MULTIFREQUENCY STUDIES OF BRIGHT RADIO SUPERNOVA-REMNANTS .2. W49B

We report radio observations of the supernova remnant W49B using the V ery Large Array of the NRAO(1) at 90, 20, and 6 cm. This work continue s the study of the properties of young, bright supernova remnants (SNR s) begun with 3C 391 (Moffett and Reynolds 1994). Here we present high -resolution images of total intensity, polarization, and spectral inde x of W49B. In large-scale morphology it is basically a box-shaped remn ant expanding into an apparently inhomogeneous medium, but we also fin d small-scale structures, arcs and remnants, which lie interior to the outer shell in projection. The actual spatial position of the filamen ts is unknown, though we suspect that they are in the remnant interior rather than on the front or back face of the blast wave. In addition, their distribution in two dimensions suggests the projection of a hel ical structure. The shell edge, sharply rising in brightness for at le ast 3/4 of its circumference, still remains unresolved at our highest resolution (4''). Thus for our assumed distance of 8 kpc, the width of the region in which the emission appears is less than 0.16 pc, indica ting short mean free paths for shock-accelerated electrons and high le vels of MHD turbulence presumably causing the scattering. We find no p olarized flux at 90 or 20 cm, with 3 sigma upper limits in polarized i ntensity of 36 mJy at 90 cm (45'' resolution) and 7.2 mJy at 20 cm (5' ' resolution), or 22 mu Jy arcsec(-2) and 370 mu Jy arcsec(-2), respec tively, for any emission extended on those scales. Polarized flux is p resent at 6 cm, but at a very low mean polarized fraction (total polar ized flux divided by total flux) of 0.44% +/- 0.06%, which, as for 3C 391, is much lower than typical for bright SNRs at this frequency. The morphology in polarized intensity is poorly correlated with that in t otal intensity. We see excursions in the polarized fraction up to at l east 10% in a few locations, but even where polarization is seen, typi cal levels are a few percent. Tangled or disordered magnetic fields in the emitting region of the radio shell may be responsible for depolar izing the radio synchrotron radiation, but some form of internal Farad ay depolarization may also occur. We estimate the foreground Faraday r otation measure to be about -450 rad m(-2) similar to that found for 3 C 391, which is also in the Galactic plane and just about as distant. Spectral index images created from the total intensity images show tha t the spectral index across W49B is constant to within about Delta alp ha similar to 0.1 in bright regions. This result weakly supports a com mon origin of the radio-emitting electrons, as in the blast wave, rath er than in inhomogeneous turbulent regions of differing properties due to the stochastic (second-order Fermi) acceleration process. Variatio ns at the level of Delta alpha similar to 0.1 are seen, but their sign ificance is doubtful. New observations at 90 cm, using experimental th ree-dimensional imaging technology, may improve on this limit.