1998ApJ...508..180X

We have collected 26 gamma-ray-loud blazars (15 BL Lac objects and 11 flat-spectrum radio quasars) with both observed near-IR and γ-ray flux densities, with the following main results: (1) there is a very strong correlation between F_γ and F_IR in the low state, and a weaker but also significant correlation between F_γ and F_IR in the high state for 20 objects with both ``high'' and ``low'' state fluxes; (2) there is a very significant correlation between F_γ and F_IR in the low state, and a weak correlation between F_γ and F_IR in the high state for 26 sources; (3) there is a weak correlation between F_γ and F_X, but not between F_γ and F_O, and F_R for both the low and high state of 26 sources; (4) there is a strong correlation between F_γ and F_IR in the low and high state for 15 BL Lac objects, but not for 11 flat-spectrum radio quasars. Possible constraints on the γ-ray emission mechanism are discussed. We suggest that the main gamma-ray radiation mechanism is probably the synchrotron self-Compton process. The inverse Compton scattering of the radiation from hot circumnuclear dust with T_g_ = 2000 K, which is located within the region of r ~ 3 pc, by beamed ultrarelativistic electrons is likely to be an important complementary mechanism. In addition, the γ-ray emission may be somewhat different for BL Lac objects and flat-spectrum radio quasars.