Polarization properties of iron emission lines as a tool to identify the finite thickness of disks with moderately super-Eddington accretion rate

Abstract

The Fe-K[Formula: see text] fluorescence lines are commonly observed in AGNs and X-ray binaries. The lines are believed to be originated from the reflection of the hard X-ray continuum near the inner-most region of the accretion disks of black holes. The geometry of the accretion disk is usually assumed to be infinitely thin, but this assumption is not appropriate when the accretion rate is moderately super-Eddington. With the increase of the accretion rate, the disk becomes thick, which will significantly affect the properties of the fluorescence lines. For instance, the polarized radiation is strongly depended on the geometry of the accretion disk. In this work, based on the lamp-post model, we study the polarization properties of the relativistic Fe-K[Formula: see text] lines from thick disks in the framework of fully general relativity. We find that with the increase of the disk thickness, the polarization degree (PD) at the blue edge of the iron line increase significantly, and there appears a peak at the profile of the PD of the iron emission line, which at most is one order higher that of the line from the thin disk. Thus, the polarization properties of relativistic broad Fe-K[Formula: see text] lines can be used to as a tool to diagnose the disk thickness.