Evidence for a Bright-edged Jet in the Optical/Near-infrared Afterglow of GRB 160625B

Using deep and high-cadence gamma-ray burst (GRB) afterglow data from the Reionization And Transients InfraRed/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 degrees) 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 similar to(t/[120 days])(alpha)(lambda/[800 nm])(beta) with a temporal slope alpha = -0.956 +/- 0.003 and a gradually time-varying spectral slope beta = (0.60. +/- 0.07 +/- (0.26 +/- 0.06)log(t/20 days).