Constraining the Kerr parameters via X-ray reflection spectroscopy

TL;DR

This paper explores how future X-ray reflection spectroscopy observations, specifically with the LAD instrument on eXTP, can constrain parameters describing deviations from the Kerr black hole model, enhancing tests of gravity in strong fields.

Contribution

It demonstrates that 200 ks observations with LAD/eXTP can effectively constrain most Kerr parameters, except for the weakly impacting parameter b_{11}.

Findings

Most Kerr parameters can be constrained with sufficient observation time.

The parameter b_{11} remains challenging to measure due to its weak influence.

Simulations show the potential of future X-ray missions in testing black hole metrics.

Abstract

In a recent paper [Ghasemi-Nodehi & Bambi, EPJC 76 (2016) 290], we have proposed a new parametrization for testing the Kerr nature of astrophysical black hole candidates. In the present work, we study the possibility of constraining the "Kerr parameters" of our proposal using X-ray reflection spectroscopy, the so-called iron line method. We simulate observations with the LAD instrument on board of the future eXTP mission assuming an exposure time of 200~ks. We fit the simulated data to see if the Kerr parameters can be constrained. If we have the correct astrophysical model, 200~ks observations with LAD/eXTP can constrain all the Kerr parameters with the exception of $b_{11}$, whose impact on the iron line profile is extremely weak and its measurement looks very challenging.