Stellar Wind-Blown Bubbles as Environments for Late-Time Rebrightening of Gamma-Ray Burst Afterglows

TL;DR

The paper addresses late-time gamma-ray burst afterglow rebrightening by proposing a unified framework in which a structured jet propagates through a wind-blown bubble around the progenitor. It combines ASGARD-based forward modeling with wind-bubble circumburst environments to fit multi-wavelength data for three high-redshift GRBs, uncovering narrow jet cores and steep angular energy profiles that imply high magnetization. The analysis finds transition radii $R_{tr}$ from free to shocked wind typically $<0.5$ pc (median \(\sim0.1\) pc), challenging some numerical simulations and linking rebrightening to the jet’s interaction with the ambient medium. The results offer a coherent mechanism for rebrightening, enabling constraints on progenitor winds and environments and highlighting the potential of future EP/SVOM observations to refine jet-launch and propagation models.

Abstract

We presented the multi-wavelength afterglow fitting results for three events that exhibit late afterglow re-brightening behavior: EP240414a ($z=0.402$), GRB 240529A ($z=2.695$), and GRB 240218A ($z=6.782$), which span a broad range of redshifts, from the local to the high-redshift universe. We prove that the peculiar afterglow light curves of three bursts can be well fitted by structured jets propagated in free-to-shocked stellar wind environment of stellar wind blown bubbles. This scenario offers a self-consistent explanation for the observed subclass of afterglows that exhibit rebrightening that characterized by steep rises and rapid decays. It also provides a unified solution for such events and offers pathways to study both the jet generation mechanism and the propagation process of jets through the envelope of progenitor. This study reveals that the structured jets produced by such events exhibit a narrow jet core and a steep angle-dependent energy decay index, suggesting highly magnetized jets. The derived transition radii from free stellar winds to shocked stellar winds for all three events are smaller than 0.5 pc, with statistical analysis of similar events indicating a median value of 0.1 pc, which conflicts with numerical simulation results. We anticipate that future observations by EP/SVOM missions will enhance the understanding of analogous events and further reveil information about progenitors and the circum-environments.