Observations of the Optical Afterglow of GRB 050319 : Wind to ISM transition in view

TL;DR

This paper reports the first observational evidence of a wind to ISM transition in the afterglow of GRB 050319, supporting the massive star collapse model for long GRBs through optical afterglow analysis.

Contribution

It presents the first detection of a wind to constant density medium transition in a GRB afterglow, confirmed by optical light curve analysis and modeling.

Findings

Identification of a light curve break at 0.02 days as wind to ISM transition.

Modeling supports the transition explanation for the observed optical afterglow features.

Provides evidence for massive star collapse as GRB progenitor independent of supernova detection.

Abstract

The collapse of a massive star is believed to be the most probable progenitor of a long GRB. Such a star is expected to modify its environment by stellar wind. The effect of such a circum-stellar wind medium is expected to be seen in the evolution of a GRB afterglow, but has so far not been conclusively found. We claim that a signature of wind to constant density medium transition of circum-burst 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 optical light curve at $\sim$ 0.02 day could be explained as being due to wind to constant density medium transition of circum-burst 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 a confirmation of massive star collapse scenario for GRB progenitors, independent of supernova signatures.