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Astron. Astrophys. 337, 441-446 (1998)

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Magnetic field evolution of accreting neutron stars

K.S. Cheng 1 and C.M. Zhang 1, 2

1 Department of Physics, University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China
2 Department of Physics, Hebei University of Technology, Tianjin-300130, P.R. China

Received 30 December 1997 / Accepted 1 June 1998

Abstract

We study the evolution of the magnetic field of an accreting neutron star in the frozen field and incompressible fluid approximations. The plasma is accreted onto two polar caps and squeezes some of the surface material of the neutron star toward the equator. The frozen B-field is then pushed toward the equator and is eventually buried there. The magnetic field within the polar cap areas, which is defined by the Alfv[FORMULA]n radius, decreases due to the expansion of the polar cap areas resulting from the physical motion of the accreted material, which conserves the magnetic flux. But the decrease of the magnetic field also changes the Alfv[FORMULA]n radius which modifies the size of the polar cap and also affects the decrease of the magnetic flux within the polar caps. Therefore, the magnetic field enclosed by the polar caps appears to decay rapidly with a time scale of [FORMULA] years. As a consequence the magnetic field outside the polar cap is increasing because the total flux of the entire stellar surface is conserved in our approximations. The decrease of the polar cap magnetic field will stop and reach a minimum value [FORMULA]G when the magnetic field outside the polar cap reaches [FORMULA]G, which is strong enough to stop the motion of the accretion material across the stellar surface. However, this strong [FORMULA] cannot be observed because the accreted matter stopped by this strong field cannot move toward the equator. Instead it moves inward and pulls this field inside the crust with a time scale [FORMULA] yr. Pulsars accreting similar masses but having very different magnetic field may result from different equations of state.

Key words: stars:neutron – stars: magnetic fields – X-rays: stars – stars: binaries: close

Send offprint requests to: K.S. Cheng of Dept. of Phys., The University of Hongkong

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© European Southern Observatory (ESO) 1998

Online publication: August 17, 1998
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