The European Large‐AreaInfrared Space ObservatorySurvey V: ABeppoSAXHard X‐Ray Survey of the S1 Region
Open Access
- 10 June 2001
- journal article
- research article
- Published by American Astronomical Society in The Astrophysical Journal
- Vol. 554 (1) , 18-26
- https://doi.org/10.1086/321351
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 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.Keywords
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This publication has 49 references indexed in Scilit:
- The Nonstellar Infrared Continuum of Seyfert GalaxiesThe Astronomical Journal, 2001
- The Nature of the Hard X-Ray Background Sources: Optical, Near-Infrared, Submillimeter, and Radio PropertiesThe Astronomical Journal, 2001
- The unified model and the Seyfert 2 infrared dichotomyMonthly Notices of the Royal Astronomical Society, 2001
- Optical Identification of theASCALarge Sky SurveyThe Astrophysical Journal, 2000
- A Local Infrared Perspective to Deeper ISO SurveysLecture Notes in Physics, 2000
- Observations and a model for the infrared continuum of Centaurus AMonthly Notices of the Royal Astronomical Society, 1999
- Dust emission from quasars and quasar host galaxiesMonthly Notices of the Royal Astronomical Society, 1999
- [OIII] 5007, near-IR and X-ray properties of Seyfert 2 galaxiesMonthly Notices of the Royal Astronomical Society, 1997
- Hard X-Ray Emission from Extragalactic IRAS 12 Micron Sources: Constraints on the Unified Active Galactic Nucleus Model and the Synthesis of the X-Ray BackgroundThe Astrophysical Journal, 1995
- Unified Models for Active Galactic Nuclei and QuasarsAnnual Review of Astronomy and Astrophysics, 1993