The Monogem Ring, a large angular diameter nearby old SNR, provides an excellent example of how the RASS provided new insights into low surface brightness objects . This object, also known as the Gemini-Monoceros X-ray enhancement, is a 25° diameter bright region in the 0.1-0.5 keV band. It was imaged for the first time by the RASS. It is shown to have an average temperature of 10615 K, and a column density of approximately 5 x 1019 cm~2. The PSPC observations confirm that the ring is most likely a SNR. Models suggest it is still in the adiabatic stage. If a distance of 300 pc is assumed, then the age is 86 000 yrs, and remnant is expanding in a cavity of very low ambient density, 5 x 10~3 cm~2. The overall pressure of the shocked, X-ray emitting gas is only 2-4 times that of the ambient medium. The remnant is likely to reach pressure equilibrium with the surrounding medium before it reaches the radiative evolutionary stage. This has potentially important implications for SNR evolution models in suggesting that SNRs in low-density media may end their lives differently from remnants in higher density media.
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