Photon Emission from Circular Equatorial Kerr Orbiters

Abstract: In this work we consider monochromatic and isotropic photon emission from circular equatorial Kerr orbiters. Calculating the critical curve delineating the region of photon escape from that of photon capture in each emitter’s sky, allows us to derive analytic expressions for the photon escape probability and the redshift-dependent total flux collected on the celestial sphere as a function of emission radius and black hole parameters. This critical curve generalizes to finite orbital radius the usual Kerr critical curve and displays interesting features in the limit of high spin. These results confirm that the near-horizon geometry of a high-spin black hole is in principle observable.