  Optical spectroscopy has enabled us to identify the optical counterparts to
over 200 faint X-ray sources to a flux limit of S_(0.5-2keV)=4x10^-15 erg s^-1
cm^-2 on 5 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 0.5-2 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^-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 $\Gamma=1.51 \pm 0.1$ from 0.1-2 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.
