Is the line-like optical afterglow SED of GRB 050709 due to a flare?

TL;DR

This paper explores whether the line-like optical SED observed in GRB 050709 can be explained by an optical flare, challenging previous interpretations of a macronova origin for the spectral feature.

Contribution

It demonstrates that an optical flare origin for the peculiar SED is plausible, while still supporting a macronova/kilonova as the source of late-time emission.

Findings

Optical flare can produce the observed line-like SED.

Late-time HST F814W-band emission likely from macronova/kilonova.

Flare model offers an alternative explanation to line signal interpretation.

Abstract

Recently Jin et al. (2016) reanalyzed the optical observation data of GRB 050709 and reported a line-like spectral energy distribution (SED) component observed by the Very Large Telescope at $t\sim 2.5$ days after the trigger of the burst, which had been interpreted as a broadened line signal arising from a macronova dominated by iron group. In this work we show that an optical flare origin of such a peculiar optical SED is still possible. Interestingly, even in such a model, an "unusual" origin of the late-time long-lasting Hubble Space Telescope F814W-band emission is still needed and a macronova/kilonova is the natural interpretation.