Constraining the structure of gamma-ray burst jets through the afterglow light curves

We investigate the effect that the structure of gamma-ray burst (GRB) jets has on the afterglow light curves for observers located at different viewing angles, θ_obs, from the jet symmetry axis. The largest uncertainty in the jet dynamics is the degree of lateral energy transfer. Thus, we use two simple models that make opposite and extreme assumptions for this point and calculate the light curves for an external density that is either homogeneous or decreases as the square of the distance from the source. The Lorentz factor, Γ, and kinetic energy per unit solid angle, ε, are initially taken to be power laws of the angle θ from the jet axis: ε α θ^-a, Γ α θ^-b. We perform a qualitative comparison between the resulting light curves and afterglow observations. This constrains the jet structure, and we find that a ≈ 2 and 0 ≤ b ≤ 1 are required to reproduce typical afterglow light curves. Detailed fits to afterglow data are needed to determine whether a "universal" jet model, in which all GRB jets are assumed to be intrinsically identical and differ only by our viewing angle, θ _obs, is consistent with current observations.