Tim Johannsen, Dimitrios Psaltis
According to the no-hair theorem, astrophysical black holes are fully characterized by their masses and spins and are described by the Kerr metric. This theorem can be tested observationally by measuring (at least) three different multipole moments of the spacetimes of black holes. In this paper, we calculate the profiles of fluorescent iron lines emitted from the accretion flows around black holes within a framework that allows us to perform the calculation as a function of the mass and spin of a black hole as well as of a free parameter that measures potential deviations from the Kerr metric. We show that such deviations lead to line profiles that are significantly altered and exhibit a modified flux ratio of the two peaks in their characteristic double-peaked shape. We estimate the precision that near-future X-ray missions such as Astro-H and ATHENA are required to achieve in order to resolve deviations from the Kerr metric in iron line profiles and show that constraints on such deviations will be strongest for rapidly spinning black holes. More generally, we show that measuring the line profile with a precision of ~5% at a disk inclination of 30{\deg} constraints the deviation parameter to order unity irrespectively of the spin of the black hole.
View original:
http://arxiv.org/abs/1202.6069
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