Spectral and polarization properties of black hole accretion disc emission: including absorption effects

0549680 - ASÚ 2022 RIV GB eng J - Článek v odborném periodiku
Taverna, R. - Marra, L. - Bianchi, S. - Dovčiak, Michal - Goosmann, R. W. - Marin, F. - Matt, G. - Zhang, Wenda
Spectral and polarization properties of black hole accretion disc emission: including absorption effects.
Monthly Notices of the Royal Astronomical Society. Roč. 501, č. 3 (2021), s. 3393-3405. ISSN 0035-8711. E-ISSN 1365-2966
Grant CEP: GA ČR(CZ) GA18-00533S
Institucionální podpora: RVO:67985815
Klíčová slova: accretion discs * polarization * stars abundances
Obor OECD: Astronomy (including astrophysics,space science)
Impakt faktor: 5.235, rok: 2021
Způsob publikování: Omezený přístup
https://doi.org/10.1093/mnras/staa3859

The study of radiation emitted from black hole (BH) accretion discs represents a crucial way to understand the main physical properties of these sources, and in particular the BH spin. Beside spectral analysis, polarimetry is becoming more and more important, motivated by the development of new techniques which will soon allow to perform measurements also in the X- and gamma-rays. Photons emitted from BH accretion discs in the soft state are indeed expected to be polarized, with an energy dependence which can provide an estimate of the BH spin. Calculations performed so far, however, considered scattering as the only process to determine the polarization state of the emitted radiation, implicitly assuming that the temperatures involved are such that material in the disc is entirely ionized. In this work, we generalize the problem by calculating the ionization structure of a surface layer of the disc with the public code cloudy, and then by determining the polarization properties of the emerging radiation using the Monte Carlo code stokes. This allows us to account for absorption effects alongside scattering ones. We show that including absorption can deeply modify the polarization properties of the emerging radiation with respect to what is obtained in the pure-scattering limit. As a general rule, we find that the polarization degree is larger when absorption is more important, which occurs, for example, for low accretion rates and/or spins when the ionization of the matter in the innermost accretion disc regions is far from complete.
Trvalý link: http://hdl.handle.net/11104/0325624