About the accuracy of the relxill/relxill_nk models in view of the next generation of X-ray missions

TL;DR

This study evaluates the accuracy of relxill and relxill_nk reflection models for black hole X-ray spectra, highlighting the importance of interpolation and emission angle treatment for precise measurements with future X-ray missions.

Contribution

It provides a detailed assessment of relxill models' accuracy and proposes improvements for their application in next-generation X-ray observations.

Findings

Residuals in fits when inner disk emission is high

Increasing interpolation points reduces residuals

Proper treatment of emission angle improves spectral modeling

Abstract

X-ray reflection spectroscopy is a powerful tool to study the strong gravity region of black holes. The next generation of astrophysical X-ray missions promises to provide unprecedented high-quality data, which could permit us to get very precise measurements of the properties of the accretion flow and of the spacetime geometry in the strong gravity region around these objects. In this work, we test the accuracy of the relativistic calculations of the reflection model relxill and of its extension to non-Kerr spacetimes relxill_nk in view of the next generation of X-ray missions. We simulate simultaneous observations with Athena/X-IFU and LAD of bright Galactic black holes with a precise and accurate ray-tracing code and we fit the simulated data with the latest versions of relline and relline_nk. While we always recover the correct input parameters, we find residuals in the fits when the emission from the inner part of the accretion disk is higher. Such residuals disappear if we increase the number of interpolation points on the disk in the integral of the transfer function. We also simulate full reflection spectra and find that the emission angle from the accretion disk should be treated properly in this case.