The European Large-Area Infrared Space Observatory Survey V: A BeppoSAX Hard X-Ray Survey of the S1 Region

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 D1.7 deg2 and reach an on-axis (D0.7 deg2) 2-10 keV (hard X-ray, or HX) sensitivity of D10~13 ergs s~1 cm~2. This is the Ðrst HX analysis of an ISOCAM survey. We detect nine sources with a signal-to-noise ratio four SNR HX [ 3, additional sources with a 1.3È10 keV (total X-ray, or T ) and two additional sources that SNR T [ 3, seem to be associated with QSOs having The number densities of the sources SNR T [ 2.9. SNR HX [ 3 are consistent with the ASCA and BeppoSAX log NÈlog S functions. Six BeppoSAX sources have reliable ISOCAM 15 km counterparts within D60A. All these ISOCAM sources have optical counterparts of R \ 20 mag. Five of these sources have been previously optically classiÐed, four as QSOs and one as a broad absorption line (BAL) QSO at z \ 2.2. The remaining unclassiÐed source has X-ray and photometric properties consistent with those of a nearby Seyfert galaxy. One further HX source has a 2.6 p 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 Ñux ratio similar to those of QSOs but di†erent from those found for low-redshift BAL QSOs. This di†erence 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 Ñux 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 conÐrm this with a comparison of the B/MIR Ñux ratios of MIR and blue-bandÈselected QSOs. Subject headings : cosmology : observations È galaxies : active È infrared : galaxies È surveys È X-rays : galaxies