Green bank telescope observations of IC 443: The nature of OH (1720 MHz) masers and OH absorption

We present spectral line observations of the ground-state transitions of hydroxyl (OH) toward supernova remnant (SNR) IC 443 carried out with the Green Bank Telescope. Weak, extended OH (1720 MHz) maser emission with OH (1667, 1665, 1612 MHz) absorption is detected along the southern extent of the remnant where no bright compact maser sources have been previously observed. These newly detected SNR-type masers are coincident with well-known molecular clumps and a ridge of shocked H₂ emission indicative of the SNR shock front interacting with the adjacent molecular cloud. Simultaneous observation of all four ground-state transitions of OH permits us to fit physical conditions of the shocked gas at the interaction site. A simple two-component model for the line profiles yields the physical parameters for detected regions of maser emission, including excitation temperature, OH column density, and filling factor. Observed line profiles suggest the shock is largely propagating toward the line of sight in the region of these newly identified weak masers. The implications of shock geometry and physical parameters in producing spatially extended OH maser emission in SNRs are explored. We also present VLA radio continuum observations at 330 MHz for comparison with OH line observations of the remnant.