# About the Kerr nature of the stellar-mass black hole in GRS 1915+105

## TL;DR

This study tests the Kerr black hole hypothesis in GRS 1915+105 using X-ray reflection spectroscopy and the RELXILL_NK model, revealing variable deformation parameters that depend on the astrophysical model, highlighting the complexity of such tests.

## Contribution

First application of the RELXILL_NK model to GRS 1915+105 with analysis of deformation parameters, emphasizing model-dependent results and challenges in testing the Kerr metric.

## Key findings

- Deformation parameters vary with astrophysical models.
- No similar difficulties found in previous source tests.
- Results inform source selection and model improvements for Kerr tests.

## Abstract

We employ the accretion disk reflection model RELXILL_NK to test the spacetime geometry around the stellar-mass black hole in GRS 1915+105. We adopt the Johannsen metric with the deformation parameters $\alpha_{13}$ and $\alpha_{22}$, for which the Kerr solution is recovered when $\alpha_{13} = \alpha_{22} = 0$. We analyze a NuSTAR observation of 2012, obtaining vanishing and non-vanishing values of the deformation parameters depending on the astrophysical model adopted. Similar difficulties were not found in our previous tests with other sources. The results of this work can shed light on the choice of sources suitable for testing the Kerr metric using X-ray reflection spectroscopy and on the parts of our reflection models that more urgently require improvement.