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Free keywords:
Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE, Astrophysics, Solar and Stellar Astrophysics, astro-ph.SR,General Relativity and Quantum Cosmology, gr-qc
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.
