Viewing angle and environment effects in gamma-ray bursts: sources of afterglow diversity

We discuss the afterglows from the evolution of both spherical and anisotropic fireballs decelerating in an inhomogeneous external medium. We consider both the radiative and adiabatic evolution regimes and analyze the physical conditions under which these regimes can be used. Afterglows may be expected to differ widely among themselves, depending on the angular anisotropy of the fireball and the properties of the environment. They may be entirely absent or may be detected without a corresponding gamma -ray event. A tabulation of different representative light curves is presented, covering a wide range of behaviors that resemble what is currently observed in the gamma -ray bursts (GRBs) GRB 970228, GRB 970508, and other objects.