Observation of the optical afterglow of GRB 050319: The wind-to-ISM transition in view

Abstract

The collapse of a massive star is believed to be the most probable progenitor of a long gamma-ray burst

(GRB). Such a star is expected to have its environment modified by the stellar wind. The effect of such a circumstellar wind medium is expected to be seen in the evolution of a GRB afterglow, but so far this has not been conclusively found. We claim that a signature of the transition from wind to constant density medium of the circumburst medium is visible in the afterglow of GRB 050319. Along with the optical observations of the afterglow of GRB 050319, we present a model for the multiband afterglow of GRB 050319. We show that the break seen in the optical light curve at ∼0.02 days could be explained as being due to the transition from wind to constant density medium of the circumburst medium, in which case, to our knowledge, this could be the first ever detection of such a transition at any given frequency band. Detection of such a transition could also serve as confirmation of the massive star collapse scenario for GRB progenitors, independent of supernova signatures.