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dc.contributor.authorAlexander, David M.
dc.contributor.authorLa Franca, Fabio
dc.contributor.authorFiore, Fabrizio
dc.contributor.authorBarcons, X.
dc.contributor.authorCiliegi, Paolo
dc.contributor.authorDanese, Luigi
dc.contributor.authorDella Ceca, Roberto
dc.contributor.authorFranceschini, Alberto
dc.contributor.authorGruppioni, Carlotta
dc.contributor.authorMatt, Giorgio
dc.contributor.authorMatute, Israel
dc.contributor.authorOliver, Sebastian James
dc.contributor.authorPompilio, Fulvio
dc.contributor.authorWolter, Anna
dc.contributor.authorEfstathiou, Andreas
dc.contributor.authorHéraudeau, Ph
dc.contributor.authorPerola, Giuseppe Cesare
dc.contributor.authorPerri, Matteo
dc.contributor.authorRigopoulou, Dimitra
dc.contributor.authorRowan-Robinson, Michael
dc.contributor.authorSerjeant, Stephen B.G.
dc.creatorAlexander, David M.
dc.description.abstractWe present BeppoSAX observations of the southern S1 region in the European Large-Area Infrared Space Observatory (ISO) Survey (ELAIS). These observations cover an area of ∼1.7 deg2 and reach an on-axis (∼0.7 deg2) 2-10 keV (hard X-ray, or HX) sensitivity of ∼10-13 ergs s-1 cm-2. This is the first HX analysis of an ISOCAM survey. We detect nine sources with a signal-to-noise ratio SNRHX > 3, four additional sources with a 1.3-10 keV (total X-ray, or T) SNRT > 3, and two additional sources that seem to be associated with QSOs having SNRT > 2.9. The number densities of the SNRHX > 3 sources are consistent with the ASCA and BeppoSAX log N-log S functions. Six BeppoSAX sources have reliable ISOCAM 15 μm counterparts within ∼60″. All these ISOCAM sources have optical counterparts of R < 20 mag. Five of these sources have been previously optically classified, four as QSOs and one as a broad absorption line (BAL) QSO at z = 2.2. The remaining unclassified source has X-ray and photometric properties consistent with those of a nearby Seyfert galaxy. One further HX source has a 2.6 σ ISOCAM counterpart associated with a galaxy at z = 0.325. If this ISOCAM source is real, the HX/MIR properties suggest either an unusual QSO or a cD cluster galaxy. We have constructed MIR and HX spectral energy distributions to compute the expected HX/MIR ratios for these classes of objects up to z = 3.2 and assess the HX/MIR survey depth. The BAL QSO has an observed X-ray softness ratio and HX/MIR flux ratio similar to those of QSOs but different from those found for low-redshift BAL QSOs. This difference can be explained in terms of absorption, and it suggests that high-redshift BAL QSOs should be comparatively easy to detect in the HX band, allowing their true fraction in the high-redshift QSO population to be determined. The QSOs cover a wide redshift range (0.4 < z < 2.6) and have HX/MIR flux ratios consistent with those found for nearby IRAS and optically selected Palomar-Green QSOs. This suggests that MIR-selected QSOs of R < 20 mag come from the same population as optically selected QSOs. We confirm this with a comparison of the B/MIR flux ratios of MIR and blue-band-selected QSOs.
dc.relation.ispartofAstrophysical Journal
dc.titleThe European large-area infrared space observatory survey V: A beppoSAX hard X-ray survey of the S1 region
elsevier.issue.identifier1 PART 1
elsevier.coverdisplaydate10 June 2001

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