Theory of quasi-spherical accretion in X–ray pulsars
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Transcript of Theory of quasi-spherical accretion in X–ray pulsars
Theory of quasi-spherical accretion in X–ray pulsars
Shakura et al.
Reporter: Zhang Zhen
11.11.28
• A theoretical model for quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars
• Quasi-spherical shell
• Convective motions
Outlines
• Introduction
• Quasi-spherical accretion
• Spin-up/spin-down transitions
• Specific X-ray pulsars
INTRODUCTION
• Disk accretion
• Wind accretion
• Spin-up: be proportional toFryxell & Taam (1988), Ruffert (1997), Ruffert (1999), etc
Eccentricity & inhomogeneous
• Spin-down: even more uncertain
INTRODUCTION
• Different ways for wind accreted on to NS• Burnard et al. (1983). • X-ray flux cools down & freely falls down• Supersonic a shock above the magnetosphere
• Davies & Pringle (1981)• X-ray flux weak to some extend • the radial velocity of the plasma is subsonic,• A hot quasi-static shell forms around the magnet
osphere.
QUASI-SPHERICAL ACCRETION
• Aim:
• Two equations:
The structure of the shell
Hydrostatic equilibrium equation
zeroth approximation
QUASI-SPHERICAL ACCRETION
• First approximation:
the entropy distribution in the shell is constant
• The Alfven surface
QUASI-SPHERICAL ACCRETION
• Prandtl law and isotropic turbulence
• ω > ω∗ m: transfer of angular momentum from the magnetosphere to the shell
• ω < ω∗ m: from the shell to the magnetosphere if
• ω : angular velocity of NS∗
QUASI-SPHERICAL ACCRETION
• Strong coupling
• Propeller / Equilibrium state
• Moderate coupling
• the plasma can enter the magnetosphere
• instabilities on a timescale shorter than that needed for the toroidal field to grow to the value of the poloidal field
The case of strong coupling
• The turbulent magnetic field diffusion coefficient η Lovelace et al. 1995
• Energy: from the rotation of a magnetospheric surface to turbulence
• ~ kinematic turbulent viscosity
The case of strong coupling
• RA > Rc, matter does not fall onto the neutron star, there are no accretion-generated X-rays from the neutron star, the shell rapidly cools down and shrinks and the standard Illarionov and Sunyaev propeller (1975)
• Equilibrium state• RA > Rc
The case of moderate coupling
• Rayleigh-Taylor instability
Spin-up/spin-down transitions
Specific X-ray pulsars
• GX 301-2
Doroshenko
et al. 2010
• Vela X-1
Reference
• Arxiv: 1110.3701v1
• Arxiv: 1111.1382v1
没讲清的问题• The plasma enters the magnetosphere of t
he slowly rotating neutron star due to the interchange instability.
Thanks