SHOCK-EXCITED OH MASER EMISSION OUTLINING THE GALACTIC-CENTER SUPERNOVA REMNANT G359.1-0.05

A search using the Very Large Array was performed for 1720-megahertz O H maser line emission from a number of nonthermal radio continuum sour ces in the galactic center region. The 1720-megahertz transition has r ecently been noted for its potential as a tracer of shock activity. Th e most striking result was the detection of extended 1720-megahertz OH maser emission, as well as a number of compact OH maser features, alo ng the interface between a large-scale continuum shell (G359.1-0.5) an d its surrounding ring of high-velocity molecular gas. The morphologic al correlation among the neutral gas, the nonthermal shell, and the ma ser features provides strong support for the hypothesis that the 1720- megahertz maser line of OH arises from gas shocked by the impact of th e expanding supernova remnant into the molecular material. However, th e radial velocities of the molecular cloud surrounding G359.1-0.5 are more negative than that of the OH maser spots by more than 50 kilomete rs per second. Here it is suggested that only the low-radial-velocity component of the carbon monoxide material at the limb of the remnant s atisfies the physical conditions required for collisional pumping of t he OH 1720-megahertz line behind the expanding shock front.