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The shallow decay phase or plateau phase of early afterglows of gamma-raybursts (GRBs), discovered by Swift, is currently understood as being due toenergy injection to a relativistic blast wave. One natural scenario for energyinjection invokes a millisecond magnetar as the central engine of GRBs, becausethe conventional model of a pulsar predicts a nearly constantmagnetic-dipole-radiation luminosity within the spin-down timescale. However,we note that significant brightening occurs in some early afterglows, whichapparently conflicts with the above scenario. Here we propose a new model toexplain this significant brightening phenomena by considering a hyperaccretingfallback disk around a newborn millisecond magnetar. We show that for typicalvalues of the model parameters, sufficient angular momentum of the accretedmatter is transferred to the magnetar and spins it up. It is this spin-up thatleads to a dramatic increase of the magnetic dipole radiation luminosity withtime and thus significant brightening of an early afterglow. Based on thismodel, we carry out numerical calculations and fit well early afterglows of 12GRBs assuming sufficiently strong fallback accretion. If the accretion is veryweak, our model turns out to be the conventional energy-injection scenario of apulsar. Therefore, our model can provide a unified explanation for the shallowdecay phase, plateaus, and significant brightening of early afterglows.

Spin Evolution of Millisecond Magnetars with Hyperaccreting Fallback Disks: Implications for Early Afterglows of Gamma-Ray Bursts