Reverse shock emission driven by post-merger millisecond magnetar winds: Effects of the magnetization parameter

Abstract

The study of short-duration gamma-ray bursts provides growing evidence that a good fraction of double neutron star mergers lead to the formation of stable millisecond magnetars. The launch of Poynting flux by the millisecond magnetars could leave distinct electromagnetic signatures that reveal the energy dissipation processes in the magnetar wind. In previous studies (Wang & Dai 2013b; Wang et al. 2015), we assume that the magnetar wind becomes completely lepton-dominated so that electrons/positrons in the magnetar wind are accelerated by a diffusive shock. However, theoretical modeling of pulsar wind nebulae shows that in many cases the magnetic field energy in the pulsar wind may be strong enough to suppress diffusive shock acceleration. In this paper, we investigate the reverse shock emission as well as the forward shock emission with an arbitrary magnetization parameter $\sigma$ of a magnetar wind. We find that the reverse shock emission strongly depends on $\sigma$, and in particular, $\sigma \sim 0.3$ leads to the strongest reverse shock emission. Future observations would be helpful to diagnose the composition of the magnetar wind.