2006ApJ...644L.105B

Assuming the emitted gamma-ray burst (GRB) radiation is not spherically symmetric but is confined into a narrow jet, we present the exact analytic expressions to compute the value of the detector arrival time at which we start to ``see'' the sides of the jet, in both the fully radiative and adiabatic regimes. We obtain this result using our exact analytic expressions for the equitemporal surfaces in GRB afterglows. We reexamine the validity of three different approximate formulae currently adopted for the adiabatic regime in the GRB literature. We also present an empirical fit of the numerical solutions of the exact equations, compared and contrasted with these three approximate formulae. The extent of the differences is such as to require a reassessment of the existence and entity of beaming in the cases considered in the current literature, as well as of its consequences on the GRB energetics.