The Event Horizon of Sagittarius A*

TL;DR

Recent observations of Sagittarius A* strongly support the existence of an event horizon by constraining the luminosity of any visible emitting region to be extremely low, thus implying the presence of a true black hole horizon.

Contribution

This study provides observational evidence for the event horizon of Sagittarius A* and demonstrates that such evidence applies broadly across various theories of gravity.

Findings

Luminosity of visible region < 0.4% of accretion luminosity

Efficiency of energy conversion > 99.6%

Results apply to all stationary solutions in geometric gravity theories

Abstract

Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of General Relativity (GR). Until recently, their compact size has prevented efforts to study them directly. Here we show that recent millimeter and infrared observations of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, all but requires the existence of a horizon. Specifically, we show that these observations limit the luminosity of any putative visible compact emitting region to below 0.4% of Sgr A*'s accretion luminosity. Equivalently, this requires the efficiency of converting the gravitational binding energy liberated during accretion into radiation and kinetic outflows to be greater than 99.6%, considerably larger than those implicated in Sgr A*, and therefore inconsistent with the existence of such a visible region. Finally, since we are able to frame this argument entirely in terms of observable quantities, our results apply to all geometric theories of gravity that admit stationary solutions, including the commonly discussed f(R) class of theories.