Name of the Speaker(s):Fossati G., Celotti A., Ghisellini G. and Maraschi L.
Institution: International School for Advenced Studies, Trieste
Title of the communication: On the relation between radio selcted and x-ray selecte BL Lacertae objects
Preliminary abstract (12 lines max.):
We discuss alternative interpretations of the differences in the Spectral Energy Distributions (SEDs) of BL Lacs found in flux limited Radio or X-ray surveys. Using a Montecarlo approach we compare the predictions of the two proposed scenarios, the first based on orientation, the second on different cut off energies, with the observed properties of BL Lacs in the complete radio sample of Stickel et al. (1991)and in the complete X-ray sample of Perlman et al. (1996). We find that both models reproduce reasonably well a number but not ALL the observed properties of the samples. In particular in both models the redshift distributions conflict with the observed ones. We then explore an alternative approach based on the idea that a physical parameter should govern the shape of the SEDs. Assuming an empirical relation between spectral shape and luminosity we show that the properties of the two surveys can be reproduced with only minor discrepancies.