A key question is, ``How are the gamma-rays produced?'' One possibility is the BLR photons impinge on the jet, appearing more intense in the jet frame due to its relative bulk motion, and are then scattered by relativistic electrons in the jet to gamma-ray energies. The BLR clouds nearest the jet may even be photoionized to a significant extent by the beamed jet emission, with essentially zero lag (Ghisellini & Madau 1996). So an important experiment is to monitor broad-emission-line variability and gamma-ray variability simultaneously. It is also critical to monitor the synchrotron light curve since (a) this reflects the underlying electron population and (b) these may be the seed photons or may contribute to photoionizing BLR clouds that then produce the seed photons.
Depending on the origin of the blazar variability, different correlations among wavebands are predicted. First we consider the case where gamma-ray variability is caused by variability of the seed photons. The synchrotron luminosity is proportional to the magnetic field strength and to the number density of electrons radiating at the particular energy; the inverse-Compton luminosity is proportional to the number density of electrons and to the luminosity of the seed photons. If the seed photons are synchrotron photons (SSC model) then the strength of the Compton component is proportional to electron density squared (equivalently, to the square of the synchrotron luminosity). That is, changing only the number density of energetic electrons creates a much larger variation in gamma-rays than in the IR/optical/UV.
This same effect can be caused in other ways, however. For example, if the seed photons are roughly constant but the jet Doppler factor increases slightly, ambient photons impinging on the jet appear brighter and the Compton emission grows again by the square of the synchrotron luminosity. (The Doppler factor is defined as ( (1 - cos ))-1, where (1 - 2)-1/2 is the bulk Lorentz factor of the jet.) This is only the case if the seed photons hit the jet from the front (or side) rather than behind, as might occur for seed photons from a disk; in that case, an increase in would reduce, rather than enhance, their apparent intensity to the jet electrons.
If the photoionization of the BLR by the jet is significant, an increase in either the electrons or the Doppler factor could cause an intrinsic increase in BLR photons, and thus an even greater increase in the Compton output.
|Figure 3: Spectral energy distribution of 3C 279 at two epochs. The high state was in 1991 June, when 3C 279 was discovered with EGRET, and the low state was in 1993 January. The variability in the Compton component (X-gamma-ray energies) is considerably greater than the variability in the synchrotron component (IR/optical/UV). (From Maraschi et al. 1994; copyright American Astronomical Society, reproduced with permission.)|
3C 279 is one of the few blazars for which there are extensive radio through gamma-ray data at multiple epochs. Figure 3 shows the spectrum during a high state in 1991 and a low state ~ 18 months later (Maraschi et al. 1994). There are two points to note from this figure. First, there is much less variability below the synchrotron peak than above it. This is true over many years of optical and gamma-ray monitoring; a large flare in early 1996 shows directly that the variability in the Compton component is much greater than in the synchrotron component (Wehrle et al. 1997). Second, the relative variations of Compton and synchrotron components are consistent with the N2 or 2 predictions, or with even larger relative changes. New data are required to determine which scenario pertains (the 1996 observations may be sufficient).