The supernova remnant Cassiopeia A at millimeter wavelengths

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 pointin g 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 spatia lly filtered and convolved to a common sampled UV range corresponding to an gular scales from 7 " to 95 ". The images give direct evidence for a steepe r spectral index in the knots outside the bright ring. Analysis of the 26 b rightest peaks gives statistical evidence for spectral index variations thr oughout the remnant. The high-frequency spectra confirm and extend the spec tral variations seen previously at lower frequencies. The spectra are large ly consistent with different power laws and not curved spectra. This provid es strong support for local variations in the acceleration of relativistic particles in Cas A.