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

Abstract

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 $\text{Flux}\sim (\frac{t}{[20\text{days}]}{)}^{\hat{I}}\pm (\frac{\hat{I}}{»}[800\text{nm}]{)}^2$, with a temporal slope $\hat{I}\pm =-0.956\pm 0.003$ and a gradually time-varying spectral slope ${\hat{I}}^2=(0.60\pm 0.07)+(0.26\pm 0.06)\text{log}\left(\frac{t}{20\mathrm{days}}\right)$.

10 pages, 3 figures, accepted to ApJ