Rosat observations of the eridanus soft X-ray enhancement

We present maps of the Eridanus soft X-ray enhancement made from the ROSAT all-sky survey data. The maps are in two energy bands: 1/4 keV (0.12-0.284 keV at 10% response, the ROSAT R1 and R2 bands), and 3/4 keV (0.47-1.20 keV at 10% response, the ROSAT R4 and R5 bands). The entire enhancement spans a total of ∼35° in Galactic latitude and ~20° in Galactic longitude, centered at l ∼ 200°, b ∼ -32°. While the higher spatial resolution of the ROSAT data reveals a pattern of detailed anticorrelations with 100 μm intensity that was not evident in the HEAO 1 or earlier data, the basic morphology found in previous maps of this enhancement is confirmed. The ROSAT data also clearly show that the X-ray enhancement continues up to the Orion region in the 3/4 keV band, confirming the structure suggested by the Wisconsin sky survey maps. The ROSAT data are consistent with the interpretation of Reynolds & Ogden and Burrows et al. that this enhancement, the higher temperature component of Burrows et al., is generated for the most part by several-million-degree gas filling a huge interstellar cavity. However, we interpret the low-temperature component discussed by Burrows et al. as part of a more distant large scale (≳ 30°) diffuse background enhancement. The part of this softer X-ray enhancement examined here appears to be produced by million degree gas in the galactic halo, and is therefore unlikely to be an isolated bubble as suggested by Burrows et al. on the basis of lower resolution HEAO 1 data.