Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session D4: Probing Strong-Field Gravity with Observations of the Galactic Center Black Hole |
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Sponsoring Units: GGR Chair: Scott Hughes, Massachusetts Institute of Technology Room: Thurgood Marshall North |
Saturday, February 13, 2010 1:30PM - 2:06PM |
D4.00001: Probing strong-field gravity at the galactic center using stellar motions Invited Speaker: The center of our galaxy contains the nearest supermassive black hole, with a mass four million times that of the Sun. The black hole's location and mass have been accurately determined by tracing the motions of a handful of bright young stars that move in tight orbits about the Galactic center, some with periods as short as 15 years. Until now, the measured orbits have been found to be consistent with Keplerian ellipses about a Newtonian point mass. But the stellar orbits potentially contain much more information: about the distributed mass in the inner parsec (consisting of faint stars, dark stellar remnants, and possibly particle dark matter); and also about the non-Newtonian contributions to the gravitational potential from the supermassive black hole. For stars nearer than about one milli-parsec from the singularity, frame-dragging torques should induce precession of orbital planes at a rate that is potentially observable after a few years' monitoring using the next generation of optical astrometric instruments, allowing a direct determination of the black hole's spin. Even more challenging would be a test of 'no hair' theorems by comparing the frame-dragging precession with that induced by the black hole's quadrupole moment. Results of detailed numerical simulations of the nuclear star cluster that include relativistic terms will be presented, which demonstrate the feasibility of testing theories of gravity using stellar orbits, given the inevitable noise from star-star perturbations and perturbations due to the unseen stellar remnants. [Preview Abstract] |
Saturday, February 13, 2010 2:06PM - 2:42PM |
D4.00002: Observing an Event Horizon: (sub)mm Wavelength VLBI of SgrA* Invited Speaker: A long--standing goal in astrophysics is to directly observe the immediate environment of a black hole with angular resolution comparable to the event horizon. Realizing this goal would open a new window on the study of General Relativity in the strong field regime, accretion and outflow processes at the edge of a black hole, the existence of event horizons, and fundamental black hole physics (e.g., spin). Steady long--term progress on improving the capability of Very Long Baseline Interferometry (VLBI) at short wavelengths has now made it extremely likely that this goal will be achieved within the next decade. The most compelling evidence for this is the recent observation, using 1.3mm wavelength VLBI, of Schwarzschild radius scale structure in SgrA*, the compact source of radio, submm, NIR and xrays at the center of the Milky Way. There is now very strong evidence that SgrA* marks the position of a $\sim4$ million solar mass black hole, which, due to its proximity and estimated mass, presents us with the largest apparent event horizon size of any black hole candidate in the Universe. By extending the observing wavelength of VLBI to the sub--mm bands, we will achieve angular resolution sufficient to detect strong field GR effects on the appearance of the plasma surrounding the black hole. Short wavelength VLBI can also be used to directly detect signatures of matter spiraling into the black hole with the potential to estimate the periods of orbits close to the event horizon. I will discuss what current VLBI observations of SgrA* tell us about this closest super--massive black hole, describe the exciting potential of future work, and outline plans to assemble a Global submm-VLBI ``Event Horizon Telescope". [Preview Abstract] |
Saturday, February 13, 2010 2:42PM - 3:18PM |
D4.00003: Nature of the black hole in the center of the Milky Way Invited Speaker: Recently, mm-VBLI observations capable of resolving sub-horizon structure in the emission from Sgr A*, the supermassive black hole at the center of the Milky Way, have become possible. These promise to open a new window upon the physics of black hole accretion, jet formation and gravity itself. Already, when combined with existing observations at other wavelengths, it is possible to place extraordinary constraints upon the existence of a horizon in Sgr A*, subject only to the assumption that gravity is a metric theory admitting stationary solutions. I will describe what we expect to see, how this will inform our understanding of gas dynamics near black holes, and what we've learned about the fundamental nature Sgr A* already. [Preview Abstract] |
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