1212.5848 (Zilong Li et al.)

Super-spinning compact objects generated by thick accretion disks    [PDF]

Zilong Li, Cosimo Bambi

If astrophysical black hole candidates are the Kerr black holes predicted by General Relativity, the value of their spin parameter must be subject to the {\it theoretical bound} $|a_*| \le 1$. In this work, we consider the possibility that these objects are either non-Kerr black holes in an alternative theory of gravity or exotic compact objects in General Relativity. Such a possibility is not in contradiction with current data and it can be tested with future observational facilities. We study the accretion process when their accretion disk is geometrically thick with a simple version of the Polish doughnut model. The picture of the accretion process may be qualitatively different from the one around a Kerr black hole. The inner edge of the accretion disk may not have the typical cusp on the equatorial plane any more, but there may be two cusps, respectively above and below the equatorial plane. We discuss the evolution of the spin parameter as a consequence of the accretion process and we estimate the maximum value of the spin parameter of these objects as a function of their deformation. Lastly, we compare our results with the current estimates of the mean radiative efficiency of AGNs. We find the observational bound $|a_*| \lesssim 1.3$ for the spin parameter of the super-massive black hole candidates at the centers of galaxies, which we argue to be almost independent of the exact nature of these objects.

View original: http://arxiv.org/abs/1212.5848