Black hole accretion rings revealed by future X-ray spectroscopy

Abstract

Spectral features can arise by reflection of coronal X-rays on a black hole accretion disc. The resulting profile bears various imprints of strong gravitational field acting on the light emitting gas. The observed shape of the reflection line is formed by integrating contributions over a range of radii across the accretion disc plane, where the individual photons experience different level of energy shifts, boosting, and amplification by relativistic effects. These have to be convolved with the intrinsic emissivity of the line, which is a function of radius and the emission angle in the local frame. We study if the currently discussed instruments on-board X-ray satellites will be able to reveal the departure of the line radial emissivity from a simple smooth power-law function, which is often assumed in data fitting and interpretation. Such a departure can be a result of excess emission occurring at a certain distance. This could be used to study variations with radius of the line production or to constrain the position of the inner edge of the accretion disc. By simulating artificial data from a bright active galactic nucleus of a type-1 Seyfert galaxy (inclination ~ 30 deg, X-ray flux 1-2 mCrab in keV energy band) we show that the required sensitivity and energy resolution could be reached with Large Area Detector of the proposed LOFT mission. Galactic black holes will provide another category of potentially suitable targets if the relativistic spectral features are indeed produced by reflection from their accretion discs.