GRB 050319 afterglow : wind to ISM transition in view?

Abstract

The core collapse of a Massive Star is assumed to be the most probable progenitor of GRBs. Such a star is expected to modify its environment by stellar wind. The signature of such a circumstellar wind medium on the evolution of a GRB afterglow has so far not been conclusively seen. We claim a clear signature of `wind to homogenous interstellar medium transition' of circum-burst medium in the afterglow of GRB 050319. The unusual features observed in GRB 050319 afterglow are: (i) X-ray light curve with two breaks and a very fast decay of X-ray flux at very early times; (ii) an unusual transition in the optical light curves from a steep to a flat decay with a break at sim 0.02 day after the burst onset. This break is absent in the X-ray light curve. (iii) Overall flatter decay of flux in comparison to the majority of GRB afterglows. We present a model for the multiband afterglow of GRB 050319. We show that the break seen in optical light curve at sim 0.02 day is due to `wind to homogenous interstellar medium transition' of circum-burst medium. To our knowledge, this is the first ever detection of such a transition. This can also serve as a confirmation of massive star collapse scenario for GRB progenitors, independent of Supernova signatures.