The BeppoSAX High Energy Large Area Survey (HELLAS) – III. Testing synthesis models for the X-ray background |
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| Authors: | A Comastri F Fiore C Vignali G Matt GC Perola F La Franca |
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| Institution: | 1Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy;2Osservatorio Astronomico di Roma, Via Frascati 33, I-00044 Monteporzio, Italy;3BeppoSAX Science Data Center, Via Corcolle 19, I-00131 Roma, Italy;4Dipartimento di Astronomia, Universitàdi Bologna, via Ranzani 1, I-40127 Bologna, Italy;5Dipartimento di Fisica, Universitàdegli Studi 'Roma Tre', Via della Vasca Navale 84, I-00146 Roma, Italy |
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| Abstract: | The BeppoSAX High Energy Large Area Survey (HELLAS) has surveyed several tens of deg2 of the sky in the  band down to a flux of about  . The source surface density of  at the survey limit corresponds to a resolved fraction of the  X-ray background (XRB) of the order of  per cent. The extrapolation of the HELLAS  towards fainter fluxes with a Euclidean slope is consistent with the first XMM–Newton measurements, in the same energy band, which are a factor of 20 times more sensitive. The source counts in the hardest band so far surveyed by X-ray satellites are used to constrain XRB models. It is shown that in order to reproduce the  counts over the range of fluxes covered by BeppoSAX and XMM–Newton a large fraction of highly absorbed  , luminous  active galactic nuclei is needed. A sizeable number of more heavily obscured, Compton-thick, objects cannot be ruled out but they are not required by the present data. The model predicts an absorption distribution consistent with that found from the hardness ratios analysis of the so far identified HELLAS sources. Interestingly enough, there is evidence of a decoupling between X-ray absorption and optical reddening indicators, especially at high redshifts/luminosities where several broad-line quasars show hardness ratios typical of absorbed power-law models with  . |
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| Keywords: | galaxies: active X-rays: diffuse background X-rays: galaxies X-rays: general |
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