Testing the Kerr metric with the iron line and the KRZ parametrization

TL;DR

This paper investigates how X-ray iron line observations can test the Kerr black hole hypothesis using the KRZ parametrization, comparing current and future X-ray missions' capabilities to constrain deviations from Kerr.

Contribution

It introduces the use of the KRZ parametrization for testing Kerr black holes via iron line profiles and compares the effectiveness of NuSTAR and eXTP in constraining deviations.

Findings

NuSTAR has limited ability to constrain deviations from Kerr.

eXTP can provide stringent constraints on deviations.

The choice of observational platform significantly affects testing sensitivity.

Abstract

The spacetime geometry around astrophysical black holes is supposed to be well approximated by the Kerr metric, but deviations from the Kerr solution are predicted in a number of scenarios involving new physics. Broad iron K$\alpha$ lines are commonly observed in the X-ray spectrum of black holes and originate by X-ray fluorescence of the inner accretion disk. The profile of the iron line is sensitively affected by the spacetime geometry in the strong gravity region and can be used to test the Kerr black hole hypothesis. In this paper, we extend previous work in the literature. In particular: i) as test-metric, we employ the parametrization recently proposed by Konoplya, Rezzolla, and Zhidenko, which has a number of subtle advantages with respect to the existing approaches; ii) we perform simulations with specific X-ray missions, and we consider NuSTAR as a prototype of current observational facilities and eXTP as an example of the next generation of X-ray observatories. We find a significant difference between the constraining power of NuSTAR and eXTP. With NuSTAR, it is difficult or impossible to constrain deviations from the Kerr metric. With eXTP, in most cases we can obtain quite stringent constraints (modulo we have the correct astrophysical model).