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 days } \big ) .