Testing the Kerr black hole hypothesis: comparison between the gravitational wave and the iron line approaches

TL;DR

This paper compares gravitational wave and X-ray iron line methods for testing the Kerr black hole hypothesis, finding that high-quality X-ray data could provide constraints stronger than current gravitational wave analyses.

Contribution

It demonstrates that the iron line method can be competitive with gravitational wave observations in constraining deviations from the Kerr metric.

Findings

Simulated constraints from eXTP are stronger than current gravitational wave constraints.

High-quality X-ray data can effectively test the Kerr black hole hypothesis.

Iron line method may rival gravitational wave approaches with proper systematics control.

Abstract

The recent announcement of the detection of gravitational waves by the LIGO/Virgo collaboration has opened a new window to test the nature of astrophysical black holes. Konoplya & Zhidenko have shown how the LIGO data of GW 150914 can constrain possible deviations from the Kerr metric. In this letter, we compare their constraints with those that can be obtained from accreting black holes by fitting their reflected X-ray spectrum, the so-called iron line method. We simulate observations with eXTP, a next generation X-ray mission, finding constraints much stronger than those obtained by Konoplya & Zhidenko. Our results can at least show that, contrary to what is quite commonly believed, it is not obvious that gravitational waves are the most powerful approach to test strong gravity. In the presence of high quality data and with the systematics under control, the iron line method may provide competitive constraints.