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

7. MULTIWAVELENGTH STUDIES OF BLAZARS

7.1. Broadband Continuum Snapshots

Spectral variability studies make it apparent that blazars are more variable at wavelengths shorter than the peak of the synchrotron emission, with amplitude increasing with decreasing wavelength (Bregman et al 1982, 1984, Maraschi et al 1983, Makino et al 1987, George et al 1988b, Falomo et al 1988, Treves et al 1989, Kawai et al 1991, Pian et al 1994). This is particularly clear for HBL in the X-ray range (Sambruna et al 1994, 1995), whereas for LBL this trend is observed at IR through UV wavelengths (Impey & Neugebauer 1988).

Recently, multiwavelength snapshots have been extended to gamma-ray energies. For example, the BL Lac object AO 0235+164 has a gamma-ray spectrum that matches well with an extrapolation from the flat X-ray spectrum (Madejski et al 1996), whereas nonsimultaneous ROSAT observations show that the soft X-ray flux is strongly variable and has a steep spectrum. The simplest interpretation is that in this object the synchrotron emission extends to the soft X-ray band, arguing for a classification intermediate between LBL and HBL.

In a few cases there exist simultaneous multiwavelength spectra of the same source at two epochs, one with high and one with low gamma-ray flux, notably 3C 279 (Maraschi et al 1994), Mrk 421 (Schubnell et al 1996; Figure 7), and PKS 0528+134 (Sambruna et al 1996b; Figure 7). The overall spectral variations of these three blazars show remarkable similarities. In all cases when the source was brighter in gamma rays, it was also brighter at longer wavelengths. The largest variations occurred at wavelengths shorter than the synchrotron and gamma-ray peaks. For Mrk 421 and PKS 0528+134, above gamma-ray peak frequency (10²⁵ Hz and 10²² Hz, respectively), the spectrum was harder in the brighter state. (For 3C 279, the gamma-ray spectrum in the faint state could not be determined owing to the low intensity.) Note that the spectral variation of Mrk 421 in the 0.5- to 10-keV range is analogous to that of 3C 279 in the IR to UV range. (PKS 0528+134 is highly reddened and therefore weak in the optical.)

In sum, the broadband variability behavior of different objects is similar when "normalized" to their respective peak frequencies. In each object, variations of the two spectral components are larger above the peak frequencies and appear to be correlated.