7 Discussion

The large database of X-ray spectra of blazars we have collected has allowed to test the blazar sequence scheme aiming to unify the different behaviors of blazars on the basis of a single parameter, i.e. the bolometric observed luminosity. We have found that the proposed parameterization can account for the average properties of the blazars in our sample, even if the scatter around the predicted average quantities is sometimes large.

We confirm, on a statistical basis, that more powerful blazars emit the X-rays by the inverse Compton process, while in less powerful objects the dominant mechanism is synchrotron, and the transition is smooth, with LBL possibly showing both contributions.

The Fossati et al. (1998) parameterization scheme to reproduce the blazar SED is able to fit also our new hard X-ray data and the correlations between broad band indices of the sources in our sample. However, we propose to slightly change this parameterization especially for the low power objects (i.e. the HBLs), in line with the idea that HBLs are characterized by a pure synchrotron self-Compton spectrum, without extra contributions produced by non locally produced seed photons.

In this paper, and in Fossati et al. (1998), the power law relation between the synchrotron peak frequency and the radio luminosity changes slope at some critical radio power, of the order of 3- $10\times 10^{42}$ erg s^-1. This agrees with the absence of broad emission lines in these objects. It has still to be proven, however, if the non visibility of emission lines in low power objects is due to a genuine lack of emitting clouds, or is due to a weak ionizing continuum. If the latter hypothesis is true, then we expect that the broad line region indeed exists, but at smaller radii than in more powerful objects. In this case, the lack of the external Compton component is not due to the lack of external photons, but possibly to the fact that the dissipation region in these sources is beyond the broad line region: in this case the corresponding energy density of line photons is seen, in the comoving frame of the blob, depressed by the square of the bulk Lorentz factor.

Acknowledgements

We thank the anonimous referee for useful suggestions and Luigi Costamante for discussions. This research made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, Caltech, under contract with the National Aeronautics and Space Administration. We acknowledge finantial support from the MURST.