Evidence for isotropic emission in GRB991216

The issue of the possible presence or absence of jets in GRBs is here re-examined for GRB991216, which has a claimed half-opening beaming angle of θ◦ = 3 ◦. We compare and contrast our theoretically predicted afterglow luminosity in the 2–10 keV band for spherically symmetric versus jetted emission. Jetted emission can be excluded and data analysis confirms spherical symmetry. These theoretical fits are expected to be largely improved by the forthcoming data of the Swift mission. In addition to the source GRB991216, our model has been applied successfully, assuming spherical symmetry, to GRB980519, GRB980425, GRB030329 and to GRB970228.