Theory of quasi-spherical accretion in X–ray pulsars

1. Theory of quasi-spherical accretion in X–ray pulsars Shakura et al. Reporter: Zhang Zhen 11.11.28

2. A theoretical model for quasi-spherical subsonic accretion onto slowly rotating magnetized neutron stars • Quasi-spherical shell • Convective motions

3. Outlines • Introduction • Quasi-spherical accretion • Spin-up/spin-down transitions • Specific X-ray pulsars

4. INTRODUCTION • Disk accretion • Wind accretion • Spin-up: be proportional to Fryxell & Taam (1988), Ruffert (1997), Ruffert (1999), etc Eccentricity & inhomogeneous • Spin-down: even more uncertain

5. 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 magnetosphere.

6. QUASI-SPHERICAL ACCRETION • Aim: • Twoequations: The structure of the shell Hydrostatic equilibrium equation zeroth approximation

7. QUASI-SPHERICAL ACCRETION • First approximation: the entropy distribution in the shell is constant • The Alfven surface

8. QUASI-SPHERICAL ACCRETION • Prandtl law and isotropic turbulence • ω∗ > ωm: transfer of angular momentum from the magnetosphere to the shell • ω∗ < ωm: from the shell to the magnetosphere if • ω∗: angular velocity of NS

9. 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

10. 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

11. 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

12. The case of moderate coupling • Rayleigh-Taylor instability

13. Spin-up/spin-down transitions

14. Specific X-ray pulsars • GX 301-2 Doroshenko et al. 2010

15. Vela X-1

17. Reference • Arxiv: 1110.3701v1 • Arxiv: 1111.1382v1

18. 没讲清的问题 • The plasma enters the magnetosphere of the slowly rotating neutron star due to the interchange instability.

19. Thanks