Far Ultraviolet Spectroscopic Explorer survey of Magellanic Cloud supernova remnants

We report the progress to date from an ongoing unbiased ultraviolet survey of supernova remnants in the Magellanic Clouds using the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. Earlier work with FUSE and other instruments has indicated that optical and/or X-ray characteristics of supernova remnants are not always good predictors of their brightness in the ultraviolet. This survey is obtaining spectra of a random large sample of Magellanic Cloud supernova remnants with a broad range of radio, optical, and X-ray properties. We proposed 39 objects in the Large Magellanic Cloud and 11 objects from the SmallMagellanic Cloud, with a standard request of 10 ks per object using the FUSE 3000 square aperture. To date, 39 objects have been observed in the survey (38 in the LMC and 1 in the SMC) and 15 have been detected, a detection rate of nearly 40%. Our survey has nearly tripled the number of UV-detected SNRs in the Magellanic Clouds (from 8 to 22). Because of the diffuse source sensitivity of FUSE, upper limits on nondetected objects are quite sensitive in many cases, dependent on night observing fraction and whether stellar light contamination plays a role for a given object. Estimated total luminosities in O VI, based simply on scaling the flux at the observed positions to an entire object, span a broad range from considerably brighter to many times fainter than the inferred soft X-ray luminosities, indicating that O VI can be an important and largely unrecognized coolant in certain objects. We compare the optical and X-ray properties of the detected and nondetected objects but do not find a simple indicator for ultraviolet detectability. Nondetections may be due to clumpiness of the emission, high foreground extinction, slow shocks whose emission gets attenuated by the Magellanic interstellar medium, or a combination of these effects. The characteristics of individual detected supernova remnants are summarized in an Appendix.