Variability of the blazar 3C 454.3 during 2007–2010

Results are presented of multicolor observations of the blazar 3C 454.3 carried out at the Astronomical Institute of St. Petersburg State University and the Central Astronomical Observatory of the Russian Academy of Sciences in 2007–2010. The color variability of the blazar is analyzed. Several outbursts were observed. The existence of two variable synchrotron sources is inferred. The first is responsible for the small-amplitude flux variability, and the second for flares. In each flare, the relative spectral energy distribution (SED) of the variable source is found to be constant. All the SEDs are power laws, but with different spectral indexes in different flares. This indicates the impossibility of explaining the global variability only via a difference in Doppler boosting due to variations of the angle between the line of sight and the velocity of the electrons responsible for the synchrotron radiation. The polarimetric and photometric observations are used to derive the absolute SED of constant component. A comparison of the observed SEDs for different brightness levels with the SED of the constant componentmakes it possible to explain the observed color variability as due to the superposition of a bluer variable source with a constant SED and variable flux onto the constant component.