The Crab-like Supernova Remnant G21.5−0.9 at Millimeter Wavelengths

We present a BIMA array image of the supernova remnant G21.5-0.9 at 94 GHz with angular resolution 8 6 × 4 5. On scales larger than our synthesized beam, our results do not indicate any radial or local variations in the acceleration or synchrotron loss processes for the relativistic particles emitting at radio wavelengths. However, the FWHM size of the radio remnant is significantly greater than that of the X-ray remnant. Either any low-frequency spectral break is distributed across the remnant or, more probably, the break frequency is in fact higher than ~100 GHz.