Sgr A*: Observations, models, and imaging of the event horizon with VLBI.

Details Sgr A*: Observations, models, and imaging of the event horizon with VLBI. Sgr A*: Observations, models, and imaging of the event horizon with VLBI.

Jan 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001iaus..205...28f&link_type=abstract

Galaxies and their Constituents at the Highest Angular Resolutions, Proceedings of IAU Symposium #205, held 15-18 August 2000 at

Physics

2

Scientific paper

We show and discuss results and prospects of high-resolution imaging of the supermassive black hole candidate Sgr A*. The latest millimeter-VLBI observations show compact radio emission from within a region of about 15 Schwarzschild radii. This compact component is most likely responsible for the so-called sub-mm bump in the spectrum and perhaps even for the recently discovered `anomalous' circular polarization and some X-ray emission. Most importantly, however, the sub-mm emission from Sgr A* opens the door to observe, for the first time, the event horizon of a black hole directly. Using a general-relativistic ray tracing code we show that to a distant observer, the event horizon casts a relatively large ``shadow'' with an apparent diameter of ~ 10 gravitational radii due to bending of light by the black hole, nearly independent of the black hole spin or orientation. The predicted angular size for the Galactic Center black hole is ~ 30±3 μarcseconds, a mere factor of two smaller than the highest currently achieved resolution with VLBI techniques. The main uncertainty currently is the distance to the Galactic Center. Taking into account scatter-broadening of the image in the interstellar medium and the finite achievable telescope resolution, we show that the shadow of Sgr A* will be observable at suitably high frequencies, if an event horizon exists. High-resolution radio observations at mm and sub-mm wavelength will therefore play an important role in answering fundamental physical questions in the coming years.

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