  The postburst object of a GRB is likely to be a highly magnetized, rapidly
rotating compact object (e.g., a millisecond magnetar), which could produce an
ultrarelativistic electron-positron-pair wind. The interaction of such a wind
with an outwardly expanding fireball ejected during the burst leads to a
relativistic wind bubble (RWB). We numerically calculate the dynamics and
radiative properties of RWBs and use this model to explain the shallow decay
phase of the early X-ray afterglows observed by Swift. We find that RWBs can
fall into two types: forward-shock-dominated and reverse-shock-dominated
bubbles. Their radiation during a period of $\sim 10^{2}-10^{5}$ seconds is
dominated by the shocked medium and the shocked wind, respectively, based on
different magnetic energy fractions of the shocked materials. For both types,
the resulting light curves always have a shallow decay phase. In addition, we
provide an example fit to the X-ray afterglows of GRB 060813 and GRB 060814 and
show that they could be produced by forward-shock-dominated and
reverse-shock-dominated bubbles, respectively. This implies that, for some
early afterglows (e.g., GRB 060814), the long-lasting reverse shock emission is
strong enough to explain their shallow decay phase.
