We report the localization of the forward and reverse shock fronts in the young supernova remnant Cassiopeia A using X-ray data obtained with the Chandra X-Ray Observatory. High-resolution X-ray maps resolve a previously unseen X-ray feature encompassing the extremity of the remnant. This feature consists of thin, tangential wisps of emission bordering the outer edge of the thermal X-ray and radio remnant, forming a circular rim, ∼2Ј .7 in radius. Radio images show a sharp rise in brightness at this X-ray rim along with a large jump in the synchrotron polarization angle. These characteristics suggest that these wisps are the previously unresolved signature of the forward, or outer, shock. Similarly, we identify the sharp rise in emissivity of the bright shell for both the radio and X-ray line emission associated with the reverse shock. The derived ratio of the averaged forward and reverse shock radii of ∼3 : 2 constrains the remnant to have swept up roughly the same amount of mass as was ejected; this suggests that Cas A is just entering the Sedov phase. Comparison of the X-ray spectra from the two shock regions shows that the equivalent widths of prominent emission lines are significantly lower exterior to the bright shell, as expected if they are respectively identified with the shocked circumstellar material and shocked ejecta. Furthermore, the spectrum of the outer rim itself is dominated by power-law emission, likely the counterpart of the nonthermal component previously seen at energies above ∼10 keV.

We present the results of VLA observations in the ground-state hydroxyl (OH) transition at 1720 MHz toward 20 supernova remnants (SNRs). We detect compact emission from four objects. For three of these objects (G32.8−0.1, G337.8−0.1, and G346.6−0.2), we argue that the emission results from masers which are shock-excited due to the interaction of the SNR and an adjacent molecular cloud. We observe a characteristic Zeeman profile in the Stokes V spectrum, which allows us to derive a magnetic field of 1.5 and 1.7 mG for G32.8−0.1 and G346.6−0.2, respectively. The velocity of the masers also allows us to determine a kinematic distance to the SNR. Our criteria for a maser to be associated with an SNR along the line of sight are that the position and velocity of the maser and SNR must agree, and the OH(1720) emission must be unaccompanied by other OH lines.

A new epoch of VLA measurements of Kepler's supernova remnant was obtained to make accurate measurements of the radio spectral index variations and polarization. We have compared these new radio images with Hα, infrared, and X-ray data to better understand the three dimensional structure and dynamics of Kepler, and to better understand the physical relationships between the various nonthermal and thermal plasmas. Spatial variations in the radio spectral index from −0.85 to −0.6 are observed between 6 cm and 20 cm. The mean spectral index is −0.71. The mean percent polarization is 3.5% at 20 cm and 6% at 6 cm. There is a strong correspondence between the radial and azimuthal profiles of the radio, X-ray, Hα, and infrared emission in different locations around the remnant although there is no single, global pattern. Spectral tomography shows that the flat-and steep-spectrum radio emissions have distinct structures. The flat-spectrum radio emission is found either at a larger radius than or coincident with the steep-spectrum emission. We interpret these spectral components as tracing forward-and reverse-shocked material, respectively. The flat-spectrum radio emission can alternatively be interpreted as the bow-shocked material (reshocked by the forward shock) from the progenitor's motion through the interstellar medium. The Hα and IR images are very similar. Their leading edges are coincident and are either in front of or coincident with the leading edges of the X-ray and radio emission. The X-ray emission matches the Hα and IR emission in places, and in other places traces the steep-spectrum radio emission. In the north there is also an anticorrelation in the azimuthal profiles around the -2remnant of the flat-spectrum radio emission and the thermal X-ray, Hα, and IR emissions. We suggest that this could be due to a relative weakening of the particle acceleration at the forward shock due to Alfvén wave damping in regions of high density.

We used the BIMA array to map the supernova remnant Cas A from 28 to 87 GHz with 2 ′′ to 7 ′′ angular resolution. Data from 75 to 87 GHz, with 19 pointing centers were combined with single dish data to form a completely sampled image. These new BIMA images were compared with VLA images at 1.5 and 5 GHz to look for spectral index variations across Cas A. The images were spatially filtered and convolved to a common sampled uv-range corresponding to angular scales from 7 ′′ to 95 ′′ . The images give direct evidence for a steeper spectral index in the knots outside the bright ring. Analysis of the 26 brightest peaks gives statistical evidence for spectral index variations throughout the remnant. The high frequency spectra confirm and extend the spectral variations seen previously at lower frequencies. The spectra are largely consistent with different power laws, and not curved spectra. This provides strong support for local variations in the acceleration of relativistic particles in Cas A.

We present the first X-ray expansion measurements of the young, bright supernova remnant Cas A. We have acquired a deep (232 ks) ROSAT high-resolution image of Cas A that is compared with archival images of similar quality taken 17 yr earlier with the Einstein Observatory. The average expansion rate between the two epochs is found to be yr Ϫ1, twice as fast as the expansion of the bright radio ring and two-thirds as fast 0.20% ‫ע‬ 0.01% as the ensemble of optical fast-moving knots. This argues that we are observing a fundamentally different hydrodynamical structure in each band, although all of these components are located on the same patchy ring. In addition, we detect significant variations in the expansion rate as a function of azimuth around the bright Xray ring. These results are discussed in the context of two classes of models, one involving a preexisting circumstellar shell and the other having a uniform or gradiated density profile that decreases monotonically away from the center.