Abstract
We study the spectrum and polarisation properties of X-ray emission from accretion discs in active galactic nuclei (AGN). The reflected radiation from an accretion disc illuminated by a corona above is studied using the Monte Carlo radiative transfer code STOKESWe study the spectrum and polarisation properties of X-ray emission from accretion discs in active galactic nuclei (AGN).
The reflected radiation from an accretion disc illuminated by a corona above is studied using the Monte Carlo radiative transfer code STOKES developed by Goosmann & Gaskell (2007), Marin et al. (2012, 2015) and Marin (2018). In our application the code models polarization induced mainly by scattering off free electrons.
The structure of the disc, modelled as a constant density slab, has been computed with the code for optically thick and hot photo-ionized media TITAN (Dumont et al., 2003). The resulting simulated spectra locally emitted by the accretion disc in 0.1-100 keV energy band are compared to the results obtained by by various, published radiative transfer codes (e.g.
Garcia et al. 2013). In addition to these, the STOKES code provides also the polarisation degree and polarisation angle of the outcoming radiation that depend on geometry of scattering and other key parameters such as ionisation of the disc.
Further, we use the relativistic code KYNSTOKES based on our well-testd relativistic KY package (see Dovciak et al. 2004, 2011) to account for all relativistic effects on radiation near a black hole and to compute the total disc emission as would be detected by a distant observer. In this way it is then possible to predict X-ray spectral and polarisation properties of an accretion disc in AGN and their dependence on various parameters of the system (spin of the black hole, ionization of the accretion disc, observer inclination).
This is fundamental for the interpretation of AGN spectra and polarization obtained by the upcoming X-ray polarimetric missions, such as NASA IXPE mission (Weisskopf et al. 2016) or Chinese-lead eXTP mission (Zhang et al. 2016).