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

8.4. Inverse Compton Models: The Gamma-Ray Jet

Understanding the gamma-ray emission in blazars is particularly important because, at least during flare states, it dominates the bolometric luminosity. Here we discuss models that attribute the low-energy component to synchrotron emission and the high-energy component to inverse Compton emission from the same population of relativistic electrons.

For a homogeneous emission region, neglecting Klein-Nishina effects and assuming the seed photon spectrum is not too broad, the Compton emission closely mimics the synchrotron spectral shape - both depend primarily and in the same way on the electron spectrum. To first order, the peak of the Compton spectrum is due to the same electrons (of energy _e) as the synchrotron peak, independent of the seed photons. Pairs of wavelengths in the two spectral components in the same ratio as the peak wavelengths are also produced by the same electrons. Thus, if variations are due to changes in the electron spectrum, this class of models predicts a close correlation between flux variations for each wavelength pair. The coherent variability of the synchrotron and Compton components of LBL and HBL summarized at the beginning of the present section supports this simple scheme in broad terms. More realistic models should take into account the effects of inhomogeneity in the jet.

The seed photons could come from various sources. Synchrotron photons are produced copiously within the jet (Maraschi et al 1992) but we measure only their apparent density. The greater the beaming (bulk Lorentz factor ), the lower the intrinsic photon energy density in the jet frame ( ^-4). If an accretion disk is present, it is a strong source of photons (Dermer & Schlickeiser 1993), but at the distance implied by the gamma-ray transparency condition, their intensity as seen in the jet frame is greatly reduced. Alternatively, photons produced at the disk or nucleus can be reprocessed and/or scattered and therefore isotropized in a region of appropriate scale (Sikora et al 1994); their energy density is then amplified in the jet frame by a factor ².

The relative importance of the possible sources of soft photons - the jet itself, the accretion disk, or the broad emission line region, illuminated either by the disk or by the jet - must depend on the particular characteristics of the source. In different blazars, the origin of the seed photons may well be different: Perhaps the external-Compton (EC) case is more important in strong emission-line objects like 3C 279 and PKS 0528+134 (Sambruna et al 1996b), whereas the SSC model is applicable to weak-lined blazars like Mrk 421 and PKS 2155-304 (Zdziarski & Krolik 1993, Ghisellini & Madau 1996, Ghisellini & Maraschi 1996).

In a homogeneous SSC model, the ratio between the two peak wavelengths uniquely determines the energy of the radiating electrons: _S / _C ~ _e². For EC models, the relation is less straightforward and obviously leads to different physical parameters for the emitting region. For 3C 279, homogeneous SSC models yield lower magnetic fields than EC models and have difficulties accounting for the shortest variability time scales (Ghisellini & Maraschi 1996). Both models are consistent with the (presently) observed spectral variability; the larger amplitude in the Compton component compared to the synchrotron component is explained naturally by the SSC model (Maraschi et al 1994) but is also consistent with an EC scenario where nonlinearity can be caused by a variation of the bulk flow speed or by an external mirror effect for the synchrotron photons (Ghisellini & Madau 1996).