2001/01/30 by D. M. Alexander, F. La Franca, F. Fiore +21 · 3 citations
Physics and Astronomy · #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Galaxies: Formation, Evolution, Phenomena #Galaxy #Gamma-ray bursts and supernovae #Infrared #Observatory #Physics #QSOS #Redshift #Space observatory #astro-ph
paper · pdf · doi:10.1086/321351
27 pages (draft format), 10 figures, ApJ, in press (May 20 edition)
arxiv created 2001/01/30 · openalex publication_date 2001/06/10 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
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 deg 2 and reach an on-axis (~0.7 deg 2 ) 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 SNR HX > 3, four additional sources with a 1.3-10 keV (total X-ray, or T ) SNR T > 3, and two additional sources that seem to be associated with QSOs having SNR T > 2.9. The number densities of the SNR HX > 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.