Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc])

Super-spinning compact objects generated by thick accretion disks. (arXiv:1212.5848v1 [gr-qc]):
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.