The colour of the deep ROSAT X-ray sky fluctuations

We have carried out a fluctuation analysis in four bands (R4, R5, R6 and R7 corresponding to 0.44-1.01, 0.56-1.21, 0.73-1.56 and 1.05-2.04 keV respectively) of two very deep ROSAT Position Sensitive Proportional Counter (PSPC) observations in directions where the Galactic H I column is minimal. This has enabled us to study the average spectrum of the sources contributing to 0.5-2 keV fluxes similar to 10(-15) erg cm(-2) s(-1). The best-fitting spectral energy index for the average faint source spectrum, alpha=0.95(-0.15)(+0.10), is still steeper than the one measured for the extragalactic Xray background (XRB) at these energies alpha similar to 0.4-0.7, but flatter than the typical active galactic nuclei (AGNs) spectral index alpha similar to 1-1.5. This result has allowed us to constrain the existence of a population of sources harder than the AGNs contributing to the source counts at these fluxes. We find that a population of X-ray sources with energy spectral index alpha similar to 0.4-0.6 would contribute (30+/-20) per cent to the source counts at these fluxes if the rest of them are AGNs with energy spectral index alpha similar to 1.2.