Research Articles

A Transient Radio Source near the Center of the Milky Way Galaxy

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Science  20 Mar 1992:
Vol. 255, Issue 5051, pp. 1538-1543
DOI: 10.1126/science.255.5051.1538


In late December 1990, a new radio source appeared near the center of our galaxy rivaling the intensity of Sgr A* (the compact radio source at the galactic center). Following its first detection, the flux density of the galactic center transient (GCT) increased rapidly to a maximum 1 month later, and then declined gradually with a time scale of about 3 months. Surprisingly, the GCT maintained a steep radio spectrum during both its rising and decay phases. The neutral hydrogen (HI) absorption shows similar absorption to that in front of Sgr A*; this indicates that the GCT lies near the galactic center. Furthermore, both HI and OH observations show an additional deep absorption at +20 kilometers per second with respect to the local standard of rest. Thus, the GCT is either embedded in or located behind a molecular cloud moving with that velocity. The cloud can be seen on infrared images. Its opacity is shown to be inadequate to conceal a supernova near the galactic center. It is argued that the GCT was probably transient radio emission from synchrotron-radiating plasma associated with an x-ray binary system.