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The ROSAT International X-ray/Optical Survey (RIXOS): source catalogue
We describe the ROSAT International X-ray/Optical Survey (RIXOS), amedium-sensitivity survey and optical identification of X-ray sourcesdiscovered in ROSAT high Galactic latitude fields (|b|>28 deg) andobserved with the Position Sensitive Proportional Counter (PSPC)detector. The survey made use of the central 17arcmin of each ROSATfield. A flux limit of3x10-14ergcm-2s-1 (0.5-2keV) wasadopted for the survey, and a minimum exposure time of 8000s wasrequired for qualifying ROSAT observations. X-ray sources in the surveyare therefore substantially above the detection threshold of each fieldused, and many contain enough counts to allow the X-ray spectral slopeto be estimated. Spectroscopic observations of potential counterpartswere obtained of all sources down to the survey limit in 64 fields,totalling a sky area of 15.77deg2. Positive opticalidentifications are made for 94per cent of the 296 sources thusexamined. A further 18 fields (4.44deg2), containing 105sources above the 3x10-14ergcm-2s-1survey limit, are completely optically identified to a higher flux of8x10-14ergcm-2s-1 (0.5-2keV). Opticalspectroscopic data are supplemented by deep CCD imaging of many sourcesto reveal the morphology of the optical counterparts, and objects toofaint to register on Sky Survey plates. The faintest opticalcounterparts have R~22. This paper describes the survey method, andpresents a catalogue of the RIXOS sources and their opticalidentifications. Finding charts based on Sky Survey data are given foreach source, supplemented by CCD imaging where necessary.
| X-ray spectra of the RIXOS source sample
We present results of an extensive study of the X-ray spectralproperties of sources detected in the RIXOS survey, which is a large,nearly complete sample of objects detected serendipitously in ROSAT PSPCfields down to a flux limit of 3x10^-14ergcm^-2s^-1 (0.5-2keV). We showthat for X-ray surveys containing sources with low count rate, such asRIXOS, spectral slopes estimated using simple hardness ratios in theROSAT band can be biased. Instead, we analyse three-colour X-ray datausing statistical techniques appropriate to the Poisson regime whichremove the effects of this bias. We also show that the use ofthree-colour data enables some discrimination between thermal andnon-thermal spectra. We have then applied this technique to the RIXOSsurvey to study the spectral properties of the sample. For the AGN wefind an average energy index of 1.05+/-0.05, with no evidence forspectral evolution with redshift. Individual AGN are shown to have arange of properties, including soft X-ray excesses and intrinsicabsorption. Narrow-emission-line galaxies (NELGs) also seem to fit to apower-law spectrum, which may indicate a non-thermal origin for theirX-ray emission. We infer that most of the clusters in the sample have abremsstrahlung temperature >3keV, although some show evidence for acooling flow. The stars deviate strongly from a power-law model but fitto a thermal model. Finally, we have analysed the whole RIXOS sample(extending the flux cut-off to the sensitivity threshold of eachindividual observation) containing 1762 sources to study therelationship between spectral slope and flux. We find that the meanspectral slope of the sources hardens at lower fluxes, in agreement withresults from other samples. However, a study of the individual sourcesdemonstrates that the majority have relatively soft spectra even atfaint flux levels, and the hardening of the mean is caused by theappearance of a population of very hard sources at the lowest fluxes.This has implications for the nature of the soft X-ray background.
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