Iron line profiles and self-shadowing from relativistic thick accretion discs

TL;DR

This paper introduces a fully relativistic model for thick accretion discs around rotating black holes, analyzing how disc thickness affects iron line profiles and observable images, with implications for black hole parameter estimation.

Contribution

It presents the first comprehensive relativistic computation of iron line profiles from thick, sub-Keplerian accretion discs, including all relativistic effects and self-shadowing phenomena.

Findings

Thicker discs reduce the separation and height of blue and red peaks in line profiles.

Disc self-shadowing becomes significant at high inclination angles, obscuring the black hole shadow.

Line profiles and images strongly depend on disc thickness and inclination angle.

Abstract

We present Fe Kalpha line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and light bending. We find that the separation and the relative height between the blue and red peaks of the line profile diminish as the thickness of the disc increases. This code is also well-suited to produce accretion disc images. We calculate the redshift and flux images of the accretion disc and find that the observed image of the disc strongly depends on the inclination angle. The self-shadowing effect appears remarkable for a high inclination angle, and leads to the black hole shadow being completely hidden by the disc itself.