A comparison of ultraviolet, optical, and X-ray imagery of selected fields in the Cygnus Loop

During the Astro-1 and Astro-2 space shuttle missions in 1990 and 1995, far ultraviolet (FUV) images of five 40' diameter fields around the rim of the Cygnus Loop supernova remnant were observed with the Ultraviolet Imaging Telescope (UIT). These fields sampled a broad range of conditions, including both radiative and nonradiative shocks in various geometries and physical scales. In these shocks, the UIT B5-band samples predominantly C Iv lambda 1550 and the hydrogen two-photon recombination continuum. Smaller contributions are made by emission lines of [He II] lambda 1640 and O III] lambda 1665. We present these new FUV images and compare them with optical H alpha and [O III] images and ROSAT HRI X-ray images. Comparing the UIT images with those from the other bands provides new insights into the spatial variations and locations of these different types of emission. By comparing with shock-model calculations and published FUV spectroscopy at select locations, we surmise that resonance scattering in the strong FUV permitted lines is widespread in the Cygnus Loop, especially in the bright optical filaments typically selected for observation in most previous studies.