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We consider monochromatic and isotropic photon emission from circularequatorial Kerr orbiters. We derive analytic expressions for the photon escapeprobability and the redshift-dependent total flux collected on the celestialsphere as a function of emission radius and black hole parameters. Thesecalculations crucially involve the critical curve delineating the region ofphoton escape from that of photon capture in each emitter's sky. This curvegeneralizes to finite orbital radius the usual Kerr critical curve and displaysinteresting features in the limit of high spin, which we investigate bydeveloping a perturbative expansion about extremality. Although the innermoststable circular orbit appears to approach the event horizon for very rapidlyspinning black holes, we find in this regime that the photon escape probabilitytends to $5/12+1/(\sqrt{5}\pi)\arctan\sqrt{5/3}\approx54.65\%$. We also obtaina simple formula for the flux distribution received on the celestial sphere,which is nonzero. This confirms that the near-horizon geometry of a high-spinblack hole is in principle observable. These results require us to introduce anovel type of near-horizon double-scaling limit. We explain the dip observed inthe total flux at infinity as an imprint of the black hole: the black hole"bite".

Photon Emission from Circular Equatorial Kerr Orbiters