Light ring and the appearance of matter accreted by black holes

The geometry of black hole spacetimes can be probed with exquisite precision in the gravitational-wave window, and possibly also in the optical regime. We study the accretion of bright spots-objects which emit strongly in the optical or in gravitational waves-by nonspinning black holes. As the object approaches the event horizon, the emitted radiation to far-away distances is dominated by photons or gravitons orbiting the light ring, causing the total luminosity to decrease exponentially as L-o similar to e(-t/(3 root 3M)). Late-time radiation is blueshifted, due to its having been emitted during the infall, trapped at the light ring and subsequently reemitted. These universal properties are a clear signature of the existence of light rings in the spacetime, and not particularly sensitive to near horizon details.

Automated summary

The study finds that as bright spots are accreted by black holes, the emitted radiation decreases exponentially as the object approaches the event horizon, and exhibits a blueshift at late times. These universal properties serve as clear signatures of the existence of light rings in spacetime, and are not particularly sensitive to details near the horizon.