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H. Falcke (MPIFR), F. Melia (Arizona), E. Agol (JHU)
The mass of the supermassive black hole candidate Sgr A* at the Galactic Center has been determined with great precision in recent years. VLBI observations show that this source is surrounded by an ultra-compact emission region less than 15 Schwarzschildradii in diameter. We show that with the next generation of long-baseline interferometers at mm- and submm-wavelength we will able to image the shadow cast by the event horizon of this black hole, if it is one. 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\muarcseconds, a mere factor two smaller than the highest currently achieved resolution with VLBI techniques. We show how the shadow cast by the black hole changes for different emission models, be it a spherical infall, an advection-dominated accretion flow, or a jet. 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* can be observed if one goes to a suitably high frequency. The main problems are possible optical depth effects for an ADAF model and Doppler boosting for a jet model. While this does not change the basic shape and structure of the observable shadow, the nature of the emission model can have an influence on which dynamic range and which observing frequency is ultimately required to prove or disprove the existence of an event horizon.
The author(s) of this abstract have provided an email address for comments about the abstract: hfalcke@mpifr-bonn.mpg.de