Shadowing the Diffuse Extragalactic X-Ray Background

The majority of the baryons in the presentday universe are missing in thatthey are not in galaxies or as cool intergalactic gas (<1E5K). These baryons aremost likely diffuse gas at 1E6 1E7 K in regions of modest overdensity, and thesuperposition of many such regions can produce detectable Xray emission thataccounts for about 1030% of the Xray background in the 0.21 keV range. Todetect this emission, we used the shadowing properties of the gas in the edgeongalaxy NGC 891, and we find a shadow at the 99% confidence level, consistentwith a fraction of the XRB in a diffuse cosmic component. We propose additionalobservations of a better edgeon system, NGC 5907, to determine whether shadowsare universal and to better measure the level of this cosmic diffuse XRB. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]