Testing the space-time geometry around black hole candidates with the analysis of the broadKαiron line

Abstract

Astrophysical black hole candidates are thought to be the Kerr black holes predicted by General Relativity, but there is not yet clear evidence that the geometry of the space-time around these objects is really described by the Kerr metric. In order to confirm the Kerr black hole hypothesis, we have to observe strong gravity features and check that they are in agreement with the ones predicted by General Relativity. In this paper, I study the broad Kα iron line, which is often seen in the X-ray spectrum of both stellar-mass and super-massive black hole candidates and whose shape is supposed to be strongly affected by the space-time geometry. As found in previous studies in the literature, there is a strong correlation between the spin parameter and the deformation parameter; that is, the line emitted around a Kerr black hole with a certain spin can be very similar to the one coming from the space-time around a non-Kerr object with a quite different spin. Despite that, the analysis of the broad Kα iron line is potentially more powerful than the continuum-fitting method, as it can put an interesting bound on possible deviations from the Kerr geometry independently of the value of the spin parameter and without additional measurements.

Born–Infeld inspired modifications of gravity

“…This was exactly pursued in [229,191]. Let us consider the following Bianchi type I background for the dynamical metric g µν 39) where g i denote the scale factors in the x, y and z direction, respectively. By defining the quantities…”

relxill_nk: A Relativistic Reflection Model for Testing Einstein’s Gravity

“…The method was originally proposed and developed for measuring black hole spins under the assumption that the metric around astrophysical black holes is described by the Kerr solution [36,37]. More recently, the technique has been proposed for testing Einstein's gravity in the strong field regime [38][39][40][41][42][43][44][45][46][47][48][49][50][51]. Note that spin measurements (if we assume the Kerr metric) or tests of the Kerr metric require fitting the whole reflection spectrum, not just the iron line, even if often (but not always) the iron line is the feature that primarily determines the measurement of the parameters of the background metric in the strong gravity region.…”

Testing the Kerr Nature of Black Hole Candidates Using Iron Line Spectra in the CPR Framework

“…Such radiation is inevitably affected by relativistic effects and therefore carries information about the metric. Accurate measurements of the thermal spectrum of a thin disk (Zhang et al 1997;McClintock et al 2014) and of the iron Kα line (Fabian et al 1989;Reynolds 2014) are today the principal techniques for providing information about the nature of BH candidates and they can be used to test the Kerr metric (Torres 2002;Lu & Torres 2003;Bambi & Barausse 2011;Bambi 2012aBambi , 2012dBambi , 2012e, 2013bBambi , 2013cBambi , 2014bJohannsen & Psaltis 2013). However, the usual problem with such a test is often strong degeneracy among system parameters, in particular between the parameter measuring deviation from Kerr geometry, and the spin and the inclination angle.…”