The recognition of blazars and the blazar spectral sequence

We analyse a group of radio sources, a subset of the 200-mJy sample, all of which have core-jet radio structures measured with very long baseline interferometry and have flat spectra stretching from the radio to the millimetre/submillimetre band. Thus the objects have most of the properties expected of blazars. However, they display varied optical properties ranging from `Seyfert-like' objects, through BL Lac objects, to `normal' elliptical galaxies. We investigate the distribution of synchrotron peak frequencies in their spectral energy distributions (SEDs) and find a broad distribution between 10¹² and 10¹⁶ Hz. Our conclusion is that we should consider virtually all objects in the sample as blazars since much of the diversity in their classification based on traditional optical criteria arises from differences in the frequency at which the non-thermal emission begins to decline. Specifically, an object is only classified as BL Lac when its peak frequency falls in the near-infrared/optical range. We determine peak frequencies using the same method for objects from other blazar samples. An important result is that our objects do not follow the blazar spectral sequence proposed by Fossati et al. and Ghisellini et al. in which, on average, peak frequencies increase as the radio luminosity decreases. Most of our low radio luminosity sources have peaks in their SEDs at low frequencies, not at the expected high frequencies. We suggest that at least part of the systematic trend seen by Fossati et al. and Ghisellini et al. results from selection effects.