H. Tong, R. X. Xu, L. M. Song, G. J. Qiao
Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetar candidates. During their studies, the magnetic dipole braking mechanism is often assumed. This will result in a high surface dipole field for most AXPs and SGRs. It will also bring several problems challenging the magnetar interpretation. Alternatively, it is possible that AXPs and SGRs are braked down by a particle wind which also originates from magnetic field decay. In the wind braking scenario, magnetars are neutron stars with strong multipole field. A strong dipole field is no longer required. Recent challenging observations of magnetars may be explained naturally in the wind braking scenario: (1) The supernova energetics of those associated with magnetars are of normal value; (2) The non-detection in Fermi observations of magnetars; (3) The problem posed by the low-magnetic field soft gamma-ray repeater; (4) The relation between magnetars and high magnetic field pulsars ; (5) A decreasing period derivative during magnetar outbursts etc. For magnetars with $L_{\rm x}<-\dot{E}_{\rm rot}$, they may still be magnetic dipole braking. This may explain the "fundamental plane" of magnetar radio emissions. A magnetism-powered (instead of rotation-powered) pulsar wind nebula will be one of the consequences of wind braking. For a magnetism-powered pulsar wind nebula, we should see a correlation between the nebula luminosity and the magnetar luminosity. This may be the case of the extended emission around AXP 1E 1547.0-5408. A braking index different from three is also calculated. Future braking index measurement of a magnetar may tell us whether magnetars are wind braking or magnetic dipole braking.
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http://arxiv.org/abs/1205.1626
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