The XMM-Newton survey in the Marano field^*,**,***

I. The X-ray data and optical follow-up

¹ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany e-mail: mkrumpe@aip.de
² Landessternwarte Heidelberg-Königstuhl, 69117 Heidelberg, Germany
³ INAF – Osservatorio Astronomico di Bologna, Bologna, Italy
⁴ Universität Tübingen, Institut für Astronomie und Astrophysik, 72076 Tübingen, Germany
⁵ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse, Postfach 1312, 85741 Garching, Germany

Received: 16 June 2006
Accepted: 7 December 2006

Abstract

We report on a medium deep XMM-Newton survey of the Marano Field and optical follow-up observations. The mosaicked XMM-Newton pointings in this optical quasar survey field cover 0.6 deg² with a total of 120 ks good observation time. We detected 328 X-ray sources in total. The turnover flux of our sample is  erg cm^-2 s^-1 in the 0.2-10 keV band. With VLT FORS1 and FORS2 spectroscopy we classified 96 new X-ray counterparts.
The central 0.28 deg², where detailed optical follow-up observations were performed, contain 170 X-ray sources (detection likelihood ), out of which 48 had already been detected by ROSAT. In this region we recover 23 out of 29 optically selected quasars. With a total of 110 classifications in our core sample we reach a completeness of ~65%. About one-third of the XMM-Newton sources are classified as type II AGN with redshifts mostly below 1.0. Furthermore, we detect five high redshift type II AGN ().
We show that the true redshift distribution of type II AGN remains uncertain, since their lack of emission lines in a wide optical wavelength range hampers their identification in the redshift range . The optical and X-ray colours of the core sample indicate that most of the still unidentified X-ray sources are likely to be type II AGN. We calculate absorbing column densities and show that the ratio of absorbed to unabsorbed objects is significantly higher for type II AGN than for type I AGN. Nevertheless, we find a few unabsorbed type II AGN. The X-ray hardness ratios of some high redshift type I AGN also give an indication of heavy absorption. However, none of these type I objects is bright enough for spectral extraction and detailed model fitting. Types I and II AGN cover the same range in intrinsic X-ray luminosity, (), although type II AGN have a lower median intrinsic X-ray luminosity (log ) compared to type I AGN (log ).
Furthermore, we classified three X-ray bright optically normal galaxies (XBONGs) as counterparts. They show properties similar to type II AGN, probably harbouring an active nucleus.