**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.;