The Cygnus Loop is the best-studied evolved remnant. Its high surface brightness shell and modest (3°) angular diameter has made it accessible to most observatories. Its proximity (440 pc) and low column density make it accessible to soft X-ray (and UV) instruments. Its wealth and variety of shock structures make it an ideal laboratory for studying the interaction between a modest velocity shock and interstellar clouds.
The Cygnus Loop was the very first supernova remnant imaged, via a sounding rocket instrument . That image showed the remnant as a nearly complete, limb brightened shell. Subsequent, higher resolution views have refined the overall impression provided by the first image, but have not substantially revised it. Complete mosaic images were constructed using the Einstein IPC, the ROSAT PSPC during the All-Sky Survey, the ASCA GIS, and most impressively, using a million seconds of ROSAT HRI exposure (Fig. 17.1). Chandra and XMM-Newton studies have concentrated on small regions of interest around the shell.
Global studies of the Cygnus Loop using the ROSAT PSPC and ASCA have revealed a number of interesting attributes. The ROSAT image reveals an abundance of features, including the complete south blowout region and a nearly circular bubble in the western interior . The remnant is more strongly limb brightened at the lowest energy, and the interior is found to be hotter. A general correlation with radio surface brightness is found, but there are strong variations in the radio to X-ray surface brightness. Around the circumference, the displacement between the X-ray and radio edge is as much as ±2 arcmin but averages to zero. The ASCA map shows that in the 1.5-5.0 keV band the remnant appears not limb-brightened, but centrally filled . This is ascribed to a higher temperature in the interior than on the periphery.
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