X-ray Tests of General Relativity with Black Holes

Abstract

General relativity is one of the pillars of modern physics. For decades, the theory has been mainly tested in the weak-field regime with experiments in the solar system and radio observations of binary pulsars. Until 2015, the strong-field regime was almost completely unexplored. Thanks to new observational facilities, the situation has dramatically changed in the last few years. Today, we have gravitational wave data of the coalesce of stellar-mass compact objects from the LIGO-Virgo-KAGRA collaboration, images at mm wavelengths of the supermassive black holes in M87* and Sgr A* from the Event Horizon Telescope collaboration, and X-ray data of accreting compact objects from a number of X-ray missions. Gravitational wave tests and black hole imaging tests are certainly more popular and are discussed in other articles of this Special Issue. The aim of the present manuscript is to provide a pedagogical review on X-ray tests of general relativity with black holes and to compare these kinds of tests with those possible with gravitational wave data and black hole imaging.