ASCA observations of deep ROSAT fields – V. The X‐ray spectrum of hard X‐ray selected QSOs

We present an analysis of the ROSAT and ASCA spectra of 21 broad-line active galactic nuclei (AGN) quasi-stellar objects (QSOs) with detected in the band with the ASCA GIS. The summed spectrum in the ASCA band is well described by a power law with , flatter than the average spectral index of bright QSOs and consistent with the spectrum of the X-ray background in this band. The flat spectrum in the ASCA band could be explained by only a moderate absorption (∼ assuming the typical AGN spectrum, i.e. a power law with . This could in principle suggest that some of the highly obscured AGN, required by most X-ray background synthesis models, may be associated with normal blue QSOs rather than narrow-line AGN. However, the combined ASCA–ROSAT spectrum is well fit by a power-law of with a spectral upturn at soft energies. It has been pointed out that such an upturn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless, if real, it could imply that the above absorption model suggested by the ASCA data alone is ruled out. Then a large fraction of QSOs could have ‘concave’ spectra i.e. they gradually steepen towards softer energies. This result is in agreement with the BeppoSAX hardness ratio analysis of ∼100 hard X-ray selected sources.