A deep ROSAT survey - XV. The average QSO spectrum and its evolution

Using a sample of 165 X-ray selected QSOs from seven deep ROSAT fields f_(0.5-2.0keV)>~4×10^-15ergcm^{- 2}s^-1], we investigate the X-ray spectral properties of an `average' radio-quiet broad-line QSO as a function of redshift. The QSO stacked spectra, in the observer's 0.1-2keV band, in five redshift bins over the range 0.1<=z<=3.2, apparently harden from an equivalent photon index of Γ~2.6 at z=0.4 to Γ~2.1 at z=2.4 as seen in other QSO samples. In contrast, the spectra in the 0.5-2keV band show no significant variation in spectral index with redshift. This suggests the presence of a spectral upturn at low energies (<0.5keV). Indeed, while at high redshifts (z>1.0) the single power-law model gives an acceptable fit to the data over the full energy band, at lower redshifts the spectra need a second component at low energies, a `soft excess'. Inclusion of a simple model for the soft excess, i.e. a blackbody component (kT~100eV), results in a significant improvement to the model fit, and yields power-law slopes of Γ~1.8-1.9, for all redshift bins. This power law is not inconsistent, within the error bars, with those of nearby active galactic nuclei (AGN) in the 2-10keV band, suggesting that the same intrinsic power-law slope may continue from 10keV down to below ~0.5keV. We caution that there is a possibility that the spectral upturn observed may not represent a real physical component, but could be caused by co-adding spectra with a large dispersion in spectral indices. Regardless of the origin of the soft excess, the average QSO spectrum has important consequences for the origin of the X-ray background: the average spectra of a typical, faint, high-redshift QSO are significantly steeper than the spectrum of the X-ray background, extending the spectral paradox into the soft 0.1-2keV X-ray band.