An X-ray survey in SA 57 with XMM-Newton ^*,**

¹ Dipartimento di Fisica, Universitá di Roma “La Sapienza”, P.le A. Moro 2, 00185 Roma, Italy e-mail: dario.trevese@roma1.infn.it
² Dipartimento di Fisica, Universitá di Roma “Tor Vergata”, via delle Ricerca Scientifica 1, 00133 Roma, Italy
³ ASI Science Data Centre, c/o ESRIN, via G. Galilei, 00044 Frascati, Italy
⁴ INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
⁵ Department of Astronomy, University of Wisconsin, 475 North Charter Street, Madison, WI 53706, USA

Received: 26 February 2007
Accepted: 17 April 2007

Abstract

Context.The maximum number density of Active Galactic Nuclei (AGNs), as deduced from X-ray studies, occurs at 1, with lower luminosity objects peaking at smaller redshifts. Optical studies lead to a different evolutionary behaviour, with a number density peaking at 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 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.