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

Abstract

Instruments such as the ROTSE, TORTORA, Pi of the Sky, MASTER-net, and others have recorded single-band optical flux measurements starting as early as $\thicksim$ 10 seconds after a gamma-ray burst 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 Swift BAT triggerred on GRB201015A, we observed a sharp rise in flux to a peak, followed by an approximate power law decay light curve, $\propto t^{-0.81 \pm 0.03}$. Flux was measured simultaneously in three optical filter bands, g', r', and i', using our unique instrument mounted on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory (NUTTelA-TAO). Our simultaneous multi-band observations of the early afterglow show strong colour evolution from red to blue, with a change in the optical log slope (after correction for Milky Way extinction) 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 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. If we assume that the optical log slope is constant throughout this period, with the value given by the late-time slope, and we further assume 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 s. This work shows that significant information about the early emission phase (and possibly prompt emission, if observed early enough) is being missed without such early observations with simultaneous multi-band instruments