Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...465l.111w&link_type=abstract
Astrophysical Journal Letters v.465, p.L111
Astronomy and Astrophysics
Astronomy
46
Accretion, Accretion Disks, Stars: Binaries: Close, Stars: Magnetic Fields, Stars: Neutron, Stars: Rotation, X-Rays: Stars
Scientific paper
In models for disk accretion onto magnetized objects, the inner radius R0 of the Keplerian disk is conventionally expressed in the form R0 = xi rA, where rA is the Alfven radius for spherical accretion and the parameter xi is usually taken to be 0.5. We point out that the value of xi in fact depends on the fraction of the star's magnetic flux threading the disk, which is poorly known from theory: in general, xi ~= 1.35 eta 4/7, where eta <= 1 denotes the threading coefficient. Application of the beat frequency model to binary X-ray pulsars showing quasi-periodic oscillations suggests strongly that eta ~ 1 for these objects: the stellar dipole field essentially fully threads the disk and xi ~= 1. When combined with improved accretion torque models characterized by critical fastness parameters near unity, the corrected values of xi ≡ R0/rA should allow more reliable determinations of the accreting star's dipole moment and mass-radius relationship.
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