OH Masers and Supernova Remnants
  OH(1720 MHz) masers are created by the interaction of supernova remnants with
molecular clouds. These masers are pumped by collisions in warm, shocked
molecular gas with OH column densities in the range 10^{16}--10^{17} cm^{-2}.
Excitation calculations suggest that inversion of the 6049 MHz OH line may
occur at the higher column densities that have been inferred from main-line
absorption studies of supernova remnants with the Green Bank Telescope. OH(6049
MHz) masers have therefore been proposed as a complementary indicator of
remnant-cloud interaction.
  This motivated searches for 6049 MHz maser emission from supernova remnants
using the Parkes 63 m and Effelsberg 100 m telescopes, and the Australia
Telescope Compact Array. A total of forty-one remnants have been examined by
one or more of these surveys, but without success. To check the accuracy of the
OH column densities inferred from the single-dish observations we modelled OH
absorption at 1667 MHz observed with the Very Large Array towards three
supernova remnants, IC 443, W44 and 3C 391. The results are mixed -- the OH
column is revised upwards in IC443, downwards in 3C391, and is somewhat reduced
in W44. We conclude that OH columns exceeding 10^{17} cm^{-2} are indeed
present in some supernova remnants and so the lack of any detections is not
explained by low OH column density. We discuss the possibility that non-local
line overlap is responsible for suppressing the inversion of the 6049 MHz line.
