OPTICAL VARIABILITY OF THE QUASAR 3C 279: THE SIGNATURE OF A DECELERATING JET?

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 the 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 active galactic nuclei environment. Synchrotron, Synchrotron self Compton, 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 similar to 100 MeV is expected to be delayed by a few days with respect to the onset of the optical decay, while the very-high energy gamma-rays are expected to track the optical light curve closely with a delay of at most a few hours.