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 ROSA T 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 ∼ 10-15 erg cm-2 s-1. The best-fitting spectral energy index for the average faint source spectrum, α = 0.95-0.15 +0.10, is still steeper than the one measured for the extragalactic X-ray background (XRB) at these energies α ∼ 0.4-0.7, but flatter than the typical active galactic nuclei (AGNs) spectral index α ∼ 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 α ∼ 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 α ∼ 1.2.Partial financial support for this work was provided by the DGICYT under project PB92-0501.Peer Reviewe