The colour of the Deep ROSAT X-ray sky fluctuations

Abstract

We have carried out a fluctuation analysis in four bands (R4, R5, R6 and R7 corresponding to 0.44-1.01 keV, 0.56-1.21 keV, 0.73-1.56 keV and to 1.05-2.04 keV respectively) of two very deep ROSAT PSPC observations in directions where the galactic H{\small I} column is minimal. This has enabled us to study the average spectrum of the sources contributing to 0.5-2 keV fluxes $\sim 10^{-15} \rm \ergpcmsqps$. The best fit spectral energy index for the average faint source spectrum, $\hat{I}\pm =0.95_{-0.15}^{+0.10}$, is still steeper than the one measured for the extragalactic XRB at these energies $\hat{I}\pm \sim 0.4-0.7$, but flatter than the typical AGN spectral index $\hat{I}\pm \sim 1-1.5$. 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 $\hat{I}\pm \sim 0.4-0.6$ would contribute $(30\pm 20)$ per cent to the source counts at these fluxes if the rest of them are AGNs with energy spectral index $\hat{I}\pm \sim 1.2$.

5 pages, Latex (mn.sty), 3 figures included (epsf), postscript version also available via anonymous ftp at ftp://astsun1.ifca.unican.es/pub/ceballos/ .Accepted for publication in MNRAS