A Comparison of the Radio / Submillimetre Spectra of Bl-Lacertae Objects and Flat Spectrum Radio Quasars

We present quasi-simultaneous radio, millimetre and submillimetre spectra of 22 BL Lac objects and 24 radio-loud, violently variable quasars. These data are used to test the hypothesis that BL Lacs and OVV quasars are fundamentally different objects. We find a considerable degree of consistency of the overall spectral shape in all sources and conclude that the same basic physical model applies to the continuum emission over this frequency range in both cases. There is clear evidence in the data for the BL Lacs to have flatter 150-375 GHz spectra, and we attribute the effect to an intrinsic difference between the underlying jets in the two types of object. A similar difference in the 37-270 GHz spectral indices between the two types can be attributed to a simple redshifting of the synchrotron self-absorption turnover.