# Testing the Kerr hypothesis using X-ray reflection spectroscopy with NuSTAR data of Cygnus X-1 in the soft state

## TL;DR

This study uses NuSTAR X-ray reflection spectroscopy data of Cygnus X-1 to evaluate the Kerr hypothesis, revealing the method's limitations and the importance of source selection.

## Contribution

It demonstrates the dependency of Kerr tests on assumptions and identifies Cygnus X-1 as unsuitable for such tests with current methods.

## Key findings

- Measurement depends on intensity profile assumptions
- Cygnus X-1 not suitable for Kerr tests with X-ray reflection spectroscopy
- Discusses properties needed for ideal candidate sources

## Abstract

We continue exploring the constraining capabilities of X-ray reflection spectroscopy to test the Kerr-nature of astrophysical black holes and we present the results of our analysis of two NuSTAR observations of Cygnus X-1 in the soft state. We find that the final measurement can strongly depend on the assumption of the intensity profile. We conclude that Cygnus X-1 is not suitable for tests of general relativity using X-ray reflection spectroscopy and we discuss the desired properties of a source to be a good candidate for our studies.