Far Ultraviolet Spectroscopic Explorer and Hopkins Ultraviolet Telescope Observations of Radiative Shocks in the Cygnus Loop

Abstract

We report a combination of high and low spectral resolution far-ultraviolet spectral data for several groupings of bright radiative shock filaments in the Cygnus Loop supernova remnant. The high-resolution spectra were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite and cover the 905-1187 A spectral region at 0.1-0.3 A resolution, depending on observing aperture. The low-resolution data were obtained with the Hopkins Ultraviolet Telescope (HUT) and cover the 850-1850 A spectral region at ~3 A resolution. These data sets complement each other, with the HUT data providing the broader context of a wider range of emission lines and the FUSE data resolving line profiles from nearby airglow emissions and allowing detailed line profiles to be studied. Relative line intensities and line profiles change on a wide variety of spatial scales. O VI λλ1032, 1038 emission is much more generally distributed than the optical filaments. The FUSE data provide direct insight into the effects and variability of self-absorption in the strong resonance lines of O VI and C III λ977. Variability of the central reversals in O VI with position indicate a patchy O VI distribution, with self-absorption occurring locally within the remnant. Self-absorption in C III could be due to the interstellar medium, the local regions of the remnant, or a combination. We also find evidence for overlying molecular hydrogen absorption effects on some lines and suggest an interstellar (foreground) origin. It is likely that these effects have had significant impacts on many previous investigations that based their results on the interpretation of low-resolution spectral data where these effects could not be seen.