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Hydromagnetic Structure of a Neutron Star Accreting at Its Polar Caps

Published online by Cambridge University Press:  05 March 2013

A. Melatos  and
E. S. Phinney
A. Melatos
Affiliation:
School of Physics, University of Melbourne, Parkville, Vic 3010, Australia; a.melatos@physics.unimelb.edu.au Theoretical Astrophysics, Mail Code 130–33, California Institute of Technology, Pasadena, CA 91125, USA
E. S. Phinney
Affiliation:
Theoretical Astrophysics, Mail Code 130–33, California Institute of Technology, Pasadena, CA 91125, USA
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Abstract

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The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated as a function of accreted mass, Ma, starting from a polytropic sphere plus central magnetic dipole (Ma =0) and evolving the configuration through a quasistatic sequence of twodimensional, Grad–Shafranov equilibria as Ma increases. It is found that the accreted material spreads equatorward under its own weight, compressing the magnetic field into a thin boundary layer and burying it everywhere except in a narrow, equatorial belt. The magnetic dipole moment of the star is given by µ=5.2×1024(B0/1012.5G)1.3(Ma/10−8Mʘ yr−1)0.18(Ma/Mʘ)−1.3Gcm3, and the fractional difference between its principal moments of inertia is given by Є=2.1×10−5(B0/1012.5G)0.27(Ma/10−8Myr−1)0.18(Ma/Mʘ)1.7, for Ma in the range 10−5Ma/Mʘ10−1,where B0 is the pre-accretion magnetic field strength, and Ma is the accretion rate.

Keywords

accretion, accretion diskspulsars:generalstars: magnetic fieldsstars: neutron
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 18 , Issue 4 , 2001 , pp. 421 - 430
Copyright
Copyright © Astronomical Society of Australia 2001

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