Inner Accretion Disk Edges in a Kerr-Like Spacetime. (arXiv:1304.8106v1 [gr-qc])

Inner Accretion Disk Edges in a Kerr-Like Spacetime. (arXiv:1304.8106v1 [gr-qc]):
According to the no-hair theorem, astrophysical black holes are uniquely
described by the Kerr metric. In order to test this theorem with observations
in either the electromagnetic or gravitational-wave spectra, several Kerr-like
spacetimes have been constructed which describe potential deviations from the
Kerr spacetime in parametric form. For electromagnetic tests of the no-hair
theorem, such metrics allow for the proper modeling of the accretions flows
around candidate black holes and the radiation emitted from them. In many of
these models, the location of the inner edge of the accretion disk is of
special importance. This inner edge is often taken to coincide with the
innermost stable circular orbit (ISCO), which can serve as a direct probe of
the spin and the deviation from the Kerr metric. In certain cases, however, an
ISCO does not exist, and the inner edge of an accretion disk is instead
determined by an instability against small perturbations in the direction
vertical to the disk. In this paper, I analyze the properties of accretion
disks in the Kerr-like metric proposed by Johannsen and Psaltis (2011), whose
inner edges are located at the radii where this vertical instability occurs. I
derive expressions of the energy and axial angular momentum of disk particles
that move on circular equatorial orbits and calculate the locations of the
inner disk edges. As a possible observable of such accretion disks, I simulate
profiles of relativistically broadened iron lines and show that they depend
significantly on the values of the spin and the deviation parameter.