VIEWING ANGLE AND ENVIRONMENT EFFECTS IN GAMMA-RAY BURSTS - SOURCES OF AFTERGLOW DIVERSITY

We discuss the afterglows from the evolution of both spherical and ani sotropic fireballs decelerating in an inhomogeneous external medium. W e consider both the radiative and adiabatic evolution regimes and anal yze the physical conditions under which these regimes can be used. Aft erglows may be expected to differ widely among themselves, depending o n the angular anisotropy of the fireball and the properties of the env ironment. They may be entirely absent or may be detected without a cor responding gamma-ray event. A tabulation of different representative l ight curves is presented, covering a wide range of behaviors that rese mble what is currently observed in the gamma-ray bursts (GRBs) GRB 970 228, GRB 970508, and other objects.