Impact of the returning radiation on X-ray reflection spectroscopy measurements: the case of Galactic black holes

TL;DR

This study investigates the impact of returning radiation on X-ray reflection spectroscopy measurements of Galactic black holes, finding negligible differences in parameter estimates when including this effect in the analysis.

Contribution

It is the first comprehensive analysis of returning radiation's influence on black hole parameter estimation using NuSTAR spectra with the relxill model.

Findings

No significant difference in parameter estimates with or without returning radiation

Returning radiation effects are minimal even for fast-rotating black holes

The study highlights limitations and approximations in current reflection models

Abstract

The effect of the returning radiation has long been ignored in the analysis of the reflection spectra of Galactic black holes and active galactic nuclei and only recently has been implemented in the relxill package. Here we present a study on the impact of the returning radiation on the estimate of the parameters of Galactic black holes. We consider high-quality NuSTAR spectra of three Galactic black holes (GX 339-4, Swift J1658.2-4242, and MAXI J1535-571) and we fit the data with the lamppost model in the latest version of relxill, first without including the returning radiation and then including the returning radiation. We do not find any significant difference in the estimate of the parameters of these systems between the two cases, even if all three sources are fast-rotating black holes and for two sources the estimate of the height of the corona is very low, two ingredients that should maximize the effect of the returning radiation. We discuss our results and the approximations in relxill.