Testing the Kerr metric with the iron line and the KRZ parametrization

Abstract

The spacetime geometry around astrophysical black holes is supposed to be\nwell approximated by the Kerr metric, but deviations from the Kerr solution are\npredicted in a number of scenarios involving new physics. Broad iron K$\\alpha$\nlines are commonly observed in the X-ray spectrum of black holes and originate\nby X-ray fluorescence of the inner accretion disk. The profile of the iron line\nis sensitively affected by the spacetime geometry in the strong gravity region\nand can be used to test the Kerr black hole hypothesis. In this paper, we\nextend previous work in the literature. In particular: i) as test-metric, we\nemploy the parametrization recently proposed by Konoplya, Rezzolla, and\nZhidenko, which has a number of subtle advantages with respect to the existing\napproaches; ii) we perform simulations with specific X-ray missions, and we\nconsider NuSTAR as a prototype of current observational facilities and eXTP as\nan example of the next generation of X-ray observatories. We find a significant\ndifference between the constraining power of NuSTAR and eXTP. With NuSTAR, it\nis difficult or impossible to constrain deviations from the Kerr metric. With\neXTP, in most cases we can obtain quite stringent constraints (modulo we have\nthe correct astrophysical model).\n