Testing the Kerr black hole hypothesis using X-ray reflection spectroscopy

TL;DR

This paper introduces a new X-ray reflection model to test if astrophysical black holes conform to the Kerr metric, enabling more precise tests of general relativity with current and future X-ray observations.

Contribution

The authors develop the first X-ray reflection model based on the transfer function formalism for testing the Kerr black hole hypothesis, adaptable to various stationary, axisymmetric spacetimes.

Findings

Model accurately reproduces Kerr and Johannsen metrics.

Simulations demonstrate potential to constrain deviations from Kerr with current and future X-ray data.

Code can be extended to other stationary, axisymmetric spacetimes.

Abstract

We present the first X-ray reflection model for testing the assumption that the metric of astrophysical black holes is described by the Kerr solution. We employ the formalism of the transfer function proposed by Cunningham. The calculations of the reflection spectrum of a thin accretion disk are split into two parts: the calculation of the transfer function and the calculation of the local spectrum at any emission point in the disk. The transfer function only depends on the background metric and takes into account all the relativistic effects (gravitational redshift, Doppler boosting, and light bending). Our code computes the transfer function for a spacetime described by the Johannsen metric and can easily be extended to any stationary, axisymmetric, and asymptotically flat spacetime. Transfer functions and single line shapes in the Kerr metric are compared with those calculated from existing codes to check that we reach the necessary accuracy. We also simulate some observations with NuSTAR and LAD/eXTP and fit the data with our new model to show the potential capabilities of current and future observations to constrain possible deviations from the Kerr metric.