  Radio surveys of supernova remnants (SNRs) in the Galaxy have uncovered 19
SNRs accompanied by OH maser emission at 1720 MHz. This unusual class of maser
sources is suggested to be produced behind a shock front from the expansion of
a supernova remnant running into a molecular cloud. An important ingredient of
this model is that X-ray emission from the remnant enhances the production of
OH molecule. The role of X-ray emission from maser emitting (ME) SNRs is
investigated by comparing the X-ray induced ionization rate with theory. One
aspect of this model is verified: there is a strong association between maser
emitting and mixed-morphology (MM) or thermal composite SNRs --center-filled
thermal X-ray emission surrounded by shell-like radio morphology. We also
present ROSAT and ASCA observations of two maser emitting SNRs: G21.8--0.6 (Kes
69) and G357.7--0.1 (Tornado).
