A deep ROSAT survey .12. The X-ray spectra of faint ROSAT sources

Optical spectroscopy has enabled us to identify the optical counterparts to over 200 faint X-ray sources to a flux limit of S-(0.5-2 kev) = 4 x 10(-15) erg s(-1) cm(-2) on five deep ROSAT fields, Here we present a spectral analysis of all the X-ray sources to investigate claims that the average source spectra harden at faint X-ray flux. From a hardness ratio analysis we confirm that the average spectra from 0.5 to 2 keV harden from an equivalent photon index of Gamma = 2.2 at S-(0.5-2 keV) = 1 x 10(-13) erg s(-1) cm(-2) to Gamma similar or equal to 1.7 below 1 x 10(-14) erg s(-1) cm(-2). These spectral changes are due to the emergence of an unidentified source population rather than the class of X-ray QSOs already identified, The 128 QSOs detected so far show no evidence for spectral hardening over this energy range and retain a mean photon index of Gamma = 2.2, Recent work suggests that many of the remaining unidentified sources are X-ray-luminous galaxies, Taking a subset identified as the most likely galaxy candidates we find that these show significantly harder spectra than QSOs, The emission-line galaxies in particular show spectra more consistent with the residual X-ray background, with Gamma = 1.51 +/- 0.1 from 0.1 to 2 keV, Individually the galaxies appear to be a mixture of absorbed and unabsorbed X-ray sources, Combined with recent cross-correlation results and work on the source number count distribution, these results suggest that we may be uncovering the missing hard component of the cosmic X-ray background.