Thermal OH (1667/1665 MHz) absorption and nonthermal OH (1720 MHz) emission toward the W28 supernova remnant

The W28 supernova remnant (SNR) is an excellent prototype for observing shocked gas resulting from the interaction of SNRs and adjacent molecular clouds (MCs). We present two new signatures of shocked molecular gas in this remnant. One is the detection of mainline extended OH (1667 MHz) absorption with broad line widths. The column density of OH estimated from the optical depth profiles is consistent with a theoretical model in which OH is formed behind a C-type shock front. The second is the detection of extended, weak OH (1720 MHz) line emission with narrow line widths distributed throughout the shocked region of W28. These give observational support to the idea that compact maser sources delineate the brightest component of a much larger region of mainline OH absorption and nonthermal OH (1720 MHz) emission tracing the global structure of shocked molecular gas. Mainline OH (1665/1667 MHz) absorption and extended OH (1720 MHz) emission line studies can serve as powerful tools to detect SNR-MC interaction even when bright OH (1720 MHz) masers are absent.