Constraints on the disc-magnetosphere interaction in accreting pulsar 4U 1626--67

Abstract

Using the spin and flux evolution of the accreting pulsar 4U 1626$-$67 across the 2008 torque reversal, we determine the fastness parameter dependence of the dimensionless torque acting on the pulsar. We find that the dimensionless torque is qualitatively different from the existing models: it is concave-up across the torque equilibrium whereas the existing torque models predict a concave-down (convex) relation with the fastness parameter. We show that the dimensionless torque has a cubic dependence on the fastness parameter near the torque equilibrium. We also find that the torque can not attain large values away from the equilibrium, either in the positive or the negative side, but saturates at limited values. The spin-down torque can attain a 2.5 times larger magnitude at the saturation limit than the spin-up torque. From the evolution of the frequency of quasi-periodic oscillations of 4U 1626$-$67 across the torque reversal of 1990, we determine the critical fastness parameter corresponding to torque equilibrium to be ${\omega }_{\mathrm{c}}\simeq 0.75$ within the framework of the beat frequency model and boundary region model for reasonable values of the model parameters. We find that the disc magnetosphere interaction becomes unstable when the inner radius approaches the corotation radius as predicted by some models, though with a longer timescale. We also find that there is an unstable regime that is triggered when the fastness parameter is 0.8 times the critical fastness parameter ($\omega =0.6$ for ${\omega }_{\mathrm{c}}\simeq 0.75$) possibly associated with an instability observed in numerical simulations.

arXiv commentAccepted by MNRAS; 10 pages, 9 figures

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Cited by in corpus (6)

• On the Magnetic Fields, Beaming Fractions, and Fastness Parameters of Pulsating Ultra-Luminous X-Ray Sources
• On the torque reversals of 4U 1626--67
• On the peculiar torque reversals and the X-ray luminosity history of the accretion-powered X-ray pulsar 4U 1626--67
• On the minimum spin period of accreting pulsars
• Observing Rayleigh-Taylor stable and unstable accretion through a Kalman filter analysis of X-ray pulsars in the Small Magellanic Cloud
• On the Spin Period of the Neutron Star in the Ultraluminous X-Ray Source M51 ULX-8