Issue |
A&A
Volume 428, Number 2, December III 2004
|
|
---|---|---|
Page(s) | 383 - 399 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20040252 | |
Published online | 26 November 2004 |
Exploring the X-ray sky with the XMM-Newton bright serendipitous survey *,**
1
INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: [rdc;tommaso;caccia;paola;braito]@brera.mi.astro.it
2
Instituto de Fisica de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain e-mail: [barcons;carreraf]@ifca.unican.es
3
X-ray Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK e-mail: [mgw;jat]@star.le.ac.uk
4
Max-Planck-Institut für Extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: [hbrunnerl;ile]@mpe.mpg.de
5
Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK e-mail: mjp@mssl.ucl.ac.uk
6
Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany e-mail: [glamer;aschwope]@aip.de
Received:
12
February
2004
Accepted:
14
July
2004
We present here “The XMM-Newton Bright Serendipitous Survey”,
composed of two flux-limited samples: the XMM-Newton Bright Source Sample (BSS,
hereafter) and the XMM-Newton “Hard” Bright Source Sample (HBSS, hereafter)
having a flux limit of erg cm-2 s-1
in the
0.5-4.5 keV and 4.5-7.5 keV energy band, respectively. After discussing the
main goals of this project and the survey strategy, we present the basic
data on a complete sample of 400 X-ray sources (389 of them belong to
the BSS, 67 to the HBSS with 56 X-ray sources in common) derived from the
analysis of 237 suitable XMM-Newton fields (211 for the HBSS).
At the flux limit of the survey we cover a survey
area of 28.10 (25.17 for the HBSS) sq. deg.
The extragalactic number-flux
relationships (in the 0.5-4.5 keV and in the 4.5-7.5 keV energy bands) are in
good agreement with previous and new results making us confident about the
correctness of data selection and analysis.
Up to now
% (
%) of the sources have been spectroscopically
identified making the BSS (HBSS) the sample with the highest number of
identified XMM-Newton sources published so far.
At the X-ray flux limits of the sources studied here we found
that: a) the optical counterpart in the majority (
%) of cases has a
magnitude brighter than the POSS II limit (
); b) the
majority of the objects identified so far are broad line AGN both in the BSS
and in the HBSS. No obvious trend of the source spectra (as deduced from the
Hardness Ratios analysis) as a function of the count rate is measured and the
average spectra of the “extragalactic” population corresponds to a (0.5-4.5 keV) energy spectral index of
(
) for the BSS (HBSS)
sample. Based on the hardness ratios we infer that about 13% (40%) of the
sources in the BSS (HBSS) sample are
described by an energy spectral index flatter than that of the cosmic X-ray
background. Based on previous X-ray
spectral results on a small subsample of objects
we speculate that all these sources are indeed absorbed AGN with the NH
ranging from a few times 1021 up to few times 1023 cm-2. We do not find
strong evidence that the 4.5-7.5 keV survey is sampling a completely different
source population if compared with the 0.5-4.5 keV survey; rather we find that,
as expected from the CXB synthesis models, the hard survey is simply picking up
a larger fraction of absorbed AGN. At the flux limit of the HBSS sample we
measure surface densities of optically type 1 and type 2 AGN of
deg-2
and
deg-2, respectively; optically
type 2 AGN represent
of the total AGN population. Finally, we have found a clear separation, in the
hardness ratio diagram and in the (hardness ratio) vs. (X-ray to optical
flux ratio) diagram,
between Galactic “coronal emitting” stars and extragalactic sources. The
information and “calibration” reported in this paper will make
the existing and incoming
XMM-Newton catalogs a unique resource for astrophysical studies.
Key words: X-rays: diffuse background / surveys / X-rays: active galaxies
Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA). The majority of the new optical spectroscopy data used here have been obtained using the facilities of the Italian “Telescopio Nazionale Galileo” (TNG) and of the European Southern Observatory (ESO).
© ESO, 2004
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