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[sub (0.5–2.0 keV)]&gsim;4×10[sup -15] erg cm[sup -2] s[sup -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 0.1–2 keV band, in five redshift bins over the range 0.1&les;z&les;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 0.5–2 keV band show no significant variation in spectral index with redshift. This suggests the presence of a spectral upturn at low energies (<0.5 keV). 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∼100 eV), 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 2–10 keV band, suggesting that the same intrinsic power-law slope may continue from 10 keV down to below ∼0.5 keV. 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 0.1–2 keV X-ray band. [ABSTRACT FROM AUTHOR]