A deep ROSAT survey — XII. The X-ray spectra of faint ROSAT sources

Optical spectroscopy has enabled us to identify the optical counterparts toover 200 faint X-ray sources to a flux limit of S_(0.5-2keV)=4x10^-15 erg s^-1cm^-2 on 5 deep ROSAT fields. Here we present a spectral analysis of all theX-ray sources to investigate claims that the average source spectra harden atfaint X-ray flux. From a hardness ratio analysis we confirm that the averagespectra from 0.5-2 keV harden from an equivalent photon index of $\Gamma=2.2$at S_(0.5-2keV)=1x10^-13 erg s^-1 cm^-2 to $\Gamma=1.7$ below 1x10^-14 erg s^-1cm^-2. These spectral changes are due to the emergence of an unidentifiedsource population rather than the class of X-ray QSOs already identified. The128 QSOs detected so far show no evidence for spectral hardening over thisenergy range and retain a mean photon index of $\Gamma=2.2$. Recent worksuggests that many of the remaining unidentified sources are X-ray luminousgalaxies. Taking a subset identified as the most likely galaxy candidates wefind that these show significantly harder spectra than QSOs. The emission linegalaxies in particular show spectra more consistent with the residual X-raybackground, with $\Gamma=1.51 \pm 0.1$ from 0.1-2 keV. Individually thegalaxies appear to be a mixture of absorbed and unabsorbed X-ray sources.Combined with recent cross-correlation results and work on the source numbercount distribution, these results suggest that we may be uncovering the missinghard component of the cosmic X-ray background.