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The amplitude of variability in blazars is greater at frequencies higher than the peak of the synchrotron spectrum, and probably this is the case in the Compton component as well (although the data are more sparse). This explains in a phenomenological way why LBL are highly variable in the optical while HBL are not: for LBL, optical emission lies above the peak frequency while for HBL it lies below. At X-ray energies, on the other hand, HBL are among the most rapidly variable AGN known. (Unfortunately, LBL are relatively faint in the X-ray and so their variability has not been observed to equivalent levels.)

This points out a significant selection effect, relevant to both variability and spectroscopic studies currently available, due to the relative brightnesses at wavelengths of interest. Most high-energy X-ray data are available for HBL, while those blazars with well-studied radio and optical variability, particularly intraday variability, tend to be LBL. Also, the strongest EGRET sources are LBL and the only TeV gamma-ray sources are HBL. So with current instrument sensitivities there are very strong differences in samples studied in the different kinds of experiments. For example, HBL have been followed extensively with the Whipple Observatory but have rarely been monitored with EGRET; we therefore know little about the relative variability at gamma-ray and TeV energies (variability within the Compton component) or high-energy variability with respect to longer wavelengths, much less the relative behavior of LBL and HBL at gamma-ray energies.

Our basic picture of blazar continuum emission - although there is still much we do not know - is that it arises in a fast-moving jet filled with energetic electrons. Whether the jet is smooth or clumpy is not yet clear. If the magnetic field, electron density, and particle energy decrease outward along the jet, the highest energy synchrotron emission comes primarily from the innermost region and progressively longer wavelength emission from more extended regions. The Compton component is presumably produced by scattering of ambient UV or X-ray photons by the same electrons that are radiating the synchrotron photons. Whether the seed photons are the synchrotron photons themselves (the synchrotron self-Compton, or SSC, process), or X-ray or UV light from an accretion disk, or broad-line photons from the BLR has yet to be determined. Ghisellini and Madau (1996) explore these three options and suggest that the origin may vary from one kind of blazar to another. In weak-lined blazars like Mrk 421 or PKS 2155-304, the SSC model may apply, while in strong-lined blazars like 3C 279 (see below), the BLR photon density can dominate the local synchrotron photon density as seen by the jet electrons (the BLR intensity is enhanced due to the bulk relativistic motion of the jet).

There are only a few blazars for which good multiwavelength monitoring data are available, and they are very different in luminosity. Higher luminosity objects (usually RBL) may have physically larger jets while the HBL are more compact. Below we discuss the variability of two of the three best-studied blazars: 3C 279, a strong-emission-line object (FSRQ), and PKS 2155-304 (HBL).

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