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WIND ACCRETION TO MAGNETIZED STARS

SPINNING-DOWN OF MOVING MAGNETAR IN THE PROPELLER REGIME 

[abstract] [full text] [plots from the paper] [animation]

Using axisymmetric MHD simulations we have studied the supersonic propagation through the ISM of magnetars in the propeller stage. We have done many simulation runs for the purpose of determining the angular momentum loss rate of the star due to the interaction of its magnetosphere with the shocked ISM. 

We conclude, that the interaction may be highly effective in spinning-down magnetars. From many simulation runs we have derived an approximate scaling laws for the angular momentum loss rate: 
dL/dt ~ -
hm0.3 m0.6 r0.8 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 hm is magnetic diffusivity. A star with magnetic field B ~ 1013 - 1015 G is expected to spin-down in DT ~ 104 - 105 years. This time may be longer if the ISM material does not effciently interact with the external regions of the magnetar's magnetosphere. Therefore, after relatively short stages of pulsar and propeller activity, a magnetar becomes a very slowly rotating object, with a period P > 105 -106 s, which is much longer than the periods expected for ordinary pulsars. This may be a reason why the number of soft gamma repeaters, which are candidate magnetars, is so small. We should note however, that the rate of spinning-down depends on the magnetic diffusivity which is not known. At lower diffusivity the rate of spinning-down will be lower. The INS candidate RX J1856.5-3754 may be an example of a slowly rotating magnetar. However, this model does not explain the Ha nebulae. An ordinary misaligned pulsar explains the diŽerent features more easily, excluding the fact that no periodic fluctuations were observed from this object.

 

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