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 activ e galactic nuclei (AGN) quasi-stellar objects (QSOs) with z similar to 1 de tected in the 2-10 keV band with the ASCA GIS. The summed spectrum in the A SCA band is well described by a power law with Gamma = 1.56 +/- 0.18, flatt er than the average spectral index of bright QSOs and consistent with the s pectrum of the X-ray background in this band. The flat spectrum in the ASCA band could be explained by only a moderate absorption (similar to 10(22) c m(-2)) assuming the typical AGN spectrum, i.e. a power law with Gamma = 1.9 . This could in principle suggest that some of the highly obscured AGN, req uired by most X-ray background synthesis models, may be associated with nor mal blue QSOs rather than narrow-line AGN. However, the combined 0.5-8 keV ASCA-ROSAT spectrum is well fit by a power-law of Gamma = 1.7 +/- 0.2 with a spectral upturn at soft energies. It has been pointed out that such an up turn may be an artefact of uncertainties in the calibration of the ROSAT or ASCA detectors. Nevertheless, if real, it could imply that the above absor ption model suggested by the ASCA data alone is ruled out. Then a large fra ction of QSOs could have 'concave' spectra i.e. they gradually steepen towa rds softer energies. This result is in agreement with the BeppoSAX hardness ratio analysis of similar to 100 hard X-ray selected sources.