X-ray constraints on the warm-hot intergalactic medium

Three observational constraints can be placed on a warm-hot intergalactic medium (WHIM) using ROSAT Position Sensitive Proportional Counter pointed observations and survey data: the emission strength, the energy spectrum, and the fluctuation spectrum. The upper limit to the emission strength of the WHIM is keV 7.5 +/- 1.0 s(-1) cm(-2) sr(-1) keV(-1) in the 3/4 keV band, an unknown portion of which value may be due to our own Galactic halo. The spectral shape of the WHIM emission can be described as thermal emission with, although log T = 6.42 the true spectrum is more likely to come from a range of temperatures. The values of the emission strength and spectral shape are in reasonable agreement with hydrodynamical cosmological models. The autocorrelation 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 WHIM and the contributions due to point sources, is less than or similar to0.002 for in the 10' < <theta> < 20' in the 3/4 keV band. This value is lower than the Croft et al. cosmological model by a factor of <similar to>5 but is still not inconsistent with cosmological models. It is also found that the normalization of the extragalactic power-law component of the SXRB spectrum must be 9.5 +/- 0.0 keV s(-1) cm(-2) sr(-1) keV(-1) to be consistent with the ROSAT All-Sky Survey.