Magnetically-induced outflows from binary neutron star merger remnants

Abstract

Recent observations by the Swift satellite have revealed long-lasting ($\sim 10^2-10^5\,\mathrm{s}$), "plateau-like" X-ray afterglows in the vast majority of short gamma-ray bursts events. This has put forward the idea of a long-lived millisecond magnetar central engine being generated in a binary neutron star (BNS) merger and being responsible for the sustained energy injection over these timescales ("magnetar model"). We elaborate here on recent simulations that investigate the early evolution of such a merger remnant in general-relativistic magnetohydrodynamics. These simulations reveal very different conditions than those usually assumed for dipole spin-down emission in the magnetar model. In particular, the surrounding of the newly formed NS is polluted by baryons due to a dense, highly magnetized and isotropic wind from the stellar surface that is induced by magnetic field amplification in the interior of the star. The timescales and luminosities of this wind are compatible with early X-ray afterglows, such as the "extended emission". These isotropic winds are a generic feature of BNS merger remnants and thus represent an attractive alternative to current models of early X-ray afterglows. Further implications to BNS mergers and short gamma-ray bursts are discussed.