The structure of thin accretion discs around magnetised stars

Abstract

Aims. We determine the steady-state of an axisymmetric thin accretion disc with an internal dynamo around a magnetised star. Methods. Starting from the vertically integrated equations of magnetohydrodynamics we derive a single ordinary differential equation for a thin accretion disc around a massive magnetic dipole and integrate this equation numerically from the outside inwards.Results. Our numerical solution shows that the torque between the star and the accretion disc is dominated by the contribution from the dynamo in the disc. The location of the inner edge of the accretion disc varies between R A and 10 R A depending mainly on the strength and direction of the magnetic field generated by the dynamo in the disc

Thin accretion discs around millisecond X-ray pulsars

“…The models of Ghosh & Lamb (1979) and Rappaport et al (2004) have ignored the possibility of an internal dynamo in the accretion disc, and have considered only the magnetic field that is generated in the disc by the mismatch in the rotation of the neutron star and the disc. However the magnetic field due to a dynamo can strengthen the coupling between the accretion disc and the neutron star, and we (Tessema & Torkelsson 2010) have shown that the angular momentum exchange between the accretion disc and the neutron star can increase by more than an order of magnitude compared to the standard model by Ghosh & Lamb (1979). Furthermore, the dynamo is insensitive to the rotation of the neutron star, and can therefore in principle provide a spin‐up torque even at a low accretion rate.…”

Stability of Accretion Discs around Magnetized Stars

“…The instability has also been invoked as a component of a disc dynamo model, in which the instability creates radial field from vertical field, the shear in the disc creates azimuthal field from the radial component, and the Parker instability creates vertical from azimuthal field and expels flux from the disc [9]. [10] hereafter Paper I, have investigated the interaction between magnetic neutron star and its surrounding accretion disc in the case where the accretion disc is supporting an internal dynamo. They introduced a new solution for an accretion disc around a magnetic star.…”

Angular momentum transport through magnetic accretion curtains inside disrupted discs

“…Most authors have assumed, like Ghosh & Lamb, that all the angular momentum of the magnetically channelled material is transferred to the star (e.g. Wang 1987; Armitage & Clarke 1996; Matt & Pudritz 2005; Tessema & Torkelsson 2010; Campbell 2011). However, Shu et al (1994a,b) and Ostriker & Shu (1995) developed a modified model of the star–disc interaction in which the accretion torque on the star is greatly reduced.…”