Long-term Multiband Optical Variability of Blazar S5 0716+714

Multiband optical photometry data of blazar S5 0716+714 obtained from 2002 to 2019 reveal stable color index change with flux variability. We analyzed this trend under variability caused by the Doppler factor change in the presence of a curved photon energy spectrum. A break in the energy spectrum of emitting electrons, caused by radiative losses, or log-parabolic electron energy distribution, or the synchrotron self-absorption acting in a compact jet part forms such the photon spectrum. We explained the observed color index change with variability by geometric effects only under the assumption that the radiating region is the synchrotron self-absorbed core and the bright optically thin jet. In this framework, we estimated the magnetic field strength in the optically thick part of the radiating region. These values correspond to other independent estimates of the magnetic field near the black hole, further supporting our assumption.