MAGNETIC FIELD SCREENING OF DISK ACCRETING STARS

Authors: Lovelace R. V. E., Romanova M.M., and Bisnovatyi-Kogan G.S., 


An analytical model is developed for the screening of the external magnetic field of a rotating, axisymmetric neutron star due to the accretion of plasma from a disk. The decrease of the field occurs as a result of the electric current in the infalling plasma. The deposition of this current-carrying plasma on the star's surface creates an induced magnetic moment with a sign opposite to that of the original magnetic dipole. The field decreases independent of whether the star spins up or spins down. The timescale for an appreciable decrease (factor of >100) of the field is found to be  ~1.6 × 107 yr, for a mass accretion rate M  = 10-9 M
Q yr-1 and an initial magnetic moment mi = 1030 G cm3, which corresponds to a surface field of 1012 G if the star's radius is 106 cm. The timescale varies approximately as mi 3.8/M1.9. The decrease of the magnetic field does not have a simple relation to the accreted mass. Once the accretion stops the field leaks out on an Ohmic diffusion timescale that is estimated to be >109 yr.

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Last updated on 29.01.07