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The ROSAT UK Medium Sensitivity Survey: optical identification and relation to X-ray spectral properties
We present the first results on the optical identification of 89 sourceswith S(0.5-2 keV)>=1.7x10^-14 erg s^-1 cm^-2 detected in eight ROSATPosition Sensitive Proportional Counter (PSPC) observations targeted onthe second Lockman hole region. Because of the low Galactic HI columndensity towards this field [N_HI~(6-12)x10^19 cm^-2] the X-ray spectraof the sources are relatively unaffected by Galactic absorption, and arelation between X-ray spectral properties and the nature of the sourcescan be established. The optical study is based on R-band CCD images ofalmost all of the sources, optical spectroscopy of the opticallybrightest 23 per cent of sources and colour information from the APMmeasurements of the POSS plates. We find that the fraction of the X-raysources identified with galaxies is at least 10 per cent. Similarly, aminimum of 29 per cent are QSOs and ~10 per cent are stars. The galaxiesform a heterogeneous group of sources, including ellipticals withoutemission lines, normal star-forming, starburst and Seyfert galaxies. TheX-ray spectra of the various types of galaxies are on averagesignificantly harder than those of the QSOs, and similar to those of thestars. This suggests that galaxies may make a significant contributionto the soft X-ray source counts at faint flux levels and in thesynthesis of the soft X-ray background.
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