Optical/infrared flares of GRB 080129 from late internal shocks

TL;DR

This paper analyzes optical and near-infrared flares in GRB 080129, proposing they are caused by late internal shocks, similar to X-ray flares, with detailed modeling supporting this interpretation.

Contribution

It introduces the late internal shock model as an explanation for optical/NIR flares in GRB 080129, providing detailed analysis and constraints on the outflow Lorentz factor.

Findings

Optical/NIR flares are analogous to X-ray flares in GRBs.

The Lorentz factor of the flare outflow is approximately 30.

The late internal shock model explains the sharp decline and timing of the flares.

Abstract

Strong optical and near-infrared (NIR) flares were discovered in the afterglow of GRB 080129. Their temporal behaviors, the sudden emergence and the quick disappearance, are rather similar to that of many X-ray flares (for instance, the giant flare of GRB 050502B). We argue that the optical/NIR flares following GRB 080129 are a low energy analogy of the X-ray flares and the most likely interpretation is the "late internal shock model". In this model, both the very sharp decline and the very small ratio between the duration and the occurrence time of the optical/NIR flares in GRB 080129 can be naturally interpreted. The initial Lorentz factor of the flare outflow is found to be $\sim 30$, consistent with the constraint $\leq 120$ set by the forward shock afterglow modeling. Other possibilities, like the reverse shock emission or the radiation from the continued but weaker and weaker collision between the initial GRB outflow material, are disfavored.