A Large-Area Cross-Correlation Study of High Galactic Latutude Soft and   Hard X-ray Skies
  We have made cross-correlation analyses of (2 -- 15 keV) HEAO A2 and 1 keV ROSAT PSPC All-Sky Survey maps over a selected area ($\sim$ 4000 deg$^2$) with high galactic latitude (b>40 deg). We have calculated the correlations for the bright ROSAT sources and residual background separately with the \HEAO A2 TOT (2 -- 10 keV) and HRD (5 -- 15 keV) maps. The amplitude of the bright \ROSAT source -- A2 CCFs are consistent with expectations from model populations of AGNs and clusters of galaxies, which emit in both bands. However, the residual ROSAT background -- A2 CCFs amplitude at zero degree are about a factor of three larger than that expected from the model populations. Our soft-hard zero-lag and angular CCF results have been compared with the 1 keV auto-correlation function (ACF) found by Soltan et al. (1995) for the same ROSAT data. Their significant angular CCF at a scale of < 5 deg is of an unknown origin and it could represent a new class of cosmic X-ray sources. If this 1 keV ACF has a hot plasma spectrum with kT\sim 2 keV, contribution of this component is consistent with both our zero-lag CCF in excess of the population synthesis model prediction and the upper-limit to the angular CCF at \theta \sim 2.5 deg. On the other hand, if this component has a lower temperature or a steeper spectrum, a major modification to the population synthesis model and/or an introduction of new classes would be needed. 
Takamitsu Miyaji (1), Guenther Hasinger (2), Roland Egger (1), Joachim   Truemper (1), & Michael J. Freyberg (1) ((1) Max-Pl...