Testing the Kerr black hole hypothesis with RELXILL_NK

TL;DR

This paper reviews how X-ray reflection spectroscopy, using the relxill_nk model and observational data, can test whether astrophysical black holes conform to the Kerr solution of general relativity.

Contribution

It presents new constraints on deviations from the Kerr metric using the relxill_nk model and data from multiple X-ray observatories.

Findings

Constraints on deviations from Kerr metric established

X-ray reflection spectroscopy proves effective for testing black hole nature

Relxill_nk model enhances analysis accuracy

Abstract

Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity. However, macroscopic deviations from the Kerr background are predicted by a number of scenarios beyond Einstein's gravity. X-ray reflection spectroscopy can be a powerful tool to probe the strong gravity region of these objects and test the Kerr black hole hypothesis. Here I briefly review the state of the art of this line of research and I present some constraints on possible deviations from the Kerr metric obtained with the new X-ray reflection model relxill_nk and XMM-Newton, NuSTAR, and Swift data of the supermassive black hole in 1H0707-495.