Transient emission from dissipative fronts in magnetized, relativistic outflows. II. Synchrotron flares

The time-dependent synchrotron emission from relativistic jets and the rela tion between the synchrotron and the inverse Compton scattering of external radiation emission is considered within the framework of the radiative fro nt model. The timescale and profile of the optically thin emission are show n to be determined, in this model, by the shock formation radius, the thick ness of expelled fluid slab, and the variation of the front's parameters ow ing to its transverse expansion. For a range of reasonable conditions, a va riety of flare shapes can be produced, varying from roughly symmetric hares with exponential rises and decays, as often seen in blazars, to highly asy mmetric ones with a fast rise and a much slower, power-law decay, as seen i n the afterglows of gamma-ray bursts. The onset, duration, and fluence of l ow-frequency (below the initial turnover frequency) and hard gamma-ray (abo ve the initial gamma-spheric energy) outbursts are limited by opacity effec ts; the emission at these energies is quite generally delayed and, in the c ase of outbursts of sufficiently short lengths, severely attenuated. The ob servational consequences are discussed. One distinctive prediction of this model is that in a single, powerful source, the upper cutoff of the gamma-r ay spectrum should be correlated with the timescale of the outburst and wit h the amplitude of variations at long wavelengths (typically radio to milli meter).