A Survey for OH (1720 MHz) Maser Emission Toward Supernova Remnants

The satellite line of the hydroxyl radical (OH) at 1720.5 MHz has been searched for in the direction of 66 supernova remnants in our Galaxy with the Green Bank 43-m and Parkes 64-m telescopes. We detect OH emission toward 20 supernova remnants. Follow-up observations with the VLA and ATCA resulted in the detection of compact emission from 8 supernova remnants. We argue that these features are masers and that they are physically associated with the supernova remnants. A model in which the masers are collisionally excited by the passage of the supernova remnant shock through a molecular cloud best describes the data. Thus, in addition to the stellar masers and the masers found in star forming regions, there is now a new and growing class of masers found in supernova remnants. The OH(1720 MHz) line provides a powerful new diagnostic for finding and studying the interaction of supernova remnants with molecular clouds.