2023MNRAS.520.6104K

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 ∼ 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 theSwift BAT triggered on GRB201015A, we observed a sharp rise in optical flux to a peak, followed by a power law temporal decay, ∝ 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 $A_\mathrm{v}^\mathrm{local}$ from ∼0.8 mag to 0.3 mag in ∼2500 s. This work shows that significant information about the early emission phase is being missed without such early observations with simultaneous multiband instruments.