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Paper: Imaging the Event Horizon of a Massive Black Hole: Past, Present and Future Very Long Baseline Interferometry of Sagittarius A*
Volume: 340, Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA
Page: 247
Authors: Bower, G.C.
Abstract: Sagittarius A*, the nearest massive black hole, is an important laboratory for astrophysics. Nevertheless, despite intense observational and theoretical work for 30 years, much remains unknown about this system. In particular, we still do not know the means by which gas accretes onto the black hole and the role of outflow via winds or jets. VLBI observations have been a critical probe of Sgr A* but they have long been limited by the presence of very strong interstellar scattering along the line of sight. This has pushed observations to short wavelengths where the effects of scattering are weaker but where technical limitations to image quality are greater. These problems have led to technical and algorithmic development which are at the forefront of VLBI. Principal among these are the use of ad hoc high frequency VLBI networks and the use of closure quantities to exclude errors from calibration.

I present the results of VLA and VLBA observations from 20 cm to 7 mm which measure the intrinsic size of Sgr A*. These results were achieved through the use of closure amplitude analysis. This is the highest resolution image of any black hole system in units of the Schwarzschild radius. I also discuss the exciting future of millimeter and submillilmeter wavelength VLBI, which includes the possibility of detecting general relativistic effects in the vicinity of the black hole. These results can only be achieved if there is sufficient foresight in the design of the next generation of millimeter observatories such as ALMA.

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