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

arXiv comment10 pages, 3 figures, accepted to ApJ

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• GRB 160625B: Evidence for a Gaussian-Shaped Jet
• Three-stage Collapse of the Long Gamma-Ray Burst from GRB 160625B Prompt Multiwavelength Observations
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