Joint Astrophysics Colloquium

Many galactic nuclei are now believed to harbor a supermassive black hole. Recent studies of stellar dynamics in the central parsec of the Milky Way indicate the presence of a dark massive object with a mass of 2.6 million suns. This dark object is apparently coincident with Sagittarius A* (Sgr A*), the compact nonthermal radio source located at the dynamical center of the Milky Way. By analogy with quasars and active galactic nuclei (AGN), Sgr A* is thought to be powered by matter accreting onto a supermassive black hole. However, one of the long-standing puzzles of high-energy astrophysics is that Sgr A* is many orders of magnitude fainter than expected in X-rays.

I will present results from the first high-spatial-resolution (~1 arcsecond), hard X-ray (0.5-10 keV) spectroscopic image of the central 40 pc (17 arcminutes) of the Milky Way. This image was obtained in a 40 ks observation with the Advanced CCD Imaging Spectrometer (ACIS) on the Chandra X-ray Observatory. I will argue that we have detected X-rays from Sgr A* for the first time and discuss the implications for the various theoretical models of the accretion flow onto the supermassive black hole at Sgr A*. I will also discuss the complex structure of the X-ray emission from the Sgr A radio complex and along the Galactic plane and will present morphological evidence that Sgr A* and Sgr A West, a spiral-shaped HII region feeding gas toward the dark massive object, lie within the hot plasma in the central cavity of Sgr A East, a shell-shaped nonthermal radio source which is believed to have originated in a Type II supernova about 10,000 yrs ago. This Chandra observation is, for the first time, giving us a clear picture of the complex, highly energetic processes in the cental parsecs of our Galaxy.

Thursday, March 22nd 2001, 12:30
Ernest Rutherford Physics Building, room 305