The Discovery of a Supernova Remnant Embedded in a Giant H II Region of M33

We have combined radio, optical, and X-ray data to discover a new supernova remnant (SNR) in M33. This remnant is embedded in the giant H II region NGC 592. Our VLA-WSRT radio survey of M33 showed that NGC 592 has a nonthermal component to its radio emission. Optical images of the H II region at the KPNO 4 m allowed us to subtract the thermal source to reveal the nonthermal source. NGC 592 had also been identified in Einstein data as a soft X-ray source. Our ROSAT observation, combined with the radio and optical data, provides evidence that the X-ray source is a supernova remnant. A knot of [S II] emission was isolated in the H II region, and MMT spectra confirm that the knot has the spectrum of a supernova remnant. We use these observations to investigate the properties of the remnant and its environment. We find that this SNR is a typical middle-aged remnant except that it is expanding into dense gas of the H II region NGC 592. We also find that there is a pressure difference between the hot postshock region and the cool recombination zone. This difference may be a sign that nonthermal particles and magnetic fields provide significant pressure support in the recombination zone.