The contribution of faint active galactic nuclei to the hard X-ray background |
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| Authors: | F Fiore F La Franca P Giommi M Elvis G Matt A Comastri S Molendi I Gioia |
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| Institution: | Science Data Center, Via Corcolle 19, I–00131 Roma, Italy; Osservatorio Astronomico di Roma, Via Frascati 33, I–00044 Monteporzio, Italy; Harvard-Smithsonian Center of Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA; Dipartimento di Fisica, Universitàdegli Studi 'Roma Tre', Via della Vasca Navale 84, I–00146 Roma, Italy; Osservatorio Astronomico di Bologna, via Ranzani 1, I–40127 Bologna, Italy; IFCTR/CNR, via Bassini 15, I–20133 Milano, Italy; IRA/CNR, via Gobetti 101, I–40129 Bologna, Italy; Institute for Astronomy, 2680 Woodlawn Drive, Honolulu, HI 96822, USA |
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| Abstract: | Hard X-ray selection is the most efficient way to discriminate between accretion-powered sources, such as active galactic nuclei (AGN), and sources dominated by starlight. Hard X-rays are also less affected than other bands by obscuration. We have therefore carried out the BeppoSAX High Energy Large Area Survey (HELLAS) in the largely unexplored 5–10 keV band, finding 180 sources in ～50 deg2 of sky with flux?5×10?14 erg cm?2 s?1. After correction for the non-uniform sky coverage this corresponds to resolving about 30 per cent of the hard cosmic X-ray background (XRB). Here we report on a first optical spectroscopic identification campaign, finding 12 AGN out of 14 X-ray error boxes studied. Seven AGN show evidence for obscuration in X-ray and optical bands, a fraction higher than in previous ROSAT or ASCA – ROSAT surveys (at 95–99 and 90 per cent confidence levels respectively), thus supporting the scenario in which a significant fraction of the XRB is created by obscured AGN. |
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| Keywords: | surveys galaxies: active galaxies: general X-rays: galaxies |
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