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

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.

5 pages, 3 figures. v2: updated some references as in the published version

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• Testing Einstein-dilaton-Gauss-Bonnet gravity from the reflection spectrum of accreting black holes
• Iron K$\alpha$ line of boson stars
• Merger estimates for rotating Kerr black holes in modified gravity
• Quasi-periodic oscillations as a tool for testing the Kerr metric: A comparison with gravitational waves and iron line
• Superradiance in Kerr-like black holes
• Quasinormal modes, quasibound states, scalar clouds, and superradiant instabilities of a Kerr-like black hole
• Rotating black holes in Eddington-inspired Born-Infeld gravity: an exact solution
• Iron K$\alpha$ line of Proca stars
• Experimental Relativity with Accretion Disk Observations
• Constraining the Kerr parameters via X-ray reflection spectroscopy
• Iron K$\alpha$ line of Kerr black holes with Proca hair
• Shadow Cast by the Kerr MOG Black Hole under the Influence of Plasma and Constraints from EHT Observations
• Testing beyond-Kerr spacetimes with GWTC-3
• Testing General Relativity with Black Hole X-ray Data
• Exploring black hole shadows in axisymmetric spacetimes with coordinate-independent methods and neural networks