GRB 081029: A Gamma-Ray Burst with a Multi-Component Afterglow

TL;DR

This paper analyzes the complex optical and X-ray afterglow of GRB 081029, revealing a multi-component jet structure and challenging simple external shock models, thus advancing understanding of gamma-ray burst jet composition.

Contribution

It introduces a detailed multi-wavelength analysis of GRB 081029's unusual afterglow, proposing a two-component jet model to explain the optical rise and complex light curve features.

Findings

Optical afterglow shows a rise at ~3000 s not seen in X-ray.

Data inconsistent with simple synchrotron or continuous energy injection models.

Optical light curves explained by collision within a two-component jet.

Abstract

We present an analysis of the unusual optical light curve of the gamma-ray burst GRB 081029, a long-soft burst with a redshift of z = 3.8479. We combine X-ray and optical observations from the Swift X-Ray Telescope and the Swift UltraViolet/Optical Telescope with ground-based optical and infrared data obtained using the REM, ROTSE, and CTIO 1.3-m telescopes to construct a detailed data set extending from 86 s to approximately 100,000 s after the BAT trigger. Our data cover a wide energy range, from 10 keV to 0.77 eV (1.24 to 16,000 Angstrom). The X-ray afterglow shows a shallow initial decay followed by a rapid decay starting at about 18,000 s. The optical and infrared afterglow, however, shows an uncharacteristic rise at about 3000 s that does not correspond to any feature in the X-ray light curve. Our data are not consistent with synchrotron radiation from a jet interacting with an external medium, a two-component jet, or continuous energy injection from the central engine. We find that the optical light curves can be broadly explained by a collision between two ejecta shells within a two-component jet. A growing number of gamma-ray burst afterglows are consistent with complex jets, which suggests that some (or all) gamma-ray burst jets are complex and will require detailed modelling to fully understand them.