The long-lasting optical afterglow plateau of short burst GRB 130912A

TL;DR

This study analyzes the unique long-lasting optical afterglow plateau of short GRB 130912A, demonstrating that a standard forward shock model explains the data without energy injection, and suggesting a low-density environment consistent with a merger origin.

Contribution

It shows that the optical plateau can be explained by a canonical forward shock model without energy injection, challenging previous interpretations requiring additional energy input.

Findings

The optical plateau is explained by standard forward shock emission.

The burst occurred in a very-low density interstellar medium.

High fractions of energy were allocated to electrons and magnetic fields.

Abstract

The short burst GRB 130912A was detected by Swift, Fermi satellites and several ground-based optical telescopes. Its X-ray light curve decayed with time normally. The optical emission, however, displayed a long term plateau, which is the longest one in current short GRB observations. In this work we examine the physical origin of the X-ray and optical emission of this peculiar event. We find that the canonical forward shock afterglow emission model can account for the X-ray and optical data self-consistently and the energy injection model that has been widely adopted to interpret the shallowly-decaying afterglow emission is not needed. We also find that the burst was born in a very-low density interstellar medium, consistent with the compact object merger model. Significant fractions of the energy of the forward shock have been given to accelerate the non-thermal electrons and amplify the magnetic fields (i.e., $\epsilon_{\rm e}\sim 0.37$ and $\epsilon_{\rm B}\sim 0.16$, respectively), which are much larger than those inferred in most short burst afterglow modeling and can explain why the long-lasting optical afterglow plateau is rare in short GRBs.