POLARIZATION OBSERVATIONS OF 1720 MHZ OH MASERS TOWARD THE 3 SUPERNOVA-REMNANTS W28, W44 AND IC-443

We present arcsecond resolution observations from the VLA with full St okes polarimetry of the ground-state satellite line of the hydroxyl mo lecule (OH) at 1720.53 MHz ((II3/2)-I-2, J = 3/2, F = 2 --> 1) toward three Galactic supernova remnants: W28, W44, and IC 443. The total num ber of individual OH (1720 MHz) ''spots'' we detect in each of these t hree remnants is 41, 25, and 6, respectively. The OH (1720 MHz) featur es appear to lie along the edge of radio continuum emission from the s upernova remnants, but they are displaced behind the leading edge of t he shock as traced by the synchrotron emission. The brightness tempera tures of the OH (1720 MHz) emission features range from 2 x 10(4) to 1 0(8) K, convincingly demonstrating the maser nature of the OH (1720 MH z) features. We argue that the partially resolved angular diameters th at we measure for the masers are neither intrinsic sizes nor scatterin g 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 p rofile, indicative of Zeeman splitting in the weak-field case, is iden tified 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 ar e remarkably constant in both direction and magnitude over regions sev eral parsecs apart. These are the first measurements of postshock magn etic fields in supernova remnants and demonstrate the importance of ma gnetic 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 sy stemic velocity of the remnant. We suggest that the maximum amplificat ion of the maser transition will occur when the acceleration produced by the shock is transverse to the line of sight. Additional support fo r this point comes from the location of. the masers in IC 443, and fro m molecular observations that allow the shock geometry to be determine d. All of our observations are consistent with a model in which the OH (1720 MHz) is collisionally excited by H-2 molecules in the postshock gas heated by a nondissociative shock. Finally, we end with a discuss ion of the importance of supernova remnants with OH (1720 MHz) maser e mission as promising candidates to conduct high-energy searches for th e sites of cosmic-ray acceleration.