Magnetospheric Flows in X-ray Pulsars I: Instability at super-Eddington regime of accretion
Monthly Notices of the Royal Astronomical Society(2024)
Abstract
Within the magnetospheric radius, the geometry of accretion flow in X-ray
pulsars is shaped by a strong magnetic field of a neutron star. Starting at the
magnetospheric radius, accretion flow follows field lines and reaches the
stellar surface in small regions located close to the magnetic poles of a star.
At low mass accretion rates, the dynamic of the flow is determined by
gravitational attraction and rotation of the magnetosphere due to the
centrifugal force. At the luminosity range close to the Eddington limit and
above it, the flow is additionally affected by the radiative force. We
construct a model simulating accretion flow dynamics over the magnetosphere,
assuming that the flow strictly follows field lines and is affected by gravity,
radiative and centrifugal forces only. The magnetic field of a NS is taken to
be dominated by the dipole component of arbitrary inclination with respect to
the accretion disc plane. We show that accretion flow becomes unstable at high
mass accretion rates and tends to fluctuate quasi-periodically with a typical
period comparable to the free-fall time from the inner disc radius. The
inclination of a magnetic dipole with respect to the disc plane and strong
anisotropy of X-ray radiation stabilise the mass accretion rate at the poles of
a star, but the surface density of material covering the magnetosphere
fluctuates even in this case.
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