1996A&A...312....1M

We have made cross-correlation analyses of (2-15keV) HEAO-1 A2 and 1keV ROSAT PSPC All-Sky Survey maps over a selected area (∼4000deg²) with high galactic latitude (b>40deg). We have calculated the correlations for the bright ROSAT sources and residual background separately with the HEAO-1 A2 TOT (2-10keV) and HRD (5-15keV) maps. In any case, the angular dependence of the cross-correlation function (CCF) was consistent with expected from the HEAO-1 A2 collimator response function and the smoothing function of the ROSAT 1keV map. The amplitude of the bright ROSAT source - A2 CCFs is 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 is 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 1keV auto-correlation function (ACF) found by Soltan et al. (1995) for the same ROSAT data. Their significant angular CCF at a scale of θ≲5deg is of an unknown origin and it could represent a new class of cosmic X-ray sources. If this 1keV ACF has a hot plasma spectrum with kT∼2keV, 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 θ∼2.5deg. On the other hand, if this component has a lower temperature or steeper spectrum, a major modification to the population synthesis model and/or an introduction of new classes would be needed.