Testing the Kerr Paradigm with X-ray Observations

TL;DR

This paper reviews current observational techniques, continuum-fitting and iron line analysis, used to test if astrophysical black holes conform to the Kerr solution of general relativity, highlighting their strengths and limitations.

Contribution

It provides an overview of the methods used to verify the Kerr paradigm and discusses the challenges faced due to data quality and modeling complexities.

Findings

Continuum-fitting is robust but limited in constraining deviations.

Iron line analysis has potential but is hindered by data quality.

Current techniques are not yet definitive in confirming the Kerr nature.

Abstract

Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but the actual nature of these objects has still to be confirmed. The continuum-fitting and the iron line methods are currently the only available techniques to probe the spacetime geometry around these bodies and test the Kerr black hole paradigm. The continuum-fitting method is a robust approach, but the shape of the disk's thermal spectrum is in general too simple to measure the spin and to constrain possible deviations from the Kerr solution at the same time. The iron line analysis is potentially a powerful technique, but at the moment we do not have high quality data and a robust astrophysical model.