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

TL;DR

This paper models the interaction of relativistic winds from magnetars with GRB fireballs to explain the shallow decay phase of early X-ray afterglows, proposing two types of wind bubbles with distinct radiative behaviors.

Contribution

It introduces a numerical model of relativistic wind bubbles from magnetars and links their dynamics to the shallow decay phase in GRB X-ray afterglows, providing explanations for observed phenomena.

Findings

Relativistic wind bubbles can be forward-shock or reverse-shock dominated.

Both bubble types produce shallow decay phases in X-ray afterglow light curves.

Model fits to specific GRBs suggest reverse shocks can explain long-lasting shallow decay.

Abstract

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.