Evolution of the afterglow optical spectral shape of GRB 201015A in the first hour: evidence for dust destruction

TL;DR

This paper presents early multi-band optical observations of GRB 201015A, revealing rapid spectral evolution and dust destruction within the first hour, highlighting the importance of prompt, simultaneous multi-band data for understanding GRB afterglows.

Contribution

First simultaneous multi-band optical spectral measurements within 58 seconds of GRB trigger, showing dust destruction evidence and spectral evolution during early afterglow.

Findings

Optical flux peaked 58 seconds after trigger and decayed as t^{-0.81}

Strong color evolution from red to blue indicating dust destruction

Local extinction decreased from 0.8 to 0.3 mag within 2500 seconds

Abstract

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements of gamma-ray bursts starting as early as $\thicksim$ 10 seconds after gamma-ray trigger. The earliest measurements of optical spectral shape have been made only much later, typically on hour time scales, never starting less than a minute after trigger, until now. Beginning only 58 seconds after the \emph{Swift} BAT triggerred on GRB201015A, we observed a sharp rise in optical flux to a peak, followed by a power law temporal decay, $\propto t^{-0.81 \pm 0.03}$. Flux was measured simultaneously in three optical bands, g\p, r\p, and i\p, using our Burst Simultaneous Three-channel Imager (BSTI) on the NUTTelA-TAO telescope. Our data during the decay show strong colour evolution from red to blue, with a change in the optical log slope of $+0.72 \pm 0.14$; during this time the X-ray log slope remained constant. We did not find evidence for a two-component jet structure or a transition from reverse to forward shock or a prompt emission component that would explain this change in slope. We find that the majority of the optical spectral slope evolution is consistent with a monotonic decay of extinction, evidence of dust destruction. Assuming a constant source spectral slope and an SMC-like extinction curve, we derive a change in the local extinction $A_\mathrm{v}^\mathrm{local}$ from $\thicksim$0.8 mag to 0.3 mag in $\thicksim$2500 seconds. This work shows that significant information about the early emission phase is being missed without such early observations with simultaneous multi-band instruments.