Evidence for Dust Destruction from the Early-time Colour Change of GRB 120119A

TL;DR

This study presents early-time observations of GRB 120119A showing a unique colour change consistent with dust destruction, providing strong evidence for dust being destroyed in the burst's local environment.

Contribution

First direct evidence of dust destruction in a GRB's environment based on early-time colour evolution, supported by detailed spectral analysis.

Findings

Early-time colour change coincides with prompt emission

Dust destruction model fits the data better than spectral index change alone

Supports the hypothesis of dust destruction in GRB environments

Abstract

We present broadband observations and analysis of Swift gamma-ray burst (GRB) 120119A. Our early-time afterglow detections began under 15 s after the burst in the host frame (redshift z = 1.73), and they yield constraints on the burst energetics and local environment. Late-time afterglow observations of the burst show evidence for a moderate column of dust (A_V ~ 1.1 mag) similar to, but statistically distinct from, dust seen along Small Magellanic Cloud sightlines. Deep late-time observations reveal a dusty, rapidly star-forming host galaxy. Most notably, our early-time observations exhibit a significant red-to-blue colour change in the first ~200 s after the trigger at levels heretofore unseen in GRB afterglows. This colour change, which is coincident with the final phases of the prompt emission, is a hallmark prediction of the photodestruction of dust in GRB afterglows. We test whether dust-destruction signatures are significantly distinct from other sources of colour change, namely a change in the intrinsic spectral index {\beta}. We find that a time-varying power-law spectrum alone cannot adequately describe the observed colour change, and allowing for dust destruction (via a time-varying A_V) significantly improves the fit. While not definitively ruling out other possibilities, this event provides the best support yet for the direct detection of dust destruction in the local environment of a GRB.