The XMM-Newton bright serendipitous survey ^*,**

Identification and optical spectral properties

¹ INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milan, Italy e-mail: alessandro.caccianiga@brera.inaf.it
² INAF - Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy
³ Instituto de Física de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain
⁴ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85741 Garching, Germany
⁵ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK
⁶ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
⁷ INAF - IASFPA, via Ugo La Malfa 153, 90146 Palermo, Italy
⁸ Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁹ X-ray & Observational Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK


Received: 29 August 2007
Accepted: 1 October 2007

Abstract

Aims.We present the optical classification and redshift of 348 X-ray selected sources from the XMM-Newton Bright Serendipitous Survey (XBS), which contains a total of 400 objects (identification level = 87%). About 240 are new identifications. In particular, we discuss in detail the classification criteria adopted for the active galactic nuclei (AGNs) population.

Methods.By means of systematic spectroscopic campaigns using various telescopes and through the literature search, we have collected an optical spectrum for the large majority of the sources in the XBS survey and applied a well-defined classification “flow chart”.

Results.We find that the AGNs represent the most numerous population at the flux limit of the XBS survey (~10^-13 erg cm^-2 s^-1) constituting 80% of the XBS sources selected in the 0.5–4.5 keV energy band and 95% of the “hard” (4.5–7.5 keV) selected objects. Galactic sources populate the 0.5–4.5 keV sample significantly (17%) and only marginally (3%) the 4.5–7.5 keV sample. The remaining sources in both samples are clusters/groups of galaxies and normal galaxies (i.e. probably not powered by an AGN). Furthermore, the percentage of type 2 AGNs (i.e. optically absorbed AGNs with  mag) dramatically increases going from the 0.5–4.5 keV sample (%) to the 4.5–7.5 keV sample (%). We finally propose two simple diagnostic plots that can be easily used to obtain the spectral classification for relatively low-redshift AGNs even if the quality of the spectrum is not good.