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Alexander, D.M.; La Franca, F.; Fiore, F.; Barcons, X.; Ciliegi, P.; Danese, L.; Della Ceca, R.; Franceschini, A.; Gruppioni, C.; Matt, G.; Matute, I.; Oliver, S.; Pompilio, F.; Wolter, A.; Efstathiou, A.; Héraudeau, P.; Perola, G.C.; Perri, M.; Rigopoulou, D.; Rowan-Robinson, M. and Serjeant, S.
(2001).
DOI: https://doi.org/10.1086/321351
URL: http://arxiv.org/abs/astro-ph/0101546
Abstract
We 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 mum 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 sigma 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.
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