A recent optical monitoring campaign on the prominent quasar 3C 279 revealed at least one period of a remarkably clean exponential decay of monochromatic ( BVRI ) fluxes with time , with a time constant of \tau _ { d } = 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 Fermi and simultaneous optical monitoring , is that the peak in the \gamma -ray light curve at \sim 100 MeV is expected to be delayed by a few days with respect to the onset of the 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 .