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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:
Besides being among the most promising sources of gravitational waves,
merging neutron star binaries also represent a leading scenario to explain the
phenomenology of short gamma-ray bursts (SGRBs). Recent observations have
revealed a large subclass of SGRBs with roughly constant luminosity in their
X-ray afterglows, lasting $10\!-\!10^4$ s. These features are generally taken
as evidence of a long-lived central engine powered by the magnetic spin-down of
a uniformly rotating, magnetized object. We propose a different scenario in
which the central engine powering the X-ray emission is a differentially
rotating hypermassive neutron star (HMNS) that launches a quasi-isotropic and
baryon-loaded wind driven by the magnetic field, which is built-up through
differential rotation. Our model is supported by long-term, three-dimensional,
general-relativistic, and ideal magnetohydrodynamic simulations, showing that
this isotropic emission is a very robust feature. For a given HMNS, the
presence of a collimated component depends sensitively on the initial magnetic
field geometry, while the stationary electromagnetic luminosity depends only on
the magnetic energy initially stored in the system. We show that our model is
compatible with the observed timescales and luminosities and express the latter
in terms of a simple scaling relation.
