Astrophysical Evidence for Black Hole Event Horizons

TL;DR

This paper reviews evidence from accretion models indicating that low-luminosity black holes in X-ray binaries and galactic nuclei possess event horizons, supporting their identification as true black holes.

Contribution

It demonstrates how ADAF models provide compelling evidence for the existence of event horizons in black hole candidates.

Findings

ADAF models fit low-luminosity accretion data

Evidence supports black holes having event horizons

Distinguishes black holes from neutron stars based on accretion behavior

Abstract

Astronomers have discovered many potential black holes in X-ray binaries and galactic nuclei. These black holes are usually identified by the fact that they are too massive to be neutron stars. Until recently, however, there was no convincing evidence that the objects identified as black hole candidates actually have event horizons. This has changed with extensive applications of a class of accretion models for describing the flow of gas onto compact objects; for these solutions, called advection-dominated accretion flows (ADAFs), the black hole nature of the accreting star, specifically its event horizon, plays an important role. We review the evidence that, at low luminosities, accreting black holes in both X-ray binaries and galactic nuclei contain ADAFs rather than the standard thin accretion disk.