Publication date: Nov 2018
Abstract:
For the Kerr naked singularity (KNS) spacetimes, we study properties of
spherical photon orbits (SPOs) confined to constant Boyer-Lindquist
radius r. Some new features of the SPOs are found, having no
counterparts in the Kerr black hole (KBH) spacetimes, especially stable
orbits that could be pure prograde/retrograde, or with turning point in
the azimuthal direction. At r>1 (r<1) the covariant photon energy
E> 0 (E< 0), at r=1 there is E= 0. All unstable orbits must have
E> 0. It is shown that the polar SPOs can exist only in the
spacetimes with dimensionless spin a < 1.7996. Existence of closed
SPOs with vanishing total change of the azimuth is demonstrated.
Classification of the KNS and KBH spacetimes in dependence on their
dimensionless spin a is proposed, considering the properties of the
SPOs. For selected types of the KNS spacetimes, typical SPOs are
constructed, including the closed paths. It is shown that the stable
SPOs intersect the equatorial plane in a region of stable circular
orbits of test particles, depending on the spin a. Relevance of this
intersection for the Keplerian accretion discs is outlined and
observational effects are estimated.
Authors:
Charbulák, Daniel; Stuchlík, Zdeněk;