We present maps of the "Monogem Ring," also known as the Gemini-Monoceros X-ray enhancement, extracted from the ROSAT All-Sky Survey in the R1 (110-284 eV), R2 (140-284 eV), and R4 + R5 (500- 1100 eV) bands. The Monogem Ring is an ∼25° diameter region of diffuse, soft X-ray emission centered at (1, b) ∼(203°, +12°). The Monogem Ring has a fragmented, shell-like structure in the R1 and R2 bands and extends across the Galactic plane with little or no absorption. Only parts of the ring are visible in the R4 + R5 band. There are intrinsic spectral variations within the ring on the scale of a few degrees and a coherent large-scale gradient from the low-longitude side to the high-longitude side. The minimum temperature observed is log (T/K) = 5.95, and the maximum temperature is log (T/K) = 6.34, assuming a Raymond-Smith model, with coronal equilibrium and solar abundances and a constant neutral hydrogen column density of 5.0 x 10¹⁹ cm^-2. We examined these data for evidence of absorption by an interstellar cloud visible in the IRAS 100 μm data. The X-ray intensity versus 100 am intensity data are consistent with absorption; however, the X-ray spectra do not exhibit the expected variation in hardness ratio.

We consider three explanations for this large region of hot gas: an OB association superbubble, an evaporating cloud, and a supernova remnant (SNR). We conclude that the SNR origin is the most likely explanation, in agreement with the conclusion of Nousek et al. based on HEAO 1 data. The Monogem Ring is modeled as an SNR in the adiabatic stage at various distances between 100 and 1300 pc. If the distance to the ring is 300 pc and the average temperature is log (T/K) = 6.15, the initial ambient density is 5.2 x 10^-3 cm^-3, the initial explosion energy is 1.9 x 10⁵⁰ ergs, the luminosity in the 100-1000 eV band is 6.1 x 10³⁴ ergs s^-1, the radius is 66.5 pc, and the age is ∼86,000 yr. The thermal pressure of the X-gas is 4.5 x 10⁴ K cm^-3, which is only 2-4 times higher than the estimated pressure of the undisturbed interstellar medium (ISM). As this remnant continues to evolve, it will most likely reach pressure equilibrium with the ISM before it reaches the radiative stage of SNR evolution. The Monogem Ring appears to be located in a region of the Galaxy with an unusually low density and has consequently evolved into a large, low surface brightness remnant.

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