ULTRAVIOLET IMAGING TELESCOPE OBSERVATIONS OF THE CYGNUS LOOP

Ultraviolet images of a 40' field in the northeast part of the Cygnus Loop were made by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 mission in 1990 December. This Letter compares the longest far-UV (B5) exposure with ground-based and X-ray observations and with models. At bright knots observed by IU E, the B5 bandpass includes flux from several lines. However, supernova remnant (SNR) shock models imply that C IV lambda-1550 dominates for more typical conditions in SNRs, and IU E spectra demonstrate a strong correlation between C IV lambda-1550 flux and UIT B5 flux. The B5 image resembles images in [O III] lambda-5007, but to the east of the edge of thc [O III] filaments there is a faint C IV emission, consistent with nonradiative shocked material behind the SNR blast wave. Conditions in shocked gases that lead to C IV emission are similar to those that produce [O III]. Hence the ratio of the B5 and [O III] intensities can be used to map the optical depths of the sheetlike regions that emit the C IV resonance line. A feature in the B5 emission closely follows a "bow shock" shape in Einstein HRI imagery; its morphology is consistent with an encounter of the SNR shock with a cloud outlined by a column density enhancement found in H I maps.