X-Ray Constraints on the Warm-Hot Intergalactic Medium

Three observational constraints can be placed on a warm-hot intergalacticmedium (WHIM) using \rosat PSPC pointed and survey data, the emission strength,the energy spectrum, and the fluctuation spectrum. The upper limit to theemission strength of the WHIM is 7.5 +/- 1.0 keV s^-1 cm^-2 sr^-1 keV^-1 in the3/4 keV band, an unknown portion of which value may be due to our own Galactichalo. The spectral shape of the WHIM emission can be described as thermalemission with log T=6.42, although the true spectrum is more likely to comefrom a range of temperatures. The values of emission strength and spectralshape are in reasonable agreement with hydrodynamical cosmological models. Theautocorrelation function in the 0.44 keV < E < 1.21 keV band range, w(theta),for the extragalactic soft X-ray background (SXRB) which includes both the WHIMand contributions due to point sources, is less than about 0.002 for 10arcminutes < theta < 20 arcminutes in the 3/4 keV band. This value is lowerthan the Croft et al. (2000) cosmological model by a factor of about 5, but isstill not inconsistent with cosmological models. It is also found that thenormalization of the extragalactic power law component of the soft X-raybackground spectrum must be 9.5 +/- 0.9 keV s^-1 cm^-2 sr^-1 keV^-1 to beconsistent with the ROSAT All-Sky Survey.Comment: 5 pages, 2 figures, submitted to Astrophysical Journal Letter