C. G. Campbell

The inner structure of an accretion disc around a magnetic neutron star

Abstract The effect of a magnetic binary neutron star on the inner structure of its accretion disc is considered. The stellar field penetrates the disc and toroidal field is created as a result of the vertical shearing motions, its magnitude being diffusion limited. If the disc’s magnetic diffusivity, ν. is treated as a function of ω, the distance from the stellar centre, with a radial length scale of ∼ω w, a self-consistent solution can be found for the inner region. In the orbital plane, the magnetic force has a small effect on the radial momentum, so the angular velocity is Keplerian. The vertical equilibrium is a balance between the gradients in azimuthal magnetic pressure and thermal pressure, while the inflow is caused by the transfer of angular momentum from the disc to the star, via magnetic stresses. Vertical equilibrium breaks down as the inflow speed becomes a significant fraction of the Keplerian speed, this occurring in the region of the Alfven radius based on radial free-fall. However, if ν ...

Magnetically-controlled disc accretion

Abstract A magnetic alternative to the viscous accretion disc, around a non-magnetic star, is presented. The disc field is generated by an αω-dynamo; only weak turbulence is required to produce the α-effect, so the viscous force is negligible. The magnetic force does not significantly affect the vertical equilibrium or the radial momentum, hence stellar gravity balances the thermal pressure gradient vertically and the angular velocity is Keplerian up to a boundary layer close to the stellar surface. The azimuthal magnetic force causes the outward advection of angular momentum necessary for the transfer of matter through the disc at the externally imposed rate. The magnetic field has a quadrupolar structure, so B⊘ is generated by the shearing of radial field. Eigenstate solutions result, the lowest modes corresponding to the lowest temperatures. The values of B⊘, in the central plane are ∼ 100 gauss, with essentially vanishing magnetic field at the disc surface.

Angular momentum transport through magnetic accretion curtains inside disrupted discs

The structure of accretion curtain flows, which form due to disc disruption by a strongly magnetic star, is considered. It is shown that a sub-Alfvenic, magnetically channelled flow is consistent with matching the magnetic field across the curtain base where it meets the disrupted inner region of the disc. The resulting angular velocity distribution in the curtain flow is calculated, together with the consequent angular momentum transfer rate to the star. It is shown that the transition of material from the diffusive disc flow to the channelled curtain flow results in some angular momentum being fed back into the disc. This is consistent with the total angular momentum balance, and can result in a significantly smaller accretion torque acting on the star than that given by the standard model which assumes no angular momentum feedback to the disc. The sonic point coordinates are found and a critical stellar rotation rate results below which the sonic point merges with the curtain flow base, due to a reduced centrifugal barrier to the flow. Hence, at these lower rotation rates, little thermal assistance is required for matter to make the transition to a supersonic accretion flow.

Disc structure and angular momentum transfer in X-ray binary pulsars

Abstract The angular momentum exchange between a binary magnetic neutron star and its accretion disc is considered. In a previous paper it was shown that if the discs magnetic diffusivity,η, is due to buoyancy or turbulence, disruption occurs where the magnetic extraction of angular momentum starts to dominate that due to viscous advection. The disruption radius is significantly larger than one half the spherical Alfven radius, often previously used for the discs inner edge. The consequences of the increased hole radius are investigated in the present paper, the solutions for the two forms of η being compared. The magnetic and accretion torques acting on the neutron star are calculated, and expressions are found for the equilibrium period at which they cancel. Expressions are also derived for the observed quantity |[Pdot]|/P, these being related to the disruption radius and stellar spin rate. It is shown that the azimuthal viscous and magnetic forces can become comparable in a small region beyond the co...

The stabilizing effect of viscosity on magnetic wind flows from accretion discs

Magnetically channelled winds are believed to be a feature of most accretion discs. It has been shown that such flows can remove significant amounts of angular momentum from the disc and make a major contribution to driving the inflow. For a suitable range of poloidal magnetic field bending, only a small fraction of the disc mass is lost in the wind flow, so most material reaches the inner region of the disc. However, discs driven purely by such a process are prone to a field-bending instability which can lead to runaway mass loss. It is shown here that a small amount of disc viscosity can quench such an instability and allow steady disc-wind models to be constructed. The effects of perturbations to the coupling between the radial and vertical structures are allowed for, with the thermal balance having particular relevance. Runaway increases in field bending are prevented by increases in the disc temperature and magnetic diffusivity mainly caused by viscous dissipation.

Matching disc and curtain flows in magnetically disrupted accretion discs

Abstract An accretion curtain forms when a strongly magnetic star disrupts the inner region of its surrounding disc. It was previously shown that the disc expands vertically due to rapidly growing thermal pressure caused by magnetic heating over a narrow radial transition region inside the corotation radius. This allows material to flow from the disc into a magnetically channelled curtain through which it is transferred to the star. The curtain flow is trans-sonic and sub-Alfvénic, with small distortions of the stellar magnetic field. In the present paper, the disc and curtain flows are matched across the upper boundary of the disc transition region, and this is shown to determine the width of this region as a function of the stellar rotation rate. The sonic point position can adjust to allow steady mass transfer from the disc to the curtain flow. An upper limit can be defined for the rotation period of the star below which a strong magnetic channelling regime applies, with the outer edge of the disruption region lying inside a spherical Alfvén radius. The picture of a thin, magnetically channelled curtain flow fed from a thermally disrupted disc is self-consistent in this regime. A lower limit arises for the stellar angular velocity below which the sonic point merges with the curtain base, resulting in excessive mass loss from the disc which would be inconsistent with a steady solution. This corresponds to a lower limit on the disruption radius as a fraction of the corotation radius. It is noted that the spin-up timescale of the accreting star is significantly less than the lifetime of the system so that typical observed systems should lie in the strong magnetic regime.