Towards a complete description of spectra and polarization of black hole accretion discs: albedo profiles and returning radiation

Abstract

Accretion disks around stellar-mass black holes (BHs) emit radiation peaking in the soft Xrays when the source is in the thermal state. The emerging photons are polarized and, for symmetry reasons, the polarization integrated over the source is expected to be either parallel or perpendicular to the (projected) disk symmetry axis, because of electron scattering in the disk. However, due to General Relativity effects photon polarization vectors will rotate with respect to their original orientation, by an amount depending on both the BH spin and the observer's inclination. Hence, X-ray polarization measurements may provide important information about strong gravity effects around these sources. Along with the spectral and polarization properties of radiation which reaches directly the observer once emitted from the disk, in this paper we also include the contribution of returning radiation, i.e. photons that are bent by the strong BH gravity to return again on the disk, where they scatter until eventually escaping to infinity. A comparison between our results and those obtained in previous works by different authors show an overall good agreement, despite the use of different code architectures. We finally consider the effects of absorption in the disk material by including more realistic albedo profiles for the disk surface. Our findings in this respect show that considering also the ionization state of the disk may deeply modify the behavior of polarization observables.

A polarized view of the hot and violent universe

“…Independently of the main engine, gamma rays are thought to propagate in a hot-pair fireball which expands from the star surface up to few star radii [58]. Modelling of the burst spectra indicate that under such conditions the emitted radiation may be either highly polarized in the X mode or present a shift at a certain energy between X and O mode polarization [59,60]. IXPE and eXTP might be able to catch the polarization signal only in some, relatively rare, very bright bursts (the so-called intermediate flares), which are at any rate outside the primary science targets.…”

“…The combination of Note the dip in PD at the energy where PA changes due to the contribution of reflected self-irradiation of the disc. From [86] the General Relativistic Lense-Thirring (LT) precession of matter orbiting a Kerr black hole close to its event horizon and the disc turbulence is expected to lead either to the precession of the central disc as a solid body, to warped discs, or to even more interesting unstable configurations. The LT precession of the inner disc is one of the favoured models to explain the Low Frequency Quasi-Periodic Oscillations (LFQPOs) in the range of~0.1-10 Hz that are regularly observed in X-ray binaries [87].…”

“…5 Polarization degree (PD; left panel) and polarization angle (PA; right panel) dependence on energy.Note the dip in PD at the energy where PA changes due to the contribution of reflected self-irradiation of the disc. From[86]…”

Spectral and polarization properties of reflected X-ray emission from black hole accretion discs

“…Contrarily to the mildly ( 10 per cent) polarized primary radiation (Beheshtipour et al 2017;Tamborra et al 2018;Beheshtipour 2018), the total polarization signal is expected to be significantly enhanced due to additional polarization signatures emerging from Compton scattering inside the illuminated disc (Schnittman & Krolik 2009;Dovčiak et al 2011). The total simulated spectra, possibly including also the returning radiation (Schnittman & Krolik 2010;Taverna et al 2020a), will then provide direct answer to the total polarization enhancement by reflection, which is physically well-reasoned to emerge, especially at hard X-rays. If the polarization induced by reflection pro v es to be important, the presented tables will also serve as an estimate tool for observational times needed by the upcoming X-ray polarimetric missions for the faint AGN sources in the context of highly photon-demanding polarimetry (Fabiani & Muleri 2014).…”

New Constraints on the Spin of the Black Hole Cygnus X-1 and the Physical Properties of its Accretion Disk Corona

“…The interested reader is referred to (Sunyaev & Titarchuk 1985;Haardt et al 1994;Nagirner & Poutanen 1994;Poutanen 1994; Poutanen & Svensson 1996;Zdziarski et al 1996;Poutanen et al 1997;Życki et al 1999;Schnittman & Krolik 2010;Dauser et al 2013;Gonzalez et al 2017;Beheshtipour et al 2017;Zhang et al 2019;Taverna et al 2020Taverna et al , 2021) for earlier analytical and numerical studies of the properties of the coronal emission.…”