The Effect of Returning Radiation on Relativistic Reflection

TL;DR

This paper investigates how returning radiation affects the X-ray reflection spectrum in thin accretion disks around black holes, especially for high spins and compact sources, impacting the interpretation of observational data.

Contribution

It introduces a model incorporating returning radiation into relativistic reflection spectra, highlighting its significance for high-spin black holes and compact coronae.

Findings

Returning radiation significantly alters the reflection spectrum for high spins and close sources.

Neglecting returning radiation can lead to overestimating the corona height.

An updated relxill model including returning radiation is provided.

Abstract

We study the effect of returning radiation on the shape of the X-ray reflection spectrum in the case of thin accretion disks. We show that the returning radiation mainly influences the observed reflection spectrum for a large black hole spin (a > 0.9) and a compact primary source of radiation close to the black hole at height h < 5 $r_\mathrm{g}$, and that it dominates the reflected flux for extreme values of spin and compactness. The main effect of the returning radiation is to increase the irradiating flux on to the outer parts of the accretion disk, leading to stronger reflection and a flatter overall emissivity profile. By analyzing simulated observations we show that neglecting returning radiation in existing studies of reflection dominated sources has likely resulted in overestimating the height of the corona above the black hole. An updated version of the publicly available relxill suite of relativistic reflection models which includes returning radiation is also presented.