THE LATE PEAKING AFTERGLOW OF GRB 100418A

Abstract

GRB 100418A is a long gamma-ray burst (GRB) at redshift z = 0.6235 discovered with the Swift Gamma-ray Burst Explorer with unusual optical and X-ray light curves. After an initial short-lived, rapid decline in X-rays, the optical and X-ray light curves observed with Swift are approximately flat or rising slightly out to at least similar to 7 x 10(3) s after the trigger, peak at similar to 5 x 10(4) s, and then follow an approximately power-law decay. Such a long optical plateau and late peaking is rarely seen in GRB afterglows. Observations with Rapid Eye Mount during a gap in the Swift coverage indicate a bright optical flare at similar to 2.5 x 10(4) s. The long plateau phase of the afterglow is interpreted using either a model with continuous injection of energy into the forward shock of the burst or a model in which the jet of the burst is viewed off-axis. In both models the isotropic kinetic energy in the late afterglow after the plateau phase is >= 10(2) times the 10(51) erg of the prompt isotropic gamma-ray energy release. The energy injection model is favored because the off-axis jet model would require the intrinsic T-90 for the GRB jet viewed on-axis to be very short, similar to 10 ms, and the intrinsic isotropic gamma-ray energy release and the true jet energy to be much higher than the typical values of known short GRBs. The non-detection of a jet break up to t similar to 2 x 10(6) s indicates a jet half-opening angle of at least similar to 14 degrees, and a relatively high-collimation-corrected jet energy of E-jet >= 10(52) erg.