|Annu. Rev. Astron. Astrophys. 1997. 35:
Copyright © 1997 by . All rights reserved
8.1. Summary of Variability Results
Recent observations of blazars have yielded six key points critical for understanding the continuum. First, blazar spectra are characterized by two broad spectral components (Section 6.2, Figures 6 and 7), one with peak power at low frequencies (IR to soft X rays) and one with peak power at very high energies (GeV to TeV gamma rays).
Second, variability in the low frequency component is much more pronounced above the peak frequency, and the variability amplitude increases with frequency so that the spectra harden with increasing intensity. Below the peak, both the amplitude of variability and spectral changes are much smaller.
Third, the high-energy component also seems to vary more and to harden with increasing intensity above its peak frequency. This inference is, however, based on very few observations.
Fourth, simultaneous snapshots of the full broadband spectra suggest that the intensities in the two spectral components are correlated, in the sense that when the short-wavelength (gamma-ray) component is bright, the long-wavelength emission is also bright. These snapshots do not constrain possible lags significantly.
Fifth, finite lags have been measured among flares in the optical through X-ray light curves of two HBL. For both PKS 2155-304 and Mrk 421, the soft X-ray photons lag the hard X-ray photons by ~ 1 h (Makino et al 1996, Takahashi et al 1996). In addition, for PKS 2155-304 the EUV and UV light curves lag the X-ray curves by ~ 1 and ~ 2 days, respectively (Urry et al 1997; Figure 8). For Mrk 421, the EUV and optical light curves lag the TeV by ~ 1 day (Schubnell et al 1996; Figure 9); a big gap in the X-ray light curve close to the flare peak prevents quantitative statements, but it is plausible that the behavior of Mrk 421 closely resembles that of PKS 2155-304.
Sixth, HBL and LBL exhibit completely analogous variability with respect to the (different) peak frequencies in their spectral distributions. For example, both vary more above their respective power peaks; it is just that this makes LBL highly variable in the optical and GeV gamma rays, whereas HBL are highly variable in X rays and in TeV gamma rays (and relatively quiescent in the optical and GeV ranges).
Below we discuss current theories of blazar spectra and variability (Sections 8.2 - 8.5) in the light of these six points. We start with the radio-emitting outer jet, which is reasonably well understood, and work progressively inward to smaller scales, higher energies, and lesser knowledge, with the goal of relating the observed variability to the physics of the jet. Mechanisms for variability involve essentially two possibilities, shock waves moving along the jet and rotation of the beaming cone across the line of sight; both could be relevant at different times or in different objects. In the first case, spectral variations are expected, whereas in the second achromatic variability can be produced (see Marscher 1996, Dreissigacker & Camenzind 1996 for physical models).