Polarization Observations of 1720 MHz OH Masers toward the Three Supernova Remnants W28, W44, and IC 443

We present arcsecond resolution observations from the VLA with full Stokes polarimetry of the ground-state satellite line of the hydroxyl molecule (OH) at 1720.53 MHz (2Π3/2, J = 3/2, F = 2 → 1) toward three Galactic supernova remnants: W28, W44, and IC 443. The total number of individual OH (1720 MHz) "spots" we detect in each of these three remnants is 41, 25, and 6, respectively. The OH (1720 MHz) features appear to lie along the edge of radio continuum emission from the supernova remnants, but they are displaced behind the leading edge of the shock as traced by the synchrotron emission. The brightness temperatures of the OH (1720 MHz) emission features range from 2 × 104 to 108 K, convincingly demonstrating the maser nature of the OH (1720 MHz) features. We argue that the partially resolved angular diameters that we measure for the masers are neither intrinsic sizes nor scattering disks, but result from a blend of several unresolved maser features near the same velocity. Thus, our computed brightness temperatures are lower limits to the true values. The characteristic antisymmetric S profile, indicative of Zeeman splitting in the weak-field case, is identified in the Stokes V spectrum of several of the brighter maser spots. The derived line-of-sight magnetic fields are of order 0.2 mG and are remarkably constant in both direction and magnitude over regions several parsecs apart. These are the first measurements of postshock magnetic fields in supernova remnants and demonstrate the importance of magnetic pressure in these molecular shocks. The velocity dispersion of the maser features is typically less than a few km s-1, and, except in the special case of W28, the mean maser velocity is equal to the systemic velocity of the remnant. We suggest that the maximum amplification of the maser transition will occur when the acceleration produced by the shock is transverse to the line of sight. Additional support for this point comes from the location of the masers in IC 443, and from molecular observations that allow the shock geometry to be determined. All of our observations are consistent with a model in which the OH (1720 MHz) is collisionally excited by H2 molecules in the postshock gas heated by a nondissociative shock. Finally, we end with a discussion of the importance of supernova remnants with OH (1720 MHz) maser emission as promising candidates to conduct high-energy searches for the sites of cosmic-ray acceleration.