Transient emission from dissipative fronts in magnetized, relativistic outflows. I. Gamma-ray flares

The transient emission produced behind internal shocks that are driven by o vertaking collisions of a magnetized, relativistic outflow is considered. A self-consistent model capable of describing the structure and dynamics of the shocks and the time evolution of the pair and gamma-ray distribution fu nctions is developed and applied to gamma-ray flares in blazars, in the cas e in which gamma-ray production is dominated by inverse Compton scattering of external radiation. The dependence of the hare properties on magnetic he ld dissipation rate, the intensity of ambient radiation, and the thickness of expelled fluid slabs is analyzed. It is shown that (1) the type of gamma -ray flare produced by the model is determined by the ratio of the thicknes s of ejected fluid slab and the gradient length scale of ambient radiation intensity; (2) the radiative efficiency depends sensitively on the opacity contributed by the background radiation, owing to a radiative feedback, and is typically very high for parameters characteristic to the powerful blaza rs; and (3) the emitted flux is strongly suppressed at energies for which t he pair-production optical depth is initially larger than unity; the time l ag and flare duration in this energy range increase with increasing gamma-r ay energy. At lower energies, flaring at different gamma-ray bands occurs r oughly simultaneously but with possibly different amplitudes. Some observat ional consequences are discussed.