Accretion disks around black holes in modified strong gravity

Context. Stellar-mass black holes offer what is perhaps the best scenario to test theories of gravity in the strong-field regime. In particular, f(R) theories, which have been widely discus in a cosmological context, can be constrained through realistic astrophysical models of phenomena around black holes. Aims. We aim at building radiative models of thin accretion disks for both Schwarzschild and Kerr black holes in f (R) gravity. Methods. We study particle motion in f(R)-Schwarzschild and Kerr space-times. Results. We present the spectral energy distribution of the accretion disk around constant Ricci scalar f (R) black holes, and constrain specific f (R) prescriptions using features of these systems. Conclusions. A precise determination of both the spin and accretion rate onto black holes along with X-ray observations of their thermal spectrum might allow to identify deviations of gravity from general relativity. We use recent data on the high-mass X-ray binary Cygnus X-1 to restrict the values of the parameters of a class of f (R) models.