Shallow decay phase of the early X-ray afterglow from the external shock in wind environment

TL;DR

This paper proposes a model explaining the shallow decay phase of early X-ray afterglows in gamma-ray bursts as a dynamical transition in external shocks within a wind environment, supported by application to specific GRBs.

Contribution

It introduces a model attributing the shallow decay phase to a dynamical change in external shock behavior in a wind environment, explaining observations without spectral evolution.

Findings

The model successfully explains the shallow decay phase for GRBs 050319 and 081008.

The transition corresponds to the deceleration time of the external shock.

Spectral index remains unchanged during the phase transition.

Abstract

The shallow decay phase of the early X-ray afterglow in gamma-ray bursts discovered by {\it Swift} is a widely discussed topic. As the spectral index does not change at the transferring of the shallow decay phase to a normal phase, it implies this transferring should be a dynamical change rather than a spectral evolution. We suggest both the shallow decay phase and the normal phase are from the external shock in a wind environment, while the transferring time is the deceleration time. We apply this model to GRBs 050319 and 081008, and find them can be well explained by choosing a proper set of parameters.