Escape probability of a photon emitted near the black hole horizon

We investigate the escape of photons from the vicinity of the horizon to infinity in the Kerr-Newman black hole spacetime. We assume that a light source is at rest in a locally nonrotating frame on the equatorial plane and photons are emitted isotropically. Then, we evaluate the escape probability of the emitted photons. The main result of this paper is the following. If the black hole is extremal with the nondimensional spin parameter a_*>1 /2 , however close to the horizon the light source would be, the escape probability remains nonzero. The near-horizon limit value of the escape probability is a monotonically increasing function of a_* and takes a maximum ∼29.1 % at a_*=1 , i.e., for the extremal Kerr case. On the other hand, if the black hole is extremal with 0 ≤a_*≤1 /2 or if the black hole is subextremal, the near-horizon limit value is zero.