The Event Horizon of M87

TL;DR

This paper uses observations of M87's jet and flux limits to provide indirect evidence for the presence of an event horizon, supporting the black hole model over alternatives with a surface.

Contribution

It demonstrates that the observed jet power and flux limits strongly constrain the existence of a surface, indirectly confirming the black hole's event horizon in M87.

Findings

Flux limits rule out a surface in M87's core.

Observed jet power implies a minimum accretion rate.

Results support the black hole model with an event horizon.

Abstract

The 6 billion solar mass supermassive black hole at the center of the giant elliptical galaxy M87 powers a relativistic jet. Observations at millimeter wavelengths with the Event Horizon Telescope have localized the emission from the base of this jet to angular scales comparable to the putative black hole horizon. The jet might be powered directly by an accretion disk or by electromagnetic extraction of the rotational energy of the black hole. However, even the latter mechanism requires a confining thick accretion disk to maintain the required magnetic flux near the black hole. Therefore, regardless of the jet mechanism, the observed jet power in M87 implies a certain minimum mass accretion rate. If the central compact object in M87 were not a black hole but had a surface, this accretion would result in considerable thermal near-infrared and optical emission from the surface. Current flux limits on the nucleus of M87 strongly constrain any such surface emission. This rules out the presence of a surface and thereby provides indirect evidence for an event horizon.