202556686
UR - https://hal.science/hal-05201451v1
AB - X-ray thermal emission is inherent in neutron-star and black-hole X-ray binary systems. Within these systems, it may reflect from optically thick matter, which will create characteristic observable X-ray spectro-polarimetric features. We compute rest-frame reflection spectra and the corresponding energy-dependent linear polarization degree and angle for (un)polarized single-temperature black-body 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. 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 many accreting systems by the IXPE mission.
SN - 0004-6361
SN - 1432-0746
ER -