Knox S. Long, William P. Blair, P. Frank Winkler and the ChASeM33 team
M33 contains a large number of emission nebulae identified as supernova remnants (SNRs) based on the high [S II]:H alpha ratios characteristic of shocked gas. Using Chandra data from the ChASeM33 survey with a 0.35-2 keV sensitivity of about 2 10**34 ergs/s, we have detected more than 70 of these nebulae, yielding confirmation of their SNR identifications, and providing the largest homogeneous sample of remnants detected at optical, radio, and X-ray wavelengths in any galaxy, including the Milky Way. A detailed spectral analysis of the six X-ray brightest SNRs reveals that two, G98-31 and G98-35, have spectra that appear to be dominated by ejecta from a core-collapse explosion. In general, the X-ray detected SNRs have soft X-ray spectra compared to the vast majority of sources detected along the line of sight to M33. We found no new extended X-ray sources likely to be SNRs, and it is unlikely that there remain to be discovered any other thermally dominated X-ray SNR with luminosities in excess of about 4 10**35 ergs/s in the portions of M33 covered by the ChaSeM33 survey. We have used a combination of old and new optical and radio observations to attempt to better understand why some objects are detected as X-ray sources and others are not. There are no close analogues of Cas A, Tycho's SNR or the Crab Nebula in M33, but we have found an X-ray source with a power law spectrum coincident with a small-diameter radio source that may be the first pulsar-wind nebula recognized in the galaxy.
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