Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
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6.3. Far IR-Optical-UV: The Thin Synchrotron Emission

IRAS observations have shown that for LBL, a large fraction of the bolometric luminosity is radiated at mid-IR wavelengths (Impey & Neugebauer 1988) and that rapid (time scales of weeks) large-amplitude (factors of two) far-IR variability is common, whereas the nonblazar AGN do not vary significantly in the far-IR band (Edelson & Malkan 1986, 1987). Multiwavelength observations of 3C 345 from radio to UV wavelengths, close in time to the IRAS pointing, define clearly a peak in the power per decade between 1013 and 1014 Hz (Bregman et al 1986), one of the few LBL cases in which the peak is actually measured rather than inferred.

The spectra of LBL fall off rapidly at wavelengths shorter than the synchrotron peak, i.e. in the optical-UV bands, whereas the spectra of HBL in the same bands are much flatter. Because of their relative brightness, LBL have been monitored extensively in the optical band, whereas at UV and X-ray wavelengths, mostly HBL have been observed.

The "historic" optical light curves of blazars on time scales of years show a variety of behaviors, from slow long-term trends to rapid repeated flares (Webb et al 1988). In general, long-term trends have typical time scales of 5 years in the rest frame, both for BL Lacs and FSRQ (Smith et al 1993, Smith & Nair 1995). Structure functions of the best data show slopes somewhat flatter than for radio light curves, indicating a transition from shot noise in the radio to flicker noise in the optical (Hufnagel & Bregman 1992), with relatively more power on short time scales. There are no obvious differences between BL Lac objects and FSRQ within the limited blazar samples available.

Optical variability extends to very short time scales, and intra-night small-amplitude variability has been observed in a number of blazars (Jang & Miller 1995, Heidt & Wagner 1996). The short time scale variations of radio-selected (LBL) are systematically larger in amplitude and have shorter duty cycles than those of X-ray selected (HBL) BL Lac objects (Heidt 1996). Within the radio-selected sample, there is a tendency for greater optical activity among higher luminosity sources (Heidt & Wagner 1996).

HBL are in general less polarized than LBL (Jannuzi et al 1993); however, the constancy of the polarized fraction and wavelength dependence over large flux variations observed in PKS 2155-304 argues against dilution by either starlight or an accretion disk continuum as the cause of this effect (Smith et al 1992).

High-quality UV light curves for more than a dozen blazars, obtained with IUE, show significant variability (amplitudes of 8-80%), which correlates with degree of optical polarization and with luminosity (Edelson 1992). This trend, which is opposite to the case of Seyfert galaxies, has been interpreted to mean that the most variable objects are the most beamed. Alternatively it could result from an inverse correlation between luminosity and peak frequency.

The densest IUE monitoring has revealed UV variations on extremely short time scales. In PKS 2155-304, the observed UV flux doubled in 1 h on one occasion (Pian et al 1997), comparable to the fastest X-ray variability observed in this object. A doubling time scale of 10 days is common (Urry et al 1993). At extreme ultraviolet wavelengths, rapid variations of slightly larger amplitude have been seen, although only two blazars have been monitored extensively with EUVE (HL Marshall et al, in preparation).

The spectral variability in the UV band is generally small, with only a weak tendency for larger amplitude variability at shorter UV wavelengths (Edelson 1992, Pian & Treves 1993, Paltani & Courvoisier 1994). The two blazars with the most UV observations, Mrk 421 and PKS 2155-304 (both HBL), show spectral hardening with increasing intensity only in a statistical sense (Ulrich et al 1984b, Maraschi et al 1986, George et al 1988a, Urry et al 1988). We note that LBL are faint in the UV and their slopes are difficult to measure with IUE.

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