Photon emission of extremal Kerr-Newman black holes
  In this paper, we deal with the null geodesics extending from the near-horizon region out to a distant observatory in an extremal Kerr-Newman black hole background. In particular, using the matched asymptotic expansion method, we analytically solve the null geodesics near the superradiant bound in the form of algebraic equations. For the case that the photon trajectories are limited in the equatorial plane, the shifts in the azimuthal angle and time are obtained. 
Shao-Wen Wei, Bao-Min Gu, Yong-Qiang Wang, Yu-Xiao Liu
gr-qc hep-th