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Abstract: Black hole event horizons, causally separating the external universe fromcompact regions of spacetime, are one of the most exotic predictions of GeneralRelativity GR. Until recently, their compact size has prevented efforts tostudy them directly. Here we show that recent millimeter and infraredobservations of Sagittarius A* Sgr A*, the supermassive black hole at thecenter of the Milky Way, all but requires the existence of a horizon.Specifically, we show that these observations limit the luminosity of anyputative visible compact emitting region to below 0.4% of Sgr A*-s accretionluminosity. Equivalently, this requires the efficiency of converting thegravitational binding energy liberated during accretion into radiation andkinetic outflows to be greater than 99.6%, considerably larger than thoseimplicated in Sgr A*, and therefore inconsistent with the existence of such avisible region. Finally, since we are able to frame this argument entirely interms of observable quantities, our results apply to all geometric theories ofgravity that admit stationary solutions, including the commonly discussed fRclass of theories.

Autor: Avery E. Broderick 1, Abraham Loeb 2, Ramesh Narayan 2 1 CITA,2 Harvard CfA


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