A survey of hydroxyl toward supernova remnants: Evidence for extended 1720 MHz maser emission

We present the results of GBT observations of all four ground-state hydroxyl (OH) transitions toward 15 supernova remnants (SNRs) which show OH (1720 MHz) maser emission. This species of maser is well established as an excellent tracer of an ongoing interaction between the SNR and dense molecular material. For the majority of these objects we detect significantly higher flux densities with a single dish than have been reported with interferometric observations. We infer that spatially extended, low-level maser emission is a common phenomenon that traces the large-scale interaction in maser-emitting SNRs. In addition, we use a collisional pumping model to fit the physical conditions under which OH is excited behind the SNR shock front. We find the observed OH gas associated with the SNR interaction having columns N OH ≤ 1:5 x 1017 cm-2, temperatures of 20-125 K, and densities ~105 cm-3.18 page(s