Spatially Resolved Low-Frequency Very Large Array Observations of the Supernova Remnant 3C 391

We present VLA images of the supernova remnant (SNR) 3C 391 at 74, 330, and 1465 MHz. This remnant has been known for some time to exhibit a turnover in its integrated radio continuum spectrum at frequencies <100 MHz, indicative of free-free absorption from thermal ionized gas along the line of sight. For the first time, our data reveal the spatially resolved morphology of the low-frequency free-free absorption with a resolution of ~70''. Contrary to the expectation that such absorption arises from unrelated low-density H II regions (or their envelopes) along the line of sight, these data suggest that in this case the absorbing medium is directly linked to the SNR itself. 3C 391 has been shown in a number of recent papers to be interacting with a molecular cloud. Indeed, it exhibits a number of signposts of SNR/molecular cloud shocks, including OH (1720 MHz) masers and broad molecular emission lines. Comparison of the regions of strongest 74 MHz absorption with existing X-ray, IR, and molecular data suggests that the free-free absorption originates from the SNR/molecular cloud shock boundaries due to ionized gas created from the passage of a J-type shock with a speed of ~100 km s-1. This makes it only the second SNR for which such (extrinsic) spatially resolved absorption has been measured and the only one for which the absorption is thought to arise from a SNR/molecular cloud interface region.