THE NUMBER COUNT DISTRIBUTION FOR X-RAY SELECTED BL LACERTAE OBJECTS AND CONSTRAINTS ON THE LUMINOSITY FUNCTION

In this paper we describe a sample of 34 BL Lac objects extracted from the Einstein Extended Medium Sensitivity Survey. X-ray selection has proved to be a powerful way to find new BL Lac objects and to generate complete and well-defined samples. We compute the number-count relationship and compare it with the results of other X-ray surveys. Then, we assume different models of the intrinsic X-ray luminosity function and study the effect of beamed X-ray emission on the resulting observed luminosity function, comparing the predicted number-count distribution with the observed one. Using the available redshift information, we can exclude that the AGN X-ray luminosity function (we include in the class of AGN all emission-line objects, like quasars and Seyfert galaxies), modified by beaming, can describe that of BL Lac objects, even though the predicted log N-log S relationship is in agreement with the observations.