X-ray polarization of reflected thermal emission

¹ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Praha 4, Czech Republic
² Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
³ INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Via Marzolo 8, 35131 Padova, Italy
⁵ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, Via della Vasca Navale 84, 00146 Roma, Italy
⁶ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Bartycka 18, 00-716 Warsaw, Poland

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Received: 31 July 2025
Accepted: 24 May 2026

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 computed 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 used a combination of a Monte Carlo simulation that takes into account scattering, absorption, and spectral lines, and a radiative transfer pre-computation of the ionization structure of the slab in photo-ionization equilibrium without assuming a local thermodynamic equilibrium. We discuss the impact of the reflector’s ionization and the incident spectral shape on the obtained energy dependence of polarization. For isotropic incident intensity and for parallel rather 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 a re-processing effect is particularly apparent in 2–8 keV for steep incident X-ray spectra reflecting from highly ionized and 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 rest-frame energy-dependent features will imprint to the observed X-ray polarization; and they could contribute to the increase of the total polarization degree with energy in the 2–8 keV range observed in some accreting systems by the IXPE mission.