The luminosity function and cosmological evolution of X-ray-selected BL lacertae objects

Redshifts are presented for an X-ray-selected, flux-limited sample of BL Lac objects. The median X-ray luminosity is derived. A correlation between the optical and radio luminosity of these objects is found. The complete redshift information allows measurement of the objects' evolution with redshift and calculation of a luminosity function at zero redshift. Evidence for strong negative evolution (BL Lac objects were either less common or less luminous in the recent past) is found. The differences between radio- and X-ray-selected samples of BL Lac objects are discussed