Abstract
| - Abstract. Using a sample of 165 X-ray selected QSOs from seven deep ROSAT fields f(0.5-2.0 keV)≳4×10−15 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 observer's 0.1-2 keV band, in five redshift bins over the range , 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.
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