Abstract
Abstract 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 hot-spot 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.
Highlights
Spinning supermassive black holes likely power astrophysical relativistic jets via the Blandford–Znajek mechanism (Blandford & Znajek 1977, see Blandford & Payne 1982 and Lynden-Bell 2006 for alternative jet-power mechanisms)
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
The set of echo periods and arrival locations that comprise glimmer uniquely encodes the properties of the spacetime and provides a direct way to measure black hole mass and angular momentum
Summary
Spinning supermassive black holes likely power astrophysical relativistic jets via the Blandford–Znajek mechanism (Blandford & Znajek 1977, see Blandford & Payne 1982 and Lynden-Bell 2006 for alternative jet-power mechanisms). The asymptotically bound orbits produce a characteristic critical curve whose size and shape are set by the spacetime geometry. Light signals from subsequent orbitings will be separated by a time delay set by the length of a complete winding around the hole These delays will cause the source to echo in the image plane. The Kerr geometry produces a rich spectrum of echo time delays associated with resonant bound orbits These resonant echo delays are closely related to the black hole quasinormal-mode spectrum in the eikonal limit (see Yang et al 2012), which has been studied in the context of gravitational-wave ringdown and measuring mass and spin (e.g., Berti et al 2006; Buonanno et al 2007; Berti et al 2007).
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