Publication date: Nov 2000
Abstract:
Discussion of the equatorial photon motion in Kerr-Newman black-hole and
naked-singularity spacetimes with a non-zero cosmological constant is
presented. Both repulsive and attractive cosmological constants are
considered. An appropriate `effective potential‘ governing the photon
radial motion is defined, circular photon orbits are determined, and
their stability with respect to radial perturbations is established. The
spacetimes are divided into separated classes according to the
properties of the `effective potential‘. There is a special class of
Kerr-Newman-de Sitter black-hole spacetimes with the restricted
repulsive barrier. In such spacetimes, photons with high positive and
all negative values of their impact parameter can travel freely between
the outer black-hole horizon and the cosmological horizon due to an
interplay between the rotation of the source and the cosmological
repulsion. It is shown that this type of behaviour of the photon motion
is connected to an unusual relation between the values of the impact
parameters of the photons and their directional angles relative to the
outward radial direction as measured in the locally non-rotating frames.
Surprisingly, some photons counterrotating in these frames have a
positive impact parameter. Such photons can be both escaping or captured
in the black-hole spacetimes with the restricted repulsive barrier. For
the black-hole spacetimes with a standard, divergent repulsive barrier
of the equatorial photon motion, the counterrotating photons with
positive impact parameters must all be captured from the region near the
black-hole outer horizon as in the case of Kerr black holes, while they
all escape from the region near the cosmological horizon. Further, the
azimuthal motion is discussed and photon trajectories are given in
typical situations. It is shown that for some photons with negative
impact parameter turning points of their azimuthal motion can exist.
Authors:
Stuchlík, Z.; Hledík, S.;