  Based on the stiff equations of state of neutron stars (NS) and the discovery
of high-mass NSs, a NS-NS merger will leave behind, with high probabilities, a
rapidly rotating massive magnetar. The central magnetar will dissipate its
rotational energy to the outflow by injecting Poynting flux, which will become
lepton-dominated so that a long-lasting reverse shock (RS) is developed. We
calculate the emission of the RS as well as the emission of forward shock (FS)
and find that in most cases the RS emission is stronger than FS emission. It is
found that the recently discovered transient, PTF11agg, can be neatly accounted
for by the RS emission powered by a millisecond magnetar. Other alternative
models have been considered and cannot explain the observed light curves well.
We therefore suggest that PTF11agg be the first evidence for RS emission from a
postmerger millisecond magnetar.
