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(( 0.5-2.0 keV)) greater than or similar to 4 x 10(-15) erg cm(-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 observ er's 0.1-2 keV band, in five redshift bins over the range 0.1 less than or equal to z less than or equal to 3.2, apparently harden from an equivalent photon index of Gamma similar to 2.6 at z = 0.4 to Gamma similar to 2.1 at z = 2.4 as seen in other QSO samples. In contrast, the spectra in the 0.5-2 keV band show no significant variation in spectral index with redshift. Th is suggests the presence of a spectral upturn at low energies (<0.5 keV). I ndeed, while at high redshifts (z > 1.0) the single power-law model gives a n 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'. Inclu sion of a simple model for the soft excess, i.e. a blackbody component (kT similar to 100 eV), results in a significant improvement to the model fit, and yields power-law slopes of Gamma similar to 1.8-1.9, for all redshift b ins. This power law is not inconsistent, within the: error bars, with those of nearby active galactic nuclei (AGN) in the 2-10 keV band, suggesting th at the same intrinsic power-law slope may continue from 10 keV down to belo w similar to 0.5 keV, We caution that there is a possibility that the spect ral 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 Tor the origin of the X-ray background: the average spectra of a typical, faint, high-redshift QSO are significantly steeper th an the spectrum of the X-ray background, extending the spectral paradox int o the soft 0.1-2 keV X-ray band.