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 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 $\mathrm{Flux}\sim {\left(\tfrac{t}{[20\mathrm{days}]}\right)}^{\alpha }{\left(\tfrac{\lambda }{[800\mathrm{nm}]}\right)}^{\beta }$, with a temporal slope α = −0.956 ± 0.003 and a gradually time-varying spectral slope $\beta =(0.60\,\pm 0.07)+(0.26\pm 0.06)\mathrm{log}\left(\tfrac{t}{20\,\mathrm{days}}\right)$.