Testing General Relativity with X-ray Reflection Spectroscopy of a 'Bare' Active Galactic Nucleus

TL;DR

This paper introduces a new X-ray reflection model, RELXILL_NK, to test Einstein's gravity in the strong field regime around black holes, and applies it to Suzaku data of Ark 564 to constrain deviations from general relativity.

Contribution

The paper presents RELXILL_NK, the first XSPEC reflection model designed for testing Einstein's gravity in strong fields around black holes.

Findings

Constraints on deviations from Einstein's gravity around Ark 564

First application of RELXILL_NK to observational data

Demonstrates the potential of X-ray reflection spectroscopy for strong gravity tests

Abstract

Einstein's gravity has been extensively tested in the weak field regime, primarily with experiments in the Solar System and observations of binary pulsars, and current data agree well with theoretical predictions. On the other hand, strong gravity is largely unexplored and there are a number of theories beyond Einstein's gravity that make the same predictions for weak fields and present deviations only when gravity becomes strong. The best laboratory for testing strong gravity is the spacetime around astrophysical black holes. X-ray reflection spectroscopy can be a powerful tool to probe the strong gravity region around astrophysical black holes and test the nature of these objects. In this paper, we will introduce RELXILL_NK, which is the first XSPEC reflection model to test Einstein's gravity in the strong field regime, and we will present the constraints on possible deviations from Einstein's gravity that we have obtained by analyzing Suzaku data of Ark 564.