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