A deep ROSAT survey — XII. 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 × 10−15 erg s−1cm−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 Γ = 2.2 at S(0.5–2 keV) = 1 × 10−13 erg s−1cm−2 to Γ ≃ 1.7 below 1 × 10−14 erg s−1cm−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 Γ = 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 Γ = 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.