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

Understand

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.