1999ApJ...526..874L

We present ROSAT Position Sensitive Proportional Counter (PSPC) pointed observations of the Cygnus Loop supernova remnant in two broad soft X-ray bands, characteristic of energies 0.25 and 1.25 keV. The spatial differences between 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 of the supernova remnant. It is not an observational artifact, for the angular extent is greater than the instrumental resolution. Several known interactions 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 blast wave, genuinely lower temperatures behind slower shocks soften the X-ray spectrum, and this occurs where the cavity walls of the Cygnus Loop decelerate the blast wave.