BL LACERTAE OBJECTS IN THE ROSAT ALL-SKY SURVEY - NEW OBJECTS AND COMPARISON OF DIFFERENT SEARCH TECHNIQUES

We report the discovery of 34 BL Lac objects identified during the cor relation of the ROSAT All-Sky Survey (RASS) with the Hamburg Quasar Su rvey (HQS). We find that most of the newly identified sources have hig h ratios of X-ray to optical flux, log(f(X)/f(B)) > 1.3, and use this criterion as an efficient, radio-independent means for selecting candi date BL Lac objects. Combining these new sources with 43 previously kn own BL Lac objects in the similar to 6000 deg(2) surveyed area, allowe d us to construct a new sample complete down to f(X)(0.1 - 2.4keV) sim ilar or equal to 5.5 . 10(-12) ergs cm(-2) s(-1). Using this sample, w e compare the efficiency and selection effects inherent in two differe nt search techniques: The first characterized by the selection of high log(f(X)/f(B)) objects and second by selection of objects with partic ular X-ray/radio/optical colour indices. The log S - log N- diagram of the sample is consistent with results found in previous X-ray surveys , although it shows indications of more X-ray faint BL Lacs than previ ously thought. We expect similar to 200 additional BL Lac objects amon g the remaining high log(f(X)/f(B)) BL Lac candidates in the surveyed area.