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

TL;DR

This paper investigates how matter accreting onto non-spinning black holes produces distinctive optical and gravitational wave signals dominated by photons or gravitons orbiting the light ring, revealing universal features of black hole spacetimes.

Contribution

It introduces a universal model for the emission signatures of matter falling into black holes, emphasizing the role of the light ring in shaping observable signals.

Findings

Luminosity decreases exponentially during infall, ${ m L}_o o e^{-t/(3\sqrt{3}\,M)}$.

Late-time radiation is blueshifted and re-emitted after being trapped at the light ring.

Signature features are insensitive to near-horizon details, highlighting the light ring's universal role.

Abstract

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. These universal properties are a clear signature of the existence of light rings in the spacetime, and not particularly sensitive to near horizon details.