Abstract
In this paper we investigate the properties of low X-ray-to-optical flux ratio sources detected in a wide area (2.5deg^2) shallow (f(0.5-8keV)~10e-14cgs) XMM-Newton survey. We find a total of 26 sources (5% of the total X-ray selected population) with log f_X/f_{opt}<-0.9 to the above flux limit. Optical spectroscopy is available for 20 of these low X-ray-to-optical flux ratio objects. Most of them are found to be associated with Galactic stars (total of 8) and broad line AGNs (total of 8).We also find two sources with optical spectra showing absorption and/or narrow emission lines and X-ray/optical properties suggesting AGN activity. Another two sources are found to be associated with low redshift galaxies with narrow emission line optical spectra, X-ray luminosities L_X(0.5-8keV)~10e41cgs and logf_X/f_opt ~ -2 suggesting `normal' star-forming galaxies. Despite the small number statistics the sky density of `normal' X-ray selected star-forming galaxies at the flux limit of the present sample is low consistent with previous ROSAT HRI deep surveys. Also, the number density estimated here is in good agreement with both the logN-logS of `normal' galaxies in the Chandra Deep Field North (extrapolated to bright fluxes) and model predictions based on the X-ray luminosity function of local star-forming galaxies.
Highlights
X-ray surveys with the ROSAT and more recently with the Chandra and the XMM-Newton observatories have demonstrated that the X-ray source population is a heterogeneous mix of objects comprising (i) powerful AGNs/QSOs, (ii) galaxy groups/clusters, (iii) low luminosity AGNs, (iv) ‘normal’ galaxies with X-ray emission dominated by stellar processes and (v) a small number of Galactic stars (Lehmann et al 2001; Barger et al 2002; Hornschemeier et al 2003)
In the present study we use the wide area XMM/2dF survey to study low X-ray–to–optical flux ratio sources to the survey limit f (0.5 − 8 keV) ≈ 10−14 erg s−1 cm−2. This translates to a soft band flux of f (0.5 − 2 keV) ≈ 5 × 10−15 erg s−1 cm−2 comparable to previous ROSAT PSPC surveys
The relatively mix of the low X-ray–to–optical flux ratio population found here is in fair agreement with the ROSAT HRI results of Lehmann et al (2001) with the exception of the larger fraction of galaxy groups identified by these authors
Summary
X-ray surveys with the ROSAT and more recently with the Chandra and the XMM-Newton observatories have demonstrated that the X-ray source population is a heterogeneous mix of objects comprising (i) powerful AGNs/QSOs, (ii) galaxy groups/clusters, (iii) low luminosity AGNs, (iv) ‘normal’ galaxies with X-ray emission dominated by stellar processes and (v) a small number of Galactic stars (Lehmann et al 2001; Barger et al 2002; Hornschemeier et al 2003). The ROSAT satellite with a field-of-view of ≈ 0.30 deg per pointing has probed the bright flux regime [f (0.5 − 2 keV) ≈ 10−15 erg s−1 cm−2] but has identified only a small number of low X-ray–to–optical flux ratio sources (Griffiths et al 1995; Griffiths et al 1996; Georgantopoulos et al 1996; Lehmann et al 2001) This can be attributed to (i) the poor positional accuracy of ROSAT rendering the optical identification of X-ray sources difficult and (ii) its low sensitivity making wide area surveys to the limit f (0.5 − 2 keV) ≈ 10−15 erg s−1 cm−2 time consuming. Our XMM-Newton observations (hereafter referred to as the XMM/2dF survey) cover an area of ≈ 2.5 deg much larger than any previous ROSAT surveys at the same flux limit This is essential to compile a large sample of low X-ray– to–optical flux ratio sources to explore their nature and to constrain the relative fraction of the different X-ray populations. Johnson-Cousin B-band using CCD photometry available for a subregion of the SGP field
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