... survey

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 newoptical spectroscopy data used here have been obtained using thefacilities of the Italian "Telescopio Nazionale Galileo'' (TNG) and of the European Southern Observatory (ESO).

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...

Tables 2-4 and Appendices are only available in electronic form at http://www.edpsciences.org

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... Centre

The XMM-Newton Survey Science Centre is an international collaboration involving a consortium of 10 institutions appointed by ESA to help the SOC in developing the software analysis system, to pipeline process all the XMM-Newton data, and to exploit the XMM-Newton serendipitous detections, see http://xmmssc-www.star.le.ac.uk

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... mission

One of the responsibilities of the XMM-Newton SSC is the production of the XMM-Newton Source Catalogue. This catalogue will provide a rich and unique resource for generating well-defined samples for specific studies, using the fact that X-ray selection is a highly efficient way of selecting certain types of objects, like for instance AGN, clusters of galaxies and active stars. The first XMM-Newton Serendipitous Source Catalogue (1XMM), released on 2003 April 7, contains source detections drawn from 585 XMM-Newton EPIC observations and a total of $\sim$30 000 individual X-ray sources having a likelihood value above 8 and good quality flags. The median flux (in the total photon energy band 0.2-12 keV) of the catalogue sources is ${\sim} 3\times 10^{-14}$ erg cm^-2 s^-1, with ${\sim} 12\%$ of them having fluxes below ${\sim} 1\times 10^{-14}$ erg cm^-2 s^-1, see http://xmmssc-www.star.le.ac.uk/

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... catalogue

http://www.ast.cam.ac.uk/~apmcat/

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... interval

The good-time interval is defined as the on-axis exposure time taken from the exposure map produced in the XMM-Newton pipeline processing system.

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... sample

The fraction of MOS2 images with a thin, medium and thick filter used here are ${\sim} 48$%, ${\sim} 46$% and ${\sim} 6$%, respectively.

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... extragalactic

Since we are primarily interested in the extragalactic number-flux relationship we have excluded from the computation the sources classified as stars (see Sect. 3.2). Based on the results presented in Sect. 3.5 we are confident that the large majority of the unidentified sources are associated to extragalactic objects.

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...positions

Using the optical position of the sources classified as type 1 AGN we have evaluated that the 90% confidence level error circle has a radius equal to ${\sim} 4^{\prime\prime}$; about 99% of the type 1 AGN are within $6^{\prime\prime}$ from the X-ray position. This is consistent with what found in other XMM-Newton surveys (Barcons et al. 2002; Fiore et al. 2003).

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... 2.6 m

As part of the AXIS (An XMM-Newton International Survey) project, see http://www.ifca.unican.es/~xray/AXIS

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... (XBS J014100.6-675328

XBS J014100.6-675328 (BL Hyi), also belonging to the HBSS sample, is a well known AM Herculis object (a polar) i.e. a binary system composed of a magnetic white dwarf and a low-mass star (see Caccianiga et al. 2004, and references therein).

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... HBSS sample

On the basis of the results presented in Sect. 3.5 we are confident that the bulk of unidentified sources both in the BSS and HBSS samples are associated with extragalactic objects and we know that XBS J014100.6-675328 is an accreting binary system (see Caccianiga et al. 2004).

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... processing

We have not used here the "pipeline processing product'' HR1 which is defined using the corrected count rate in the (0.15-0.5) keV and in the (0.5-2) keV energy band since the measured count rate in the (0.15-0.5) keV band is a strong function of the Galactic $N_{\rm H}$along the line of sight. Note that the effect on HR2 and HR3 due to the different $N_{\rm H_{Gal}}$ for the objects in the sample (which ranges between ${\sim} 10^{20}$ to 10²¹ cm^-2) is negligible.

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... survey

We note that in the HBS28 sample discussed in Caccianiga et al. (2004) none of the 19 type 1 AGN is absorbed with $N_{\rm H}$ in excess to 10²² cm^-2, implying that the fraction of X-ray absorbed type 1 AGN is less than 10% of the total type 1 AGN population. This result is also confirmed by the present HBSS sample since at most 4 (out of 39) broad line AGN are characterized by HR2 values typical of absorbed ( $N_{\rm H} > 10^{22}$ cm^-2) AGN. See also Sect. 3.4.

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... stars

The two sources that are outside the "stellar'' HR2 locus are XBS J014100.6-675328 and XBS J215323.7+173018. The first object, XBS J014100.6-675328, has been already discussed in footnote 9. The XMM spectrum, discussed in Caccianiga et al. (2004), is well fitted by a unabsorbed power-law model with $\Gamma \sim 1.53$ plus a thermal component having a temperature $kT\sim 55$ eV and an emission line at $E\sim 6.7$ keV. The second object (XBS J215323.7+173018) is a star having an XMM spectrum described by a thermal component with $kT\sim 1.6$ keV and a possible hard tail.

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... sample

This latter estimate is consistent with the fact that in the HBS28 sample discussed in Caccianiga et al. (2004) there is only one type 2 AGN with HR2 between -0.75 and -0.35 out of 8 type 2 objects.

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