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Ongoing Research

The Nature of Black Holes

Papers on this topic:

            Broderick, A.E., Loeb, A. & Narayan, R., 2009, ApJ, 701, 1357
The Event Horizon of Sagittarius A*

            Broderick, A.E. & Narayan, R., 2007, Classical & Quantum Gravity, 24, 650
Where are all the gravastars?  Limits upon the gravastar model from accreting black holes

            Broderick, A.E. & Narayan, R., 2006, ApJL, 638, 21
On the Nature of the Dark Mass at the Galactic Center

             Only recently has it become possible to empirically address the existence of black hole horizons.  Combined with the no-hair theorems, these are responsible for the information paradox (where does the information about the accreting materials quantum state go once accreted?), the primary sticking point in unifying general relativity and quantum mechanics, the two pillars of modern physics.

             The supermassive black hole candidate at the center of the Milky Way, Sgr A*,  is by far the best studied black hole known.  The orbits of nearby massive stars, S-15 passing within 45 AU of Sgr A*, have provided a precise determination of Sgr A*’s mass.  Long term multi-band monitoring projects have characterized its radio, infrared and X-ray spectra and variability with considerable accuracy.  Sub-mm VLBI has already resolved the emission region of Sgr A*, finding a characteristic size at 1.3mm of 37 mas, smaller even than that expected for the horizon.

             In a pair of papers with  Ramesh Narayan and Avi Loeb I have been considered the spectral implications of the absence of a horizon.  If we assume that Sgr A* is accretion powered (justified by observations), in steady state (justified by Sgr A*’s short dynamical time), and the emitting region is compact (justified by the mm-VLBI observations) then we show that a horizon must exist, regardless of whether or not general relativity describes the spacetime surrounding Sgr A*.  Note that the image of a silhouette alone is insufficient to rule out a surface with a radius less than 3GM/c2.

             With Ramesh Narayan I have also considered constraints upon explicit alternative theories to black holes.   For example, in the gravastar theory quantum mechanical  effects are invoked to argue for the formation of a surface immediately outside of the horizon, with the black hole interior being filed with something like a de Sitter space.  Such a configuration can have a large heat capacity, potentially violating the steady-state assumption.  Nevertheless, we have shown that using the supermassive black hole in the center of the Milky Way  and a particularly underluminous stellar mass black hole (J1118+480) that the most physically motivated of these theories can be excluded.

Allowed accretion flow efficiencies and emission region size, given the observed near and mid-infrared flux limits and 1.3 mm size constraints for Sgr A*.  We show that for a horizon to be absent the accretion flow must radiate more than 99.6% of the liberated gravitational binding energy, vastly in excess of the 0.1—0.01% thought to be radiated by Sgr A* and the 10% typically assumed for quasars.

Computed images of an RIAF around a black hole and a dark mass with a surface of varying radii.  Only when the radius exceeds 3M do the images differ.

The excluded energy scales of the gravastar theory we considered as a function of the fraction of its mass that the black hole has accreted over its lifetime.  Using Sgr A*, J1118+480, stellar disruptions and the observed present day accretion we are able to exclude all energy scales larger than the Planck energy!