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The postburst object of a GRB is likely to be a highly magnetized, rapidlyrotating compact object (e.g., a millisecond magnetar), which could produce anultrarelativistic electron-positron-pair wind. The interaction of such a windwith an outwardly expanding fireball ejected during the burst leads to arelativistic wind bubble (RWB). We numerically calculate the dynamics andradiative properties of RWBs and use this model to explain the shallow decayphase of the early X-ray afterglows observed by Swift. We find that RWBs canfall into two types: forward-shock-dominated and reverse-shock-dominatedbubbles. Their radiation during a period of $\sim 10^{2}-10^{5}$ seconds isdominated by the shocked medium and the shocked wind, respectively, based ondifferent magnetic energy fractions of the shocked materials. For both types,the resulting light curves always have a shallow decay phase. In addition, weprovide an example fit to the X-ray afterglows of GRB 060813 and GRB 060814 andshow that they could be produced by forward-shock-dominated andreverse-shock-dominated bubbles, respectively. This implies that, for someearly afterglows (e.g., GRB 060814), the long-lasting reverse shock emission isstrong enough to explain their shallow decay phase.

Shallow decay phase of GRB X-ray afterglows from relativistic wind bubbles