BL Lacertae objects from the EXOSAT high galactic latitude survey: Constraints on the LogN-LogS and on the cosmological evolution

Using a sample of 11 X-ray selected BL Lacertae objects extracted from the EXOSAT High Galactic Latitude Survey we derive the LogN-LogS relation of BL Lacs and we compare it with the results obtained with other X-ray surveys. All EXOSAT BL Lacs are characterized by featureless optical spectra and have been detected in the radio band. Excellent agreement is found between our results and those of the Einstein EMSS and HEAO1-A2 surveys if the energy spectral slope of BL Lacs is approximately 1.5 or greater. Evidence for a drastic difference in the number-flux relation between the population of BL Lacs and that of other types of AGN has been found. A good description of the observed LogN-LogS can be obtained under the assumption of pure luminosity evolution if the BL Lacs' luminosity function is very flat or is characterized by a break. However, in contrast to the preliminary results currently available, a large number of high redshift objects are predicted by these models. Acceptable fits can also be obtained assuming no cosmological evolution and a steep luminosity function. In this case the observed redshift distribution can easily be reproduced.