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

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 similar to 10 s 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 s after the Swift BAT triggered on GRB201015A, we observed a sharp rise in optical flux to a peak, followed by a power law temporal decay, proportional to t(-0.81 +/- 0.03). Flux was measured simultaneously in three optical bands, g', r', and i', using our Burst Simultaneous Three-channel Imager on the Nazarbayev University Transient Telescope at Assy-Turgen Astrophysical Observatory 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 +/- 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 Small Magellanic Cloud-like extinction curve, we derive a change in the local extinction A(v)(local) from similar to 0.8 mag to 0.3 mag in similar to 2500 s. This work shows that significant information about the early emission phase is being missed without such early observations with simultaneous multiband instruments.