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Substantial progress in understanding blazars has come from multiwavelength spectral studies. Early single-epoch studies led to the development of jet models for production of the broad-band radio through X-ray continuum (e.g., Marscher 1980, Königl 1981, Ghisellini et al. 1985). More recently, such models have been extended to GeV energies to explain the EGRET observations.

Figure 2 shows a schematic representation of the observed radio through ultra-high-energy gamma-ray continuum, in power per logarithmic band (nu Fnu), for the three kinds of blazars. The low-frequency component is likely due to synchrotron radiation and the high-frequency component to Compton-scattering of lower energy photons by the same electron population (see discussion of 3C 279 below).

Figure 2: Spectral energy distributions for three kinds of blazars. The synchrotron power of strong emission line blazars (FSRQ) and low-frequency peaked blazars (LBL) peaks at submillimeter to infrared wavelengths, while that of high-frequency peaked blazars (all known HBL are BL Lac objects) peak at UV to X-ray wavelengths. The Compton powers peak at GeV energies for FSRQ and LBL and at much higher (TeV) energies for HBL. In general, FSRQ and LBL (dashed lines) are more luminous than HBL (dotted lines), so that the wavelength of the peak power output correlates with luminosity.

The weak-lined blazars, or BL Lac objects, fall into two categories, defined by Padovani & Giommi (1995) as ``Low-frequency peaked BL Lacs'' (LBL) and ``High-frequency peaked BL Lacs'' (HBL) depending on whether alpharx ident log (F5 GHz / F1 keV) / 7.68 is greater than or less than 0.75, respectively. Most HBL have been found in X-ray surveys, and so have been known previously as XBL (X-ray-selected BL Lac objects), while most LBL have been found in radio surveys and so are also known as RBL. As such surveys go deeper, however, the mix of types in a given survey will change, with increasing numbers of LBL in X-ray surveys and more HBL in radio surveys, hence the need for a quantitative and clear distinction between the two types. At this point, it is not clear whether there exist two distinct classes of BL Lac object or whether there is a continuous distribution of spectral shapes between the classically discovered LBL and HBL.

The observed differences in continuum spectral shape (Fig. 2), are that the synchrotron power of LBL peaks at submm to IR wavelengths while that of HBL peaks at UV to X-ray wavelengths, and the Compton components peak at GeV energies for LBL and at much higher (TeV) energies for HBL. HBL tend to be fainter EGRET sources than LBL even though they are a lower redshift population; their ratio of peak gamma-ray flux to peak synchrotron flux is around one or less.

The strong-emission-line blazars are denoted by FSRQ (Flat-Spectrum Radio Quasars); the label FSRQ is more or less equivalent to ``blazar'' since essentially all are highly variable and (at least some of the time) highly polarized, as well as superluminal. The continuum shapes of FSRQ are very similar to those of LBL (Sambruna et al. 1996), with synchrotron peaks at 1013-1014 Hz and Compton peaks at 1022-1023 Hz.

In general, FSRQ and LBL are more luminous than HBL, so that the wavelength of the peak power output increases with luminosity. Also, for FSRQ and LBL the ratio of Compton to synchrotron power is higher than for LBL, at least in the high state, so that the Compton power increases proportionately more than the synchrotron power with increasing luminosity. This is illustrated with real multiwavelength spectra in a paper by Sambruna et al. (1996), who discuss possible physical connections among FSRQ, LBL, and HBL. One popular hypothesis is that LBL are viewed at smaller angles than HBL, so that the difference is purely an orientation effect. Sambruna et al. conclude to the contrary that there must instead be intrinsic differences because for plausible emission models it is not possible to shift the wavelength of the peak emission by as much as four orders of magnitude.

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