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

Abstract

We present an analysis of the \rosat and \asca spectra of 21 broad line AGN (QSOs) with $z\sim 1$ detected in the 2-10 keV band with the \asca \gis. The summed spectrum in the \asca band is well described by a power-law with $\Gamma =1.56\pm 0.18$, flatter that 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 ($\sim 10^{22}\mathrm{c}{\mathrm{m}}^{-2}$) assuming the typical AGN spectrum ie a power-law with $\Gamma$=1.9. 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 0.5-8 keV \asca-\rosat spectrum is well fit by a power-law of $\Gamma =1.7\pm 0.2$ 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 ie they gradually steepen towards softer energies. This result is in agreement with the \bepposax hardness ratio analysis of $\sim$ 100 hard X-ray selected sources.

7 pages, 4 figures, to be published in MNRAS

References in corpus (4)

• Resolving the extragalactic hard X-ray background
• X-ray Spectra of a large sample of Quasars with ASCA
• Optical Identification of the ASCA Large Sky Survey
• A deep ROSAT survey XV. the average QSO spectrum and its evolution

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• GMRT observations of the field of INTEGRAL X-ray sources- II (newly discovered hard X-ray sources)
• On the XMM-Newton spectra of soft X-ray selected QSOs
• Inflow Generated X-ray Corona Around Supermassive Black Holes and Unified Model for X-ray Emission