Rosat pointed observations of a complete sample of radio-selected BL
Lacs (RB Ls) have been analyzed. The 0.15 - 2.0 ke V energy distributions of
RBLs are well fitted by a single power law model with Galactic absorption; for
three bright sources more complex spectral shapes are indicated. The Rosat energy
distributions of RBLs are broad, spanning a large range of photon indices
(1 - 3). No differences are found comparing the Rosat spectral index distributions
of RBLs and those of X-ray selected BL Lacs (XBLs ), while both classes
have steeper Ros at spectra than Flat Spectrum Radio Quasars (FSRQs). A reanalysis
of the Einstein IPC data indicates that RBLs have flatter spectra in the
range 0.3 - 3.5 ke V than in the Rosat range. The Rosat photon indices correlate
with the redshifts for the three classes together. ·within the single class, a
correlation is present between the Rosat and the radio filLx density.
The multifrequency spectra of the three Blazar samples have been studied.
Radio-selected objects are characterized by steep radio-to-optical continua,
while in X-ray selected sources the bulk of the emission is at higher energies
(UV/ soft X-rays). We found that the radio-to-optical spectral index is correlated
with the shape of the X-ray continuum and with the bolometric luminosity
and/ or redshift, and that the three classes have different spectral properties but
form a continuous sequence rather than separate populations.
The radio to X-ray emission of Blazars has been interpreted using homogeneous
and inhomogeneous Synchrotron-Self Compton models. It is shown
that the observed differences of the three classes can not be reconciled within
the beaming scheme in terms of different orientation of the jet only. Rather,
a spread of intrinsic parameters is required, with X-ray selected objects having higher magnetic fields and smaller dimensions than the radio-selected ones. We
conclude that, from the point of view of their continuum emission, Blazars form
a unique population dominated by the same emission mechanism operating on
different physical scales.
