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Cross-correlation of the 2XMMi catalogue with Data Release 7 of the Sloan Digital Sky Survey
The Survey Science Centre of the XMM-Newton satellite released the firstincremental version of the 2XMM catalogue in August 2008. Containingmore than 220 000 X-ray sources, the 2XMMi was at that time the largestcatalogue of X-ray sources ever published and thus constitutes anunprecedented resource for studying the high-energy properties ofvarious classes of X-ray emitters such as AGN and stars. Thanks to thehigh throughput of the EPIC cameras on board XMM-Newton accuratepositions, fluxes, and hardness ratios are available for a substantialfraction of the X-ray detections. The advent of the 7th release of theSloan Digital Sky Survey offers the opportunity to cross-match two majorsurveys and extend the spectral energy distribution of many 2XMMisources towards the optical bands. This implies building extensivehomogeneous samples with a statistically controlled rate of spuriousmatches and completeness. We here present a cross-matching algorithmbased on the classical likelihood ratio estimator. The method developedhas the advantage of providing true probabilities of identificationswithout resorting to heavy Monte-Carlo simulations. Over 30,000 2XMMisources have SDSS counterparts with individual probabilities ofidentification higher than 90%. At this threshold, the sample has only2% spurious matches and contains 77% of all expected SDSSidentifications. Using spectroscopic identifications from the SDSS DR7catalogue supplemented by extraction from other catalogues, we build anidentified sample from which the way the various classes of X-rayemitters gather in the multi dimensional parameter space can be analysedand later used to design a source classification scheme. We illustratethe interest of this clean source sample by investigating two scientificuse cases. In the first example we show how these multi-wavelength datacan be used to search for new QSO2s. Although no specific range ofobserved properties allows us to efficiently identify Compton ThickQSO2s, we show that the prospects are much better for Compton Thin AGN2and discuss several possible multi-parameter selection strategies. In asecond example, we confirm the hardening of the mean X-ray spectrum withincreasing X-ray luminosity on a sample of over 500 X-ray active starsand reveal that on average X-ray active M stars display bluer g - rcolour indexes than less active ones. Although this catalogue of2XMM-SDSS sources cannot be used directly for statistical studies, itnevertheless represents an excellent starting point to select welldefined samples of X-ray-emitting objects.The corresponding fits file can be downloaded from the XCat-DB home page(http://xcatdb.u-strasbg.fr/).The file also contains line information for all SDSS spectroscopicentries matching a 2XMM source. Results from the cross-correlation withthe 2XMM DR3 are also available at the same location. The 2XMMi/SDSS DR7cross-correlation data file is also available at the CDS via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/527/A126
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| The Millennium Galaxy Catalogue: 16 <=BMGC < 24 galaxy counts and the calibration of the local galaxy luminosity function
The Millennium Galaxy Catalogue (MGC) is a 37.5 deg2,medium-deep, B-band imaging survey along the celestial equator, takenwith the Wide Field Camera on the Isaac Newton Telescope. The surveyregion is contained within the regions of both the Two Degree FieldGalaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey EarlyData Release (SDSS-EDR). The survey has a uniform isophotal detectionlimit of 26 mag arcsec-2 and it provides a robust,well-defined catalogue of stars and galaxies in the range 16<=BMGC < 24 mag.Here we describe the survey strategy, the photometric and astrometriccalibration, source detection and analysis, and present the galaxynumber counts that connect the bright and faint galaxy populationswithin a single survey. We argue that these counts represent the stateof the art and use them to constrain the normalizations (φ*) of anumber of recent estimates of the local galaxy luminosity function. Wefind that the 2dFGRS, SDSS Commissioning Data (CD), ESO Slice Project,Century Survey, Durham/UKST, Mt Stromlo/APM, SSRS2 and NOG luminosityfunctions require a revision of their published φ* values by factorsof 1.05 +/- 0.05, 0.76 +/- 0.10, 1.02 +/- 0.22, 1.02 +/- 0.16, 1.16 +/-0.28, 1.75 +/- 0.37, 1.40 +/- 0.26 and 1.01 +/- 0.39, respectively.After renormalizing the galaxy luminosity functions we find a mean localbJ luminosity density of .1
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