1996A&A...313..356B

ROSAT data as well as archival radio and optical data are used for a broad band study of a large group of blazars with positive detection in the soft X-ray band. It is found that X-ray selected BL Lacs differ in their properties from radio selected BL Lacs which, in turn, merge smoothly into the group of highly polarized quasars in many respects. The average soft X-ray spectra of all three object classes differ slightly and X-ray selected BL Lacs seem to require absorption in excess of the Galactic N_H-values. We propose that the two types of BL Lacs are intrinsically different and either originate from different parent populations or have emission conditions with characteristically distinct physical parameters. Radio selected BL Lacs and highly polarized quasars exhibit nearly identical continuum emission properties but differ in their redshift distribution and intrinsic luminosities. They are thus objects with physically similar parsec-scale jets and might even represent different evolutionary or environmental stages of the same class of highly beamed objects. X-ray selected and radio selected BL Lacs populate distinctly different phase space regions in flux - ratio diagrams. The corresponding values might thus serve as an unambigous class - definition for the BL Lac population.