Observational evidence for magnetars powering GRBs

The core-collapse of a massive star may lead either to a black hole or a, possibly unstable, rapidly-rotating magnetar. A magnetar provides an potential additional source of rotational energy which is available to power emission during the burst. If the energy reservoir is used up the magnetar may then suffer gravitational collapse ending the emission period. We have carried out a systematic search for the signature of a spinning-down magnetar in the long GRBs observed by the Swift satellite up to the end of 2008. We find 10 GRBs that have a feature - which we call an "internal plateau"-in their light curves that can be associated with the injection of energy into the jet by a magnetar. The duration and intensity of the internal plateau provides constraints on the initial spin period and magentic field of the magnetar. The derived magnetic fields are large compared to those of long-lived Galactic magnetars, but are within the most extreme values predicted for magnetars.