Measuring the Kerr spin parameter of a non-Kerr compact object with the continuum-fitting and the iron line methods

TL;DR

This paper investigates whether combined continuum-fitting and iron line methods can confirm the Kerr nature of black hole candidates, considering non-Kerr metrics and their impact on spin measurements.

Contribution

It analyzes the effectiveness of using both methods together to distinguish Kerr black holes from non-Kerr objects under different metric assumptions.

Findings

Methods agree for Bardeen metric, supporting Kerr hypothesis.

Methods show discrepancies for Johannsen-Psaltis metric, challenging Kerr confirmation.

Measurement results depend on the specific non-Kerr background.

Abstract

Under the assumption that astrophysical black hole candidates are the Kerr black holes of general relativity, the continuum-fitting method and the analysis of the K$\alpha$ iron line are today the only available techniques capable of providing a relatively reliable estimate of the spin parameter of these objects. If we relax the Kerr black hole hypothesis and we try to test the nature of black hole candidates, we find that there is a strong correlation between the measurement of the spin and possible deviations from the Kerr solution. The properties of the radiation emitted in a Kerr spacetime with spin parameter $a_*$ are indeed very similar, and practically indistinguishable, from the ones of the radiation emitted around a non-Kerr object with different spin. In this paper, I address the question whether measuring the Kerr spin with both the continuum-fitting method and the K$\alpha$ iron line analysis of the same object can be used to claim the Kerr nature of the black hole candidate in the case of consistent results. In this work, I consider two non-Kerr metrics and it seems that the answer does depend on the specific background. The two techniques may either provide a very similar result (the case of the Bardeen metric) or show a discrepancy (Johannsen-Psaltis background).