The disk-magnetosphere interaction close to the neutron star’s surface

Many Galactic X-ray sources host magnetized NS, which accrete matter from a companion star via a disk. The study of the interaction between the accretion disk and the NS magnetosphere is motivated by the need to understand the spin and magnetic field evolution of these systems. A still unsolved key problem in this area of research is the observed upper limit of about 730 Hz to the spin frequency of accreting magnetic neutron stars in X-ray binaries (as well as recycled radio pulsars): this is well below the breakup spin rate for most neutron star (NS) equation of state (EOS) (Chakrabarty et al. (2003); Chakrabarty (2008); Patruno (2010a)).

In the recent years it has been concluded (Haskell & Patruno (2012)) that gravitational wave emission is unlikely to account for a spin equilibrium frequency at 730 Hz, as it was previously proposed (Bildsten (1998)). A more reasonable explanation involves spin-down due to the disk- magnetosphere interaction, which was first described by P. Ghosh and F.K. Lamb in 1979 (GL).

In my seminar, after introducing the problem of the spin distribution cut-off, I will discuss the GL model focusing on the disk/magnetosphere boundary-layer, and provide a critical reading of it, underlining both its application limits and strong features.

Then I will present an extension of the GL model for the boundary-layer structure, which retains its validity even when the layer’s width is comparable to the distance between the star surface and the boundary layer. This model development is motivated by the fact that when the star’s spin frequency is about 700 Hz, the disk must extend very close to the stellar surface in order to accrete. The results of some numerical integrations are presented and their implications discussed for the observed spin limit of accreting magnetic neutron stars.