Effect of Orientation on the Variability Pattern of Gamma-Ray Sources

Abstract

Using a toy model, we demonstrate how the variability pattern of a relativistically moving source of radiation that is subject to the action of a transient force, depends on dynamical effects. The model is motivated by a recent analysis of radiative fronts in magnetized, relativistic jets, but is applicable also in other situations. The main effect analyzed in this paper is the strong dependence of the variability pattern on viewing angle, resulting from changes of the beaming cone of emission. In particular, it is shown that(1) the shape of the light curves reflects the time profile of the Doppler factor when viewing the source at large enough angles to its direction of motion, whereas at small viewing angles it is determined by the time evolution of the radiated power density; and(2) the characteristics of correlated emission depend strongly on viewing angle by virtue of either optical depth effects or the difference in beaming patterns of different radiation sources.