  Long duration gamma-ray bursts (GRBs) are thought to be produced by the
core-collapse of a rapidly-rotating massive star. This event generates a highly
relativistic jet and prompt gamma-ray and X-ray emission arises from internal
shocks in the jet or magnetised outflows. If the stellar core does not
immediately collapse to a black hole, it may form an unstable, highly
magnetised millisecond pulsar, or magnetar. As it spins down, the magnetar
would inject energy into the jet causing a distinctive bump in the GRB light
curve where the emission becomes fairly constant followed by a steep decay when
the magnetar collapses. We assume that the collapse of a massive star to a
magnetar can launch the initial jet. By automatically fitting the X-ray
lightcurves of all GRBs observed by the Swift satellite we identified a subset
of bursts which have a feature in their light curves which we call an internal
plateau -- unusually constant emission followed by a steep decay -- which may
be powered by a magnetar. We use the duration and luminosity of this internal
plateau to place limits on the magnetar spin period and magnetic field strength
and find that they are consistent with the most extreme predicted values for
magnetars.

High Energy Astrophysical Phenomena