Publication date: Nov 2010
Abstract:
We study optical phenomena related to the appearance of Keplerian
accretion discs orbiting Kerr superspinars predicted by string theory.
The superspinar exterior is described by standard Kerr naked singularity
geometry breaking the black hole limit on the internal angular momentum
(spin). We construct local photon escape cones for a variety of orbiting
sources that enable us to determine the superspinars silhouette in the
case of distant observers. We show that the superspinar silhouette
depends strongly on the assumed edge where the external Kerr spacetime
is joined to the internal spacetime governed by string theory and
significantly differs from the black hole silhouette. The appearance of
the accretion disc is strongly dependent on the value of the superspinar
spin in both their shape and frequency shift profile. Apparent extension
of the disc grows significantly with the growing spin, while the
frequency shift grows with the descending spin. This behaviour differs
substantially from the appearance of discs orbiting black holes enabling
thus, at least in principle, to distinguish clearly the Kerr
superspinars and black holes. In vicinity of a Kerr superspinar the
non-escaped photons have to be separated to those captured by the
superspinar and those being trapped in its strong gravitational field
leading to self-illumination of the disc that could even influence its
structure and cause self-reflection effect of radiation of the disc. The
amount of trapped photons grows with descending superspinar spin. We
thus can expect significant self-illumination effects in the field of
Kerr superspinars with near-extreme spin a ~ 1.
Authors:
Stuchlík, Zdeněk; Schee, Jan;