ON THE EVOLUTIONARY BEHAVIOR OF BL LAC OBJECTS

We present a new well defined sample of BL Lac objects selected from t he ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming a flux limited sample down to f(X) (0.5 - 2.0 keV) = 8 . 1 0(-13) ergs cm(-2) s(-1); redshifts are known for 33 objects (and 31 o f the complete sample). X-ray spectral properties were determined for each object individually with the RASS data. The luminosity function o f RASS selected BL Lac objects is compatible with results provided by objects selected with the Einstein observatory, but the RASS selected sample contains objects with luminosities at least tenfold higher. Our analysis confirms the negative evolution for X-ray selected BL Lac ob jects found in a sample by the Einstein observatory, the parameterizat ion provides similar results. A subdivision of the sample into halves according to the X-ray to optical flux ratio yielded unexpected result s. The extremely X-ray dominated objects have higher redshifts and X-r ay luminosities and only this subgroup shows clear signs of strong neg ative evolution. The evolutionary behaviour of objects with an interme diate spectral energy distribution between X-ray and radio dominated i s compatible with no evolution at all. Consequences for unified scheme s of X-ray and radio selected BL Lac objects are discussed. We suggest that the intermediate BL Lac objects are the basic BL Lac population. The distinction between the two subgroups can be explained if extreme ly X-ray dominated BL Lac objects are observed in a state of enhanced X-ray activity.