Jets, structured outflows and energy injection in gamma‐ray burst afterglows: numerical modelling

ABSTRACT We investigate numerically the ability of three models (jet, structured outflow and energy injection) to accommodate the optical light‐curve breaks observed in 10 gamma‐ray burst (GRB) afterglows (980519, 990123, 990510, 991216, 000301c, 000926, 010222, 011211, 020813 and 030226), as well as the relative intensities of the radio, optical and X‐ray emissions of these afterglows. We find that the jet and structured outflow models fare much better than energy injection model in accommodating the multiwavelength data of the above 10 afterglows. For the first two models, a uniform circumburst medium provides a better fit to the optical light‐curve break than a wind‐like medium with a r −2 stratification. However, in the only two cases where the energy injection model may be at work, a wind medium is favoured (an energy injection is also possible in a third case, the afterglow 970508, whose optical emission exhibited a sharp rise, but not a steepening decay). The best‐fitting parameters obtained with the jet model indicate an outflow energy of 2 × 10 50 to 6 × 10 50 ergs and a jet opening of 2°− 3°. Structured outflows with a quasi‐uniform core have a core angular size of 07–1° and an energy per solid angle of 0.5 × 10 53 to 3 × 10 53   erg  sr −1 , surrounded by an envelope where this energy falls off roughly as θ −2 with angle from the outflow axis, requiring thus the same energy budget as jets. Circumburst densities are found to be typically in the range 0.1–1 cm −3 , for either model. We also find that the reverse shock emission resulting from the injection of ejecta into the decelerating blast wave at about 1 d after the burst can explain the slowly decaying radio light curves observed for the afterglows 990123, 991216 and 010222.