The XMM-Newton bright serendipitous survey. Identification and optical spectral properties

TL;DR

This study presents optical classifications and redshifts for 348 X-ray sources from the XMM-Newton Bright Serendipitous Survey, highlighting the dominance of AGNs and the increase of type 2 AGNs in harder X-ray bands.

Contribution

It provides a systematic classification scheme and new diagnostic plots for low-redshift AGNs based on optical spectra and X-ray data.

Findings

AGNs constitute 80% of the XBS sources in the 0.5-4.5 keV band.

Type 2 AGNs increase from 7% to 32% in the 4.5-7.5 keV band.

Galactic sources are more prevalent in the softer X-ray band.

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 population. METHODS: By means of systematic spectroscopic campaigns 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 AGN 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 significantly the 0.5-4.5 keV sample (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 type2 AGN (i.e. optically absorbed AGNs with A_V>2mag) dramatically increases going from the 0.5-4.5 keV sample (f=N_AGN2/N_AGN=7%) to the 4.5-7.5 keV sample (f=32%). 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.