An X-ray survey in SA 57 with XMM-Newton

Abstract

Context: The maximum number density of Active Galactic Nuclei (AGNs), as deduced from X-ray studies, occurs at z ⪉ 1, with lower luminosity objects peaking at smaller redshifts. Optical studies lead to a different evolutionary behaviour, with a number density peaking at z ≈ 2 independently of the intrinsic luminosity, but this result is limited to active nuclei brighter than the host galaxy. A selection based on optical variability can detect low luminosity AGNs (LLAGNs), where the host galaxy light prevents the identification by non-stellar colours.
Aims: We want to collect X-ray data in a field where it exists an optically-selected sample of “variable galaxies”, i.e. variable objects with diffuse appearance, to investigate the X-ray and optical properties of the population of AGNs, particularly of low luminosity ones, where the host galaxy is visible.
Methods: We observed a field of 0.2 deg² in the Selected Area 57, for 67 ks with XMM-Newton. We detected X-ray sources, and we correlated the list with a photographic survey of SA 57, complete to B_J ∼ 23 and with available spectroscopic data.
Results: We obtained a catalogue of 140 X-ray sources to limiting fluxes 5 × 10^-16, 2 × 10^-15 erg cm^-2 s^-1 in the 0.5-2 keV and 2-10 keV respectively, 98 of which are identified in the optical bands. The X-ray detection of part of the variability-selected candidates confirms their AGN nature. Diffuse variable objects populate the low luminosity side of the sample. Only 25/44 optically-selected QSOs are detected in X-rays. 15% of all QSOs in the field have X/O < 0.1.

Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA. Table 2 is only available in electronic form at CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/469/1211