2006A&A...445..441N

We have collected a large amount of multifrequency data for objects in the Metsaehovi Radio Observatory BL Lacertae sample and computed their spectral energy distributions (SED) in the logν-log νF - representation. This is the first time the SEDs of BL Lacs have been studied with a sample of over 300 objects. The synchrotron components of the SEDs were fitted with a parabolic function to determine the synchrotron peak frequency, ν_peak. We checked the dependence between luminosities at several frequency bands and synchrotron peak frequency to test the blazar sequence scenario, which states that the source luminosity depends on the location of the synchrotron peak. We also calculated broad band spectral indices and plotted them against each other and ν_peak. The range of ν_peak in our study was considerably extended compared to previous studies. There were 22 objects for which logν_peak>19. The data shows that at 5GHz, 37GHz, and 5500Å, there is negative correlation between luminosity and ν_peak, whereas in X-rays the correlation turns slightly positive. There is no significant correlation between source luminosity at synchrotron peak and ν_peak. Several low radio luminosity-low energy peaked BL Lacs were found. The negative correlation between broad band spectral indices and ν_peak is also significant, although there is substantial scatter. Therefore we find that neither α_rx nor α_ro can be used to determine the synchrotron peak of BL Lacs. On the grounds of our results, we conclude that the blazar sequence scenario is not valid. In all our results, the BL Lac population is continuous with no hint of the bimodality of the first BL Lac samples.