On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs

TL;DR

This paper explores how gravitational effects near rotating black holes influence the variability and spectral shape of iron fluorescent lines from accretion discs, explaining observed features and complex variability patterns.

Contribution

It extends the light-bending model to include Kerr metric effects and the impact of returning radiation, providing a comprehensive explanation for observed line profiles and variability.

Findings

Bending of photon trajectories to the equatorial plane reduces line variability.

A model with a variable X-ray source explains the red wing and variability in MCG--6-30-15.

Returning radiation significantly influences the line profile, especially the blue peak.

Abstract

We investigate properties of iron fluorescent line arising as a result of illumination of a black hole accretion disc by an X-ray source located above the disc surface. We study in details the light-bending model of variability of the line, extending previous work on the subject. We indicate bending of photon trajectories to the equatorial plane, which is a distinct property of the Kerr metric, as the most feasible effect underlying reduced variability of the line observed in several objects. A model involving an X-ray source with a varying radial distance, located within a few central gravitational radii around a rapidly rotating black hole, close to the disc surface, may explain both the elongated red wing of the line profile and the complex variability pattern observed in MCG--6-30-15 by XMM-Newton. We point out also that illumination by radiation which returns to the disc (following the previous reflection) contributes significantly to formation of the line profile in some cases. As a result of this effect, the line profile always has a pronounced blue peak (which is not observed in the deep minimum state in MCG--6-30-15), unless the reflecting material is absent within the innermost 2--3 gravitational radii.