BeppoSAX observations of infrared sources in the ELAIS survey

We present BeppoSAX 1-10 keV observations of the Southern S1 region of the European Large Area ISO Survey. These are shallow observations covering an area of ~2.5 sq. deg and reaching an on-axis (~0.85 sq. deg) sensitivity of ~10<SUP>-13</SUP> ergs<SUP>-1</SUP> cm<SUP>-2</SUP>. We detect 17 hard X-ray sources, 10 of which appear to have an ELAIS ISO counterpart, we have optical spectroscopic identifications for 8 of these sources. We find 6 QSOs (one of which is a z = 2.20 broad absorption line QSO), one Seyfert 2 and an apparently normal galaxy. We construct mid-IR and hard X-ray SEDs to compute the expected hard X-ray to mid-IR flux ratios for these object types up to z = 3.2 and assess the depth of our survey. The apparently normal galaxy has an estimated hard X-ray luminosity (L<SUB>HX</SUB> > 10<SUP>44</SUP>ergs)and a flux ratio consistent with that expected for a QSO although no QSO activity is detected at optical wavelengths. Its hard X-ray luminosity appears too high for advection dominated accretion flow activity. The Seyfert 2 galaxy has a bolometric luminosity consistent with that of a ULIRG. The QSOs cover 0.39 < z < 2.59 and have flux ratios consistent with those found for nearby PG and IRAS QSOs. This suggests that hard X-ray detected, infrared selected QSOs come from the same population as optically selected QSOs. The BALQSO has an observed flux ratio similar to QSOs but is different to those found for nearby BALQSOs. This is consistent with the positive K-correction effect of a highly absorbed hard X-ray spectrum observed at high redshift, suggesting that high redshift BALQSOs should be comparitively easy to detected in the hard X-ray band.