1995A&A...298..482F

We review the currently available data on active binaries selected from Einstein observatory based X-ray surveys, and compare the inferred space density with the available estimates based on optically selected samples. It is shown that, in contrast to previous reports, no disagreement is present between the optical and X-ray based estimates, and that the space density of active binaries is likely to be smaller than previous estimates based on X-ray selected data. In particular, we show that of the 18 systems in the δ>-20deg subsample of the Einstein Extended Medium Sensitivity Survey previously classified as active binaries, 6 are not to be considered as such, while a further 5 have a somewhat doubtful status. The total space density we derive is 7.7x10^–5pc^–3 if the 5 doubtful systems are retained and 3.7x10^–5pc^–3 if they are excluded. Both values are substantially lower than the value of 4.5x10^–4pc^–3 originally derived from the same subsample of Extended Medium Sensitivity Survey fields. Also, the median X-ray luminosity of X-ray selected active binaries is found to be higher than previously reported, and again in contrast with previous reports, we find no disagreement between the X-ray luminosities of X-ray and optically selected samples. We briefly discuss the implication of the revised value of the space density on the contribution of active binaries to the soft X-ray background.