The light ring and the appearance of matter accreted by black holes

Understand

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 non-spinning black holes.
• The emission as the object falls down the hole is dominated by photons or gravitons orbiting the light ring, causing the total luminosity to decrease exponentially as ${\cal L}_o\sim e^{-t/(3\sqrt{3}\,M)}$.
• Late-time radiation is blueshifted, due to its having been emitted during the infall, trapped at the light ring and subsequently re-emitted.