Angular momentum transport through magnetic accretion curtains inside disrupted discs

Abstract

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‐Alfvénic, 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.

Fitting strategies of accretion column models and application to the broadband spectrum of Cen X-3

“…One assumption would be the presence of a more complex accretion structure. For misaligned magnetic and accretion disk axes one would expect a hollow accretion column or an accretion curtain (Basko & Sunyaev 1976;West et al 2017;Campbell 2012;Perna et al 2006). Such a geometry would lead to a much thinner accretion stream than the solid column that is assumed in the model of BW07.…”

Correlation between the luminosity and spin-period changes during outbursts of 12 Be binary pulsars observed by the MAXI/GSC and the Fermi/GBM

“…Hence, we do not employ this model. Campbell (2012) propose another disk-magnetosphere interaction model considering the angular-momentum feedback from the accreting matter to the disk. The model suggests that the accretion torque is reduced by a factor ωs ∝ L −3/7 from that in equation (6), and thus theνs-L relation becomesνs ∝ L 3/7 .…”

Studying magnetic fields of ultraluminous X-ray pulsars using different accretion torques

“…Six torque models are utilized to calculate the magnetic field, including Ghosh & Lamb [1]'s classical model (hereafter GL), Wang [2]'s model (containing 2 derived magnetic field forms, hereafter WangAB and WangC), Lovelace, Romanova & Bisnovatyi-Kogan [5]'s model (hereafter LRB), Kluźniak & Rappaport [3]'s model(hereafter KR), and Campbell [4]'s model (hereafter Camp). These models are thin-disk ac-cretion models, which are basically modified versions based on the GL model.…”

“…γ and m are dimensionless quantities, and C = 1 is assumed during the calculation. As described by Campbell [4], the accretion torque N = ω * Ṁ * √ GM * R in , which leads to a modification of Equation 5, i.e., ω * Ṁ * √ GM * R in = − Ṗ 2πI/P 2 . Thus, the derived expression of the fastness parameter is…”

“…We checked possible dependence of the magnetic field estimations on the different torque models. The calculations suggest the accretion torque models by Ghosh & Lamb [1], Wang [2], Kluźniak & Rappaport [3] and Campbell [4] are more likely to support magnetar models for ULX pulsars, while Lovelace, Romanova & Bisnovatyi-Kogan [5]'s model generally predicts the magnetic field in normal neutron stars. Implications of our results combined with other independent methods are also discussed, which will help us to understand the nature and rotational behavior of these ULX pulsars.…”