2019ApJ...873L...6S

Using deep and high-cadence gamma-ray burst (GRB) afterglow data from the Reionization And Transients Infra-Red/Optical camera, we observe a sharp and achromatic light curve break 12.6 days after the GRB, accompanied by an approximately achromatic bump. Fitting of the optical, near-infrared, and X-ray data suggest a very narrow (2°) jet that remains collimated at late time. We argue that the sharp light curve bump suggests an edge-brightened jet, perhaps emitting only during a brief period of lateral jet expansion. The light curve also exhibits a gradual spectral evolution lasting >10 days. The evolution of the flux can be modeled as Flux∼(t{[20days]})^α^{(λ{[800nm]})}β^, with a temporal slope α = -0.956 ± 0.003 and a gradually time-varying spectral slope β=(0.60±0.07)+(0.26±0.06)log(t{20days}).