The Cambridge-Cambridge ROSAT Serendipity Survey - I.X-ray-luminous galaxies

We report on the rst results obtained from a new optical identi cation programme of 123 faint X-ray sources with S(0.5{2 keV)> 2210014 erg s01 cm02 serendipitously detected in ROSAT PSPC pointed observations. We have spectroscopically identi ed the optical counterparts to more than 100 sources in this survey. Although the majority of the sample (68 objects) are QSOs, we have also identi ed 12 narrow emission line galaxies which have extreme X-ray luminosities (1042 < LX < 1043:5 erg s01). Subsequent spectroscopy reveals them to be a mixture of starburst galaxies and Seyfert 2 galaxies in approximately equal numbers. Combined with potentially similar objects identi ed in the Einstein Extended Medium Sensitivity Survey, these X-ray-luminous galaxies exhibit a rate of cosmological evolution, LX / (1 + z) 2:561:0, consistent with that derived for X-ray QSOs. This evolution, coupled with the steep slope determined for the faint end of the X-ray luminosity function (8(LX) / L 01:9 X), implies that such objects could comprise 15{35 per cent of the soft (1{2 keV) X-ray background.