Gamma-ray Burst Afterglow with Continuous Energy Injection: Signature of a Highly-Magnetized Millisecond Pulsar

We investigate the consequences of a continuously injecting central engine on the gamma-ray burst afterglow emission, focusing more specifically on a highly-magnetized millisecond pulsar engine. For initial pulsar parameters within a certain region of the parameter space, the afterglow lightcurves are predicted to show a distinctive achromatic bump feature, the onset and duration of which range from minutes to months, depending on the pulsar and the fireball parameters. The detection of or upper limits on such features would provide constraints on the burst progenitor and on magnetar-like central engine models. An achromatic bump such as that in GRB 000301C afterglow may be caused by a millisecond pulsar with P0=3.4 millisecond and Bp=2.7e14 Gauss.