2015A&A...576A..71Z

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. We examine the physical origin of the X-ray and optical emission of short GRB 130912A. The afterglow emission was analysed and the light curve fitted numerically. 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. 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.