A Catalog of Soft X-Ray Shadows, and More Contemplation of the ¼ keV Background

This paper presents a catalog of shadows in the keV soft X-ray diffuse background (SXRB) that were identified by a comparison between ROSAT All-Sky Survey maps and DIRBE-corrected IRAS 100 μm maps. These "shadows" are the negative correlations between the surface brightness of the SXRB and the column density of the Galactic interstellar medium (ISM) over limited angular regions (a few degrees in extent). We have compiled an extensive but not exhaustive set of 378 shadows in the polar regions of the Galaxy (|b| ≳ 20^°) and determined their foreground and background X-ray intensities (relative to the absorbing features) and the respective hardness ratios of that emission. The portion of the sky that was examined to find these shadows was restricted in general to regions where the minimum column density is ≲4 × 10²⁰ H cm^-2, i.e., relatively high Galactic latitudes, and to regions away from distinct extended features in the SXRB such as supernova remnants and superbubbles. The results for the foreground intensities agree well with the recent results of a general analysis of the local keV emission, while the background intensities show additional, but not unexpected scatter. The results also confirm the existence of a gradient in the hardness of the local keV emission along a Galactic center/anticenter axis with a temperature that varies from 10^6.13 to 10^6.02 K, respectively. The average temperature of the foreground component from this analysis is 10^6.08 K, compared to 10^6.06 K in the previous analysis. Likewise, the average temperature for the distant component for the current and previous analyses are 10^6.00 and 10^6.02 K, respectively. Finally, the results for the keV halo emission are compared to the observed fluxes at keV, where the lack of correlation suggests that the Galactic halo's and keV fluxes are likely produced by separate emission regions.