Thin accretion discs around millisecond X-ray pulsars

Abstract

Millisecond X-ray pulsars have weak magnetic dipole moments of ∼10 16 T m 3 compared to ordinary X-ray pulsars with dipole moments of 10 20 T m 3 . For this reason a surrounding accretion disc can extend closer to the millisecond X-ray pulsars, and thus reach a higher temperature, at which the opacity is dominated by electron scattering and radiation pressure is strong. We compute the self-similar structure of such a geometrically thin axisymmetric accretion disc with an internal dynamo. Such models produce significantly stronger torques on the neutron star than models without dynamos, and can explain the strong spin variations in some millisecond X-ray pulsars.

Effects of dynamo magnetic fields on observational properties of accreting millisecond X-ray pulsars

“…Here, as was studied by Tessema & Torkelsson (2011), we integrate Equation (39) for the inner region inwards from very small radius ∼ 12.5 and Λ = 1/3π. The dimensionless parameters γ, γ dyn and α ss are 1, 10 and 10 −2 , respectively.…”

“…This figure depicts the variations of Λ as a function of r for all ǫ = 0.45, 0.15 and 0. All solutions are case V inner boundaries except −0.15 and −0.45 (Tessema & Torkelsson 2011).…”

“…Moreover, as was investigated by Tessema & Torkelsson (2011), the standard accretion disk solution has a case D inner region boundary when the viscous torque is neglected in accretion disk theory. Hence, the angular momentum is transported from the NS to the disk when it is in the case V inner region boundary.…”

Stability of Accretion Discs around Magnetized Stars

“…They introduced a new solution for an accretion disc around a magnetic star. [11], hereafter Paper II, have also studied the thin accretion disc around millisecond X-ray pulsars, which is the extension of Paper I. In both Papers they have developed a model for steady-state disc and found that the torque generated by the internal dynamo is stronger than the torque generated due to the shear.…”