The Cygnus Loop: A soft-shelled supernova remnant

We present ROSAT Position Sensitive Proportional Counter (PSPC) pointed obs ervations of the Cygnus Loop supernova remnant in two broad soft X-ray band s, characteristic of energies 0.25 and 1.25 keV. The spatial differences be tween the bands are obvious in an X-ray softness map, which is the ratio of the 0.25 and 1.25 keV emission. The most striking feature of the softness map is a spatially thin (< 5') shell that we detect around the entire rim o f the supernova remnant. It is not an observational artifact, for the angul ar extent is greater than the instrumental resolution. Several known intera ctions with large interstellar clouds are prominent in the softest emission , and the brightest X-ray emission associated with these clouds appears to the interior of these soft features. We conclude that over most of the blas t wave, genuinely lower temperatures behind slower shocks soften the X-ray spectrum, and this occurs where the cavity walls of the Cygnus Loop deceler ate the blast wave.