M. Böttcher and D. Principe
Astrophysical Journal, 692, 1374 (2009)
A recent optical monitoring campaign on the prominent quasar 3C279 revealed at least one period of a remarkably clean exponential decay of monochromatic (BVRI) fluxes with time, with a time constant of 12.8 d, over about 14 days. This is clearly too long to be associated with radiative cooling. Here we propose that this may be the signature of deceleration of the synchrotron emitting jet component. We develop a model analogous to the relativistic blast wave model for gamma-ray bursts, including radiative energy losses and radiation drag, to simulate the deceleration of a relativistically moving plasmoid in the moderately dense AGN environment. Synchrotron, SSC and external Compton emission are evaluated self-consistently. We show that the observed optical light curve decay can be successfully reproduced with this model. The decelerating plasmoid model predicts a delayed X-ray flare, about 2 -- 3 weeks after the onset of the quasi-exponential light curve decay in the optical. A robust prediction of this model, which can be tested with GLAST and simultaneous optical monitoring, is that the peak in the gamma-ray light curve at ~ 100 MeV is expected to be delayed by a few days with respect to the onset of optical decay, while the VHE gamma-rays are expected to track the optical light curve closely with a delay of at most a few hours.