RELXILL_NK: A Black Hole Relativistic Reflection Model for Testing General Relativity

TL;DR

RELXILL_NK is a novel relativistic reflection model for black hole accretion disks that tests the Kerr hypothesis by incorporating non-Kerr solutions and relativistic ray-tracing.

Contribution

It introduces the first available model that includes non-Kerr black hole solutions for testing general relativity using X-ray reflection spectra.

Findings

RELXILL_NK can simulate relativistic reflection spectra for non-Kerr black holes.

The model allows testing deviations from the Kerr metric.

Comparison with Kerr-based models shows potential for GR tests.

Abstract

In this paper, we briefly present RELXILL_NK, the first and currently only readily available model of the relativistic reflection spectrum of black hole accretion disks that includes non-Kerr solutions for the black hole spacetime, thus allowing for tests of the Kerr hypothesis and GR. RELXILL_NK makes use of a general relativistic ray-tracing code to calculate the relativistic effects of any well-behaved, stationary, axisymmetric, and asymptotically flat black hole spacetime, while the disk physics is handled through the non-relativistic X-ray reflection code XILLVER. A number of different flavors are available within RELXILL_NK; we summarize and compare these flavors using the Johannsen metric for the black hole spacetime.