Polarization Of X-ray Radiation From High-accretion-rate Disk-corona Systems Around Black Holes

From an AGN or an X-ray binary,the X-ray emission is usually strong.The X-ray emission is believed to be originated from a disk-corona system.The corona emits the primary power-law emission with high energy cut-off,and the accretion disk reflects the emission.The reflection produces many spectral features,one of the most typical feature is the Fe-K? fluorescence line.The geometry of the accretion disk is usually assumed to be infinitely thin in the analysis of the X-ray emission,but this assumption is not appropriate when the accretion rate is very high,especially when it is in the regime of the super-Eddington accretion.With the increase of the accretion rate,the disk becomes thick,which might significantly affect the properties of the reflected emission.Detection of the polarization properties can be helpful to distinguish the disks with the different thickness,as the polarization properties are dependent on the geometrical structures of the sources.In this work,we choose two typical thick disk models.One is the slim disk,which can present real accretion disk with the high accretion rates.The other is a toy disk with unchanged relative thickness,which can be easily used in theoritical analysis.We take three different refection models into consideration.They are emission model,single-scattering reflection model and multiple-scattering reflection model,respectively.All these three models are based on the elastic scatterings in a semi-infinite atmosphere.We use the lamppost model in our work.In the lamppost model,it is assumed that the corona is an optically thin,high temperature point source,located on the rotation axis of a black hole.We calculate the polarization properties of the Fe-K? fluorescence lines in the emission model.We find that the emission line from a thick disk has a polarization degree peak accompanied with the flux density peak.In the theoritical analysis we find that this feature is only strongly dependent on the geometrical structure of the accretion disk,so it can be used to diagnose the disk thickness.In addition,the flux density peak may disappeare when the disk thickness increases,but the polarization peak is still there.It is also found that the polarizaiton propeties of the red wing of the Fe-K? fluorescence line change complicately with the disk thickness.We calculate the polarization properties of the continuum emission at the 2-6 keV band in the single-scattering reflection model and the multiple-scattering reflection model We find that for a low inclination observer the polarization degree at this band will first decrease then increase with the increase of the disk thickness,but for a high inclination observer this tendency will be broken by the relativistic and shadowing effects.However,we find that when the disk thickness is high enough,the corona is invisible to the observer.Such result means at most 90%drop of the flux and dozens of times of the increase of the polarization degree,so it could be used to distinguish the different disk thickness.We also analyze how the unstable circular orbit of photon affects the final results.