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