Name of the Speaker(s): PAOLO PADOVANI
Instiution: Dipartimento di Fisica, II Universita' di Roma (Italy)
Title of the communication: THE RELATION BETWEEN X-RAY AND RADIO SELECTED BL LACS
Preliminary abstract (12 lines max.): We are at present at a turning point in our understanding of the relation between BL Lacertae objects selected in the radio (RBLs) and X-ray (XBLs) bands. First, it is clear that this terminology is not physical, as it is not based on intrinsic properties but on the selection band. Second, the widely discussed possibility that XBLs are viewed significantly off the beaming axis and therefore represent a class of objects intermediate between RBLs and Fanaroff-Riley type I radio galaxies (the putative parent population) is being challenged. I will describe our recent results in this field, with particular emphasis on the ROSAT spectra of XBLs and RBLs and our proposed alternative interpretation of the relationship between the two classes.