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

¹ College of Science, Guilin University of Technology, 541004 Guilin, PR China
e-mail: fwzhang@hotmail.com
² Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
e-mail: jin@pmo.ac.cn
³ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, PR China
⁴ Graduate University of Chinese Academy of Sciences, 100049 Beijing, PR China

Received: 15 September 2014
Accepted: 16 January 2015

Abstract

Context. 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.

Aims. We examine the physical origin of the X-ray and optical emission of short GRB 130912A.

Methods. The afterglow emission was analysed and the light curve fitted numerically.

Results. The canonical forward-shock model of the afterglow emission accounts for the X-ray and optical data self-consistently, so the energy injection model that has been widely adopted to interpret the shallowly decaying afterglow emission is not needed.

Conclusions. The burst was born in a very-low density interstellar medium, which is consistent with the compact-object merger model. Significant amounts of the energy of the forward shock were given to accelerate the non-thermal electrons and amplify the magnetic fields (i.e., ϵ_e ~ 0.37 and ϵ_B ~ 0.16, respectively), which are much more than those inferred in most short-burst afterglow modelling and can explain why the long-lasting optical afterglow plateau is rare in short GRBs.