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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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