Evidence for a Bright-Edged Jet in the Optical/NIR Afterglow of GRB 160625B

Using deep and high-cadence gamma-ray burst (GRB) afterglow data from RATIR, 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, NIR, and X-ray data suggest a very narrow (2 degree) jet which 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 lightcurve also exhibits a gradual spectral evolution lasting $>10$ days. The evolution of the flux can be modeled as $\textrm{Flux} \sim \big(\frac{t}{[20 \textrm{days}]}\big)^\alpha \big(\frac{\lambda}{[800 \textrm{nm}]}\big)^\beta$, with a temporal slope $\alpha=-0.956 \pm 0.003$ and a gradually time-varying spectral slope $\beta = (0.60 \pm 0.07)+(0.26 \pm 0.06) \textrm{log}\big(\frac{t}{20 \rm{days}}\big)$.