The XMM-Newton Bright Serendipitous Survey

 

RATIONALE

With the aim of complementing the results obtained by medium-deep XMM-Newton and  Chandra  surveys (see e.g. Brandt and Hasinger, 2005 for a recent review) we have built up, in the last few years, a complete and representative  sample of 400 bright serendipitous XMM-Newton sources selected at high galactic latitude (|bII|>20o) using the EPIC MOS2 data.

This project, which is part of the follow-up program being conducted by the  XMM-Newton Survey Science Centre (SSC), is  led by the Osservatorio Astronomico di Brera and is  known as  the ``XMM-Newton Bright Serendipitous Survey'' (XMM BSS)

The survey consists of two complete flux-limited (Sx > 7x10-14 erg cm-2 s-1)  samples: the XMM Bright Sample (XMM BS, 389 sources on ~28 sq deg of sky)  and the XMM Hard Bright Sample  (XMM HBS, 67 sources on ~25 sq deg of sky) selected  in the 0.5-4.5 keV and 4.5-7.5 keV energy bands, respectively. This approach was dictated by the need to study the composition of the source population (in term of observed and intrinsic  energy distribution and absorption properties) as a function of the energy selection band and to reduce the strong bias against absorbed sources which occurs when selecting in soft (E<2 keV) X-rays. The survey strategy, the XMM-Newton MOS2 fields used to define the sample and the  XMM BS and XMM HBS source samples   have been reported in Della Ceca et al. (2004).   

The well defined criteria (completeness, representativeness, etc..) of these samples will allow both a detailed study of sources of high individual interest as well as statistical population studies. In particular, the XMM BSS survey will be fundamental in complementing other medium and deep XMM-newton and Chandra survey programs (having fluxes 10 to 100 times fainter and covering a smaller area of the sky) and will provide a larger baseline for all evolutionary studies. Moreover, the good X-ray statistics which characterizes most of the sources in the XMM BSS sources, combined with the relative brightness of their optical counterparts, allow us to investigate in detail their physical properties.

 

THE SPECTROSCOPIC IDENTIFICATION WORK DONE SO FAR

 

The main general properties of this sample are:

 

·        the majority of the X-ray sources have enough statistics (hundreds to thousands of counts when the data from the three EPIC CCDs --MOS1, MOS2 and pn-- are considered) to allow X-ray studies in terms of energy distributions,  absorption properties, source extent and flux variability;

·         the optical counterpart of the majority (~90%) of the X-ray sources has a magnitude brighter than the POSS II limit (R ~21mag). Among the few  blank fields, we are already finding rare and interesting classes of X-ray sources, like high z clusters of galaxies, BL Lacs, highly absorbed AGN, Type II QSOs.

 

Up to now (March 21, 2007)  349 sources have been spectroscopically identified, leading to the very high identification rate of  ~87% in the BS sample and  of ~97%  in the HBS sample. This high identification rate was reached through several observing runs carried out at TNG, ESO and Calar Alto, as well as data from the literature (for ~15% of our objects).  The optical breakdown of the sources identified so far is reported below:

 

 The current identification breakdown of the XMM BSS

 

BS sample (0.5-4.5 keV)

HBS sample (4.5-7.5 keV)a

Objects

389

67

Identified

                         338 (87%)

                          65 (97%)

Type 1 AGNsc

245

42

Type 2 AGNsc

19

19

Clusters of Galaxiesb

8

1

BL Lac Objects

6

1

Normal Galaxies

                               2

0

Stars

                              58

2

 

NOTE:

a) 57 sources are in common between the BS and HBS;

b) please note that the source detection algorithm is optimized for point-like sources, so the sample of clusters  of galaxies is not statistically complete or representative;

c) for 31 AGNs the classification is based on the X-ray analysis (Caccianiga et al., 2007, in preparation).

 

SELECTED RESULTS

 

Data from the XMM BSS project were already used to discuss some open and hot questions in the current astrophysics, like:

 

--- the hidden AGN nature  of the ``X-ray bright optically normal galaxies´´ (Severgnini et al. 2003);

 

--- the broad band properties of the X-ray selected sources (AGN, Galaxies and Stars) (Della Ceca et al. 2004);

 

--- the statistical properties of X-ray selected AGN (e.g. mean spectra index; X-ray number-flux  relationship of X-ray sources and of optically Type 1 and optically Type 2 AGN  separately) (Della Ceca et al., 2004);

 

--- the relationship between optical and X-ray absorption in AGN and the  consequence for the unification model of AGN (Caccianiga et al. 2004);

 

--- the differences between radio-loud and radio-quiet X-ray selected AGN (Galbiati et al. 2005);

 

--- the discovery of one of the very few type 2 QSO at  z~2 (Severgnini et al., 2006).

 

-- the investigation  of the recent  star formation history of the Galaxy in the vicinity of the Sun (Lopez-Santiago et al., 2007)

 

 

We stress that the XMM BSS survey is covering at the moment the largest  solid angle  (~28 sq. deg.) at fluxes of the order of  10-13 erg cm-2 s-1with an almost complete spectroscopic identification.  This sample will be fundamental  to complement other medium  and deep XMM survey  programs (having fluxes from 10 up to 100 times fainter and covering a  smaller area of the sky)  and  will provide a larger baseline for all evolutionary studies.

 

A detailed optical spectral analysis for all the newly identified BSS sources is in progress (Caccianiga et al. 2007,  in preparation) as well as the derivation of the cosmological properties of X-ray selected AGN in the HBS sample (Della Ceca et al. 2007, in preparation) together with a detailed optical and X-ray investigation of the optically elusive AGN in the BSS  (Caccianiga et al., 2007,  in preparation) and a investigation of the infrared properties (from Spitzer observations)

of the absorbed and unabsorbed AGN in the HBS sample (Severgnini et al., 2007,  in preparation).

 

 

We thank the TNG, ESO and Calar Alto Time Allocation Committee for a generous and continuous allocation of observing time.

 

 

This project has received partial financial support from ASI, MIUR and INAF grants over the last few years.