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.