Black Hole Glimmer Signatures of Mass, Spin, and Inclination

Gravitational lensing near a black hole is strong enough that light rays can circle the event horizon multiple times. Photons emitted in multiple directions at a single event, perhaps because of localized, impulsive heating of accreting plasma, take multiple paths to a distant observer. In the Kerr geometry, each path is associated with a distinct light travel time and a distinct arrival location in the image plane, producing black hole glimmer. This sequence of arrival times and locations uniquely encodes the mass and spin of the black hole and can be understood in terms of properties of bound photon orbits. We provide a geometrically motivated treatment of Kerr glimmer and evaluate it numerically for simple hotspot models to show that glimmer can be measured in a finite-resolution observation. We discuss potential measurement methods and implications for tests of the Kerr hypothesis.