All quiet on the western front? X-ray and optical observations of a prototypical cloud-blast wave interaction in the Cygnus Loop

We use the ROSAT High Resolution Imager and deep, wide-held, optical images to study a section of the western edge of the Cygnus Loop. Fast, radiative shocks result in a network of bright optical filaments. The most prominent X-rays are associated with and interior to these optical filaments. This X-ray enhancement is due to additional compression of hot, postshock gas by a reflected shock, which formed when the blast wave encountered a large molecular cloud in this region about 10(3) yr ago. This cloud is traced by far-infrared dust and CO emission that are concentrated toward the exterior of the bright filaments and extended both along the line of sight and roughly north-south in the plane of the sky. Farther west, a curved Balmer-dominated filament marks the edge of the blast wave, which has begun to wrap around the intervening cloud. The region is seen close to edge on, and this simple geometry enables us to determine its physical structure in detail: this is an example of the early stage of a cloud-blast wave interaction. Because the cloud is large, the lifetime of the interaction is long compared with the timescale over which the blast-wave properties (such as pressure) change, so evolution of the shock during the interaction is important. Bright optical filaments, associated limb-brightened X-rays, and fainter Balmer-dominated emission on the exterior of the remnant are all common around the rim of the Cygnus Loop. These phenomena are the result of interaction of the supernova blast wave with large clouds. The western edge is the prototypical example of the physical processes occurring throughout this supernova remnant.