Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
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7.2. Multiwavelength Light Curves and Correlations

RADIO-OPTICAL     On time scales of years, the optical emission from blazars is weakly correlated with the radio emission, with lead times of roughly one year (and with large uncertainties; Hufnagel & Bregman 1992). A stronger correlation appears between optical and high frequency (37 GHz) radio light curves, with lead times of months or less (Tornikoski et al 1994). Whereas the low frequency radio light curves are well sampled with respect to the variability time scales, at high radio and optical frequencies the flares are faster and variations may be simultaneous within the sampling (appeq 1-2 months). Nevertheless, not every optical flare has a radio counterpart even at high frequency, indicating that, although the particles radiating in the IR-optical and at submillimeter to centimeter wavelengths are physically related, most likely by a propagating shock, they occupy distinct spatial regions.

Simultaneous multiwavelength spectra of blazars from radio to IR bands usually show a self-absorption turnover, in some cases with multiple structure, evolving with time (Robson et al 1983, Gear et al 1986, Valtaoja et al 1988, Brown et al 1989). Systematic long-term monitoring of a number of objects from millimeter to IR bands shows that flares typically propagate from short to long wavelengths (Roellig et al 1986, Stevens et al 1994). A particularly well-studied example is 3C 279, in which the IR spectral index flattened as the intensity increased by a factor of ~ 5 (Litchfield et al 1995). The delay of the submillimeter flare with respect to the IR appears to be ~ 1 month, although the sampling is poor on such short time scales. Similar data for 3C 345 can be reconciled with a shock model only if the jet is nonadiabatic or curves away from the line of sight during the decline (Stevens et al 1996).

At millimeter wavelengths, blazar spectra generally flatten with increasing intensity, with BL Lacs (here, LBL) having flatter spectra in the submillimeter range than FSRQ (Gear et al 1994). This may depend on the different line strengths or could be related to the different luminosities, peak wavelengths, and redshifts of the two groups.

On very short (intraday) time scales, radio and optical variability seems to be correlated with no lag, at least for some blazars (Wagner & Witzel 1995).

MILLIMETER-X RAY     Some well-monitored blazars, notably 3C 279, have shown a correspondence between millimeter and X-ray emission. The first suggestion of such a correlation was for 3C 279 (eight epochs over 1988-1991, showing two flares; Makino et al 1993). Subsequent observations in 1992-1996 showed another prolonged flare at 22-37 GHz, while the X-ray intensity remained low; however, the 1994 flare had a much lower self-absorption frequency than in 1991 or 1988 (Makino et al 1996). Onecould speculate that the lack of a corresponding X-ray flare is due to the latest millimeter flare originating in the outer regions of the jet, where the Compton process may be less important.

The X-ray flux of BL Lac also appears to correlate with the submillimeter flux (Kawai et al 1991). Both 3C 279 and BL Lac have similar continuum spectra, with synchrotron peaks in the far-IR and flat X-ray spectra that lie above the extrapolation of the UV spectrum.

A study of the historic light curves of 3C 279 at radio, millimeter, optical, UV, X-ray, and gamma-ray wavelengths (Grandi et al 1994) confirms that on time scales of months to years, the X-rays correlate with the high-frequency radio flux (particularly when a linear trend is subtracted from the radio light curves), whereas the optical-UV fluxes are correlated with the gamma-ray flux.

OPTICAL-GAMMA RAY     Despite the obvious importance, it has proved very difficult to obtain simultaneous coverage of gamma-ray flares at optical wavelengths. In one case, PKS 1406-076, the optical flux rises by about 60% while the gamma rays increase by a factor of ~ 3, with the optical flare apparently leading the gamma-ray flare by about a day (Wagner et al 1995b). In another case, PKS 0420-014, gamma-ray high states correspond to optical flares, while gamma-ray nondetections coincide with optically faint states (Wagner et al 1995a).

Over long time scales, the optical and gamma-ray fluxes in 3C 279 are well correlated (Grandi et al 1994). During the rapid flare of 3C 279 in June 1991 a definite optical flare was observed, although only three optical measurements are simultaneous to the gamma-ray light curve. These show a maximum coincident with the gamma-ray maximum and a steep decline of 0.36 mag in a day, corresponding to a simultaneous decline of a factor ~3-5 in gamma rays (Hartman et al 1996).

GAMMA RAY-RADIO     Comparison of gamma-ray data for blazars (detections and nondetections) with 37 GHz light curves shows that in a statistical sense, gamma-ray detections correspond to rising radio fluxes (Valtaoja & Teräsranta 1995). Large gamma-ray flares may be connected with the birth of new VLBI components, traditionally associated with the beginning of strong radio flares, as seems to occur in 3C 279 and PKS 0528+134 (Wehrle et al 1996, Zhang et al 1994).

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