Annu. Rev. Astron. Astrophys. 1997. 35: 445-502 Copyright © 1997 by Annual Reviews. All rights reserved

6.2. Spectral Shape and Variability of the Blazar Continuum

The continuum emission of blazars is remarkably smooth and steepens gradually towards shorter wavelengths from the radio to the UV range (Impey & Neugebauer 1988). The power emitted per decade exhibits a broad peak (where ~ 1) at IR through X-ray wavelengths. This long-wavelength component is almost certainly synchrotron emission, as evidenced by its high polarization from radio through optical wavelengths. The radio core is opaque, so that radio observations do not generally probe the region of the jet closest to the central engine; in general, the synchrotron continuum becomes optically thin at submillimeter or shorter wavelengths.

The wavelength of the peak of the synchrotron luminosity in blazars anti-correlates with the ratio of X-ray to radio flux. On this basis Padovani & Giommi (1995) divided BL Lac objects into high-frequency peaked (HBL) and low-frequency peaked (LBL) objects, according to whether _rx (from 5 GHz to 1 keV) is < 0.75 or > 0.75, respectively (see schematic examples in Figure 6). The two subclasses, HBL and LBL, are sometimes referred to as XBL and RBL, respectively (see discussion in Urry & Padovani 1995).

Figure 6. Schematic broadband spectra of blazars from radio through TeV gamma rays. The low-energy component is probably due to synchrotron radiation and the high-energy component to Compton scattering of lower-energy seed photons, possibly the synchrotron photons or ambient UV/X-ray disk or line photons. Two different curves represent the average spectral shapes (Sambruna et al 1996) of HBL (high-frequency peaked BL Lac objects; dotted line) and LBL (low-frequency peaked BL Lac objects; dashed line) as defined by their ratios of X-ray to radio flux (Section 6.2). Strong emission-line blazars (i.e. flat-spectrum radio quasars, or FSRQ) have continua like those of LBL (Sambruna et al 1996).

Here we extend these definitions to all blazars. It is not yet known whether LBL and HBL represent two distinct populations or extremes of a continuous distribution of synchrotron peaks because X-ray selection favors HBL and radio selection, LBL. Arguments for continuity are given by Sambruna et al (1996a).