The physical origin of optical flares following GRB 110205A and the nature of the outflow

Abstract

The optical emission of GRB 110205A is distinguished by two flares. In this work we examine two possible scenarios for the optical afterglow emission. In the first scenario, the first optical flare is the reverse shock emission of the main outflow and the second one is powered by the prolonged activity of central engine. We however find out that it is rather hard to interpret the late ($t>0.1$ day) afterglow data reasonably unless the GRB efficiency is very high ($\sim 0.95$). In the second scenario, the first optical flare is the low energy prompt emission and the second one is the reverse shock of the initial outflow. Within this scenario we can interpret the late afterglow emission self-consistently. The reverse shock region may be weakly magnetized and the decline of the second optical flare may be dominated by the high latitude emission, for which strong polarization evolution accompanying the quick decline is possible, as suggested by Fan et al. in 2008. Time-resolved polarimetry by RINGO2-like polarimeters will test our prediction directly.