MHD simulations of spherical accretion to a star in the propeller regime

SPINNING-DOWN OF MOVING MAGNETAR IN THE PROPELLER REGIME

Authors: Toropina O.D., Romanova M.M. and Lovelace R. V. E.

We use axisymmetric magnetohydrodynamic simulations to investigate the spinning-down of magnetars rotating in the propeller regime and moving supersonically through the interstellar medium. The simulations indicate that magnetars spin-down rapidly due to this interaction, faster than for the case of a non-moving star. From many simulation runs we have derived an approximate scaling laws for the angular momentum loss rate:
dL/dt ~ - h_m^0.3 m^0.6r^0.8 M^-0.4 W_*^1.5, where r is the density of the interstellar medium, M is Mach number, m is the star's magnetic moment, W_* is its angular velocity, and h_m is magnetic diffusivity. A magnetar with a surface magnetic field of 10¹³ - 10¹⁵ G is found to spin-down to a period P > 10⁵ - 10⁶ s in ~ 10⁴ -10⁵ years. There is however uncertainty about the value of the magnetic di®usivity so that the time-scale may be longer. We discuss this model in respect of Soft Gamma Repeaters (SGRs) and the isolated neutron star candidate RXJ1856.5-3754.

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