AND ASTROPHYSICS On the evolutionary behaviour of BLLac objects?

Abstract. We present a new well defined sample of BLLac objects selected from the ROSAT All-Sky Survey (RASS). The sample consists of 39 objects with 35 forming a flux limited sample down to fX(0:5 − 2:0 keV) = 8 10−13 ergs cm−2 s−1; redshifts are known for 33 objects (and 31 of the complete sam-ple). X-ray spectral properties were determined for each ob-ject individually with the RASS data. The luminosity function of RASS selected BLLac 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 evo-lution for X-ray selected BLLac objects found in a sample by the Einstein observatory, the parameterization provides similar results. A subdivision of the sample into halves according to the X-ray to optical flux ratio yielded unexpected results. The extremely X-ray dominated objects have higher redshifts and X-ray luminosities and only this subgroup shows clear signs of strong negative evolution. The evolutionary behaviour of ob-jects with an intermediate spectral energy distribution between X-ray and radiodominated is compatiblewith no evolution at all. Consequences for unified schemes of X-ray and radio selected BLLac objects are discussed. We suggest that the intermediate BLLac objects are the basic BLLac population. The distinction between the two subgroups can be explained if extremely X-ray dominated BLLac objects are observed in a state of enhanced X-ray activity