# Public Release of RELXILL_NK: A Relativistic Reflection Model for Testing Einstein's Gravity

## TL;DR

The paper introduces relxill_nk, a publicly available X-ray reflection model for testing the Kerr hypothesis and general relativity, including new non-Kerr models and relativistic ray-tracing techniques.

## Contribution

It presents the relxill_nk package with new non-Kerr models and demonstrates the accuracy of relativistic ray-tracing in modeling black hole spectra.

## Key findings

- Relxill_nk accurately models X-ray reflection spectra for non-Kerr spacetimes.
- The numerical error from ray-tracing is negligible compared to observational errors.
- The model successfully computes spectra for the Johannsen metric.

## Abstract

We present the public release version of relxill_nk, an X-ray reflection model for testing the Kerr hypothesis and general relativity. This model extends the relxill model that assumes the black hole spacetime is described by the Kerr metric. We also present relxilllp_nk, the first non-Kerr X-ray reflection model with a lamppost corona configuration, as well as all other models available in the full relxill_nk package. In all models the relevant relativistic effects are calculated through a general relativistic ray-tracing code that can be applied to any well-behaved, stationary, axisymmetric, and asymptotically flat black hole spacetime. We show that the numerical error introduced by using a ray-tracing code is not significant as compared with the observational error present in current X-ray reflection spectrum observations. In addition, we present the reflection spectrum for the Johannsen metric as calculated by relxill_nk.