Impact of the returning radiation on the analysis of the reflection spectra of black holes

TL;DR

This study investigates how returning radiation affects the analysis of black hole reflection spectra, revealing potential biases in parameter estimation and implications for future X-ray observations.

Contribution

It quantifies the impact of returning radiation on spectral modeling of black holes, highlighting biases and residuals overlooked in previous models.

Findings

Inclination angle and spin tend to be overestimated at low viewing angles.

Returning radiation flattens the radial emissivity in extreme cases.

Neglecting returning radiation can lead to misinterpretation of spectral parameters.

Abstract

A fraction of the electromagnetic radiation emitted from the surface of a geometrically thin and optically thick accretion disk of a black hole returns to the disk because of the strong light bending in the vicinity of the compact object (returning radiation). While such radiation clearly affects the observed spectrum of the source, it is often neglected in theoretical models. In the present paper, we study the impact of the returning radiation on relativistic reflection spectra. Assuming neutral material in the disk, we estimate the systematic uncertainties on the measurement of the properties of the system when we fit the data with a theoretical model that neglects the returning radiation. Our NICER simulations show that the inclination angle of the disk and the black hole spin parameter tend to be overestimated for low viewing angles, while no clear bias is observed for high viewing angles. The iron abundance of the disk is never overestimated. In the most extreme cases (in particular, for maximally rotating black holes) the returning radiation flattens the radial emissivity beyond a few gravitational radii. In such cases, it also produces residuals that cannot be compensated by adjusting the parameters of models neglecting the returning radiation. This may be an important issue for interpretation of data from future X-ray missions (e.g. Athena). When we simulate some observations with NuSTAR and we fit data above 10 keV, we find that some conclusions valid for the NICER simulations are not true any longer (e.g., we can get a high iron abundance).