The Event Horizon of M87

The 6× {{10}⁹} {{M}_⊙ } supermassiveblack hole at the center of the giant elliptical galaxy M87 powers arelativistic jet. Observations at millimeter wavelengths with the EventHorizon Telescope have localized the emission from the base of this jetto angular scales comparable to the putative black hole horizon. The jetmight be powered directly by an accretion disk or by electromagneticextraction of the rotational energy of the black hole. However, even thelatter mechanism requires a confining thick accretion disk to maintainthe required magnetic flux near the black hole. Therefore, regardless ofthe jet mechanism, the observed jet power in M87 implies a certainminimum mass accretion rate. If the central compact object in M87 werenot a black hole but had a surface, this accretion would result inconsiderable thermal near-infrared and optical emission from thesurface. Current flux limits on the nucleus of M87 strongly constrainany such surface emission. This rules out the presence of a surface andthereby provides indirect evidence for an event horizon.