A new unifying model for X--ray and radio--selected BL Lacs

Abstract

We discuss alternative interpretations of the differences in the Spectral Energy Distributions (SEDs) of BL Lacs found in complete Radio or X--ray surveys. In order to explain the different properties of radio and X-ray selected within the assumption of a single population we propose a new approach The model is based on the idea that the BL Lacs constitute a ``parameterized family'' and that the physical parameter which governs the shape of the SEDs, is (or is associated with) the bolometric luminosity. Assuming an empirical relation between spectral shape and luminosity the observational properties of the two surveys, including the redshift distributions, can be reasonably well reproduced.

4 pages, Latex (memsait.sty included). To appear in the Proceedings of "Dal Micro al Mega Parsec: Secondo Congresso Nazionale sui Nuclei Galattici Attivi (From Micro to Mega Parsec: Second Italian National Workshop on Active Galactic Nuclei)"