Shock Excited Maser Emission from the Supernova Remnant W28

We present VLA observations in the hydroxyl line OH(1720 MHz) towards the supernova remnant W28. These high resolution images show a more extensive distribution of 1720 MHz emission than had previously been suspected from earlier single-dish observations. A total of 26 distinct maser spots were detected which lie on the edge of the radio continuum emission from W28 and are unresolved with our 12 arcsec resolution. Significant detection of linear and circular polarization in the maser spots is also made. The location of the maser spots, together with the physical conditions in the SNR and the adjacent molecular cloud, argue for a shock excitation mechanism. The possibilities of using the OH(1720 MHz) as a powerful diagnostic of the interaction of supernova remnants with molecular clouds is discussed.