The unusual afterglow of the Gamma-Ray Burst 100621A

TL;DR

This paper presents a detailed multi-wavelength analysis of the complex afterglow of GRB 100621A, revealing multiple emission components, a significant intensity jump, and high host galaxy extinction, interpreted within the fireball model.

Contribution

It introduces a multi-component interpretation of the afterglow, including a collision of shells and energy injection, with constraints on microphysical parameters and host extinction.

Findings

Identification of three emission components in the afterglow.

Detection of a sudden intensity jump one hour after prompt emission.

High host galaxy extinction (A_V = 3.6 mag) despite blue host galaxy.

Abstract

In order to constrain the broad-band spectral energy distribution of the afterglow of GRB 100621A, dedicated observations were performed in the optical/near-infrared with the 7-channel "Gamma-Ray Burst Optical and Near-infrared Detector" (GROND) at the 2.2m MPG/ESO telescope, in the sub-millimeter band with the large bolometer array LABOCA at APEX, and at radio frequencies with ATCA. Utilizing also Swift X-ray observations, we attempt an interpretation of the observational data within the fireball scenario. The afterglow of GRB 100621A shows a very complex temporal as well as spectral evolution. We identify three different emission components, the most spectacular one causing a sudden intensity jump about one hour after the prompt emission. The spectrum of this component is much steeper than the canonical afterglow. We interpret this component using the prescription of Vlasis et al. (2011) for a two-shell collision after the first shell has been decelerated by the circumburst medium. We use the fireball scenario to derive constraints on the microphysical parameters of the first shell. Long-term energy injection into a narrow jet seems to provide an adequate description. Another noteworthy result is the large ($A_V$ = 3.6 mag) line-of-sight host extinction of the afterglow in an otherwise extremely blue host galaxy.