Volume 576, April 2015
|Number of page(s)||5|
|Published online||01 April 2015|
The long-lasting optical afterglow plateau of short burst GRB 130912A
College of Science, Guilin University of Technology,
2 Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, 210008 Nanjing, PR China
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
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., ϵe ~ 0.37 and ϵB ~ 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.
Key words: gamma rays: general / X-rays: bursts
© ESO, 2015
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