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arXiv · 2609.28092

Shock-free super-Eddington accretion onto magnetized neutron star

Galina Lipunova·Pavel Abolmasov
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Abstract

Strong magnetic fields allow accreting neutron stars to form magnetically controlled funnel flows. At sufficiently high accretion rates, such flows become radiation-pressure-supported. In the conventional picture, the magnetospheric flow contains a nearly free-falling transonic outer region, a radiation-mediated shock, and a subsonic accretion column. We investigate the regime in which the height of the column is expected to exceed the magnetospheric radius. In this case a self-consistent transonic solution with a standing shock is no longer possible, and the entire magnetospheric flow remains subsonic. We construct a semi-analytical hydrodynamical solution for a fully subsonic, radiation-supported magnetospheric flow in the low-Mach-number limit. The magnetic field is treated in the force-free approximation, while the flow structure is determined by mass conservation, energy conservation, and the momentum equation along the field lines. We test the solution using one-dimensional time-dependent simulations with the code HACol. The shock-free solution is highly advective. Its enthalpy profile is close to the adiabatic limit, while the energy density is regulated by leakage of mass and heat through the sides of the accretion channel. Most of the observable power is expected to be released by plasma leaving the magnetospheric flow above the neutron-star surface. Such systems should be associated with super-Eddington accretion discs and low pulse fractions due to visibility selection and scattering in the disc wind. We suggest that many non-pulsating ultraluminous X-ray sources containing neutron stars may accrete in this shock-free regime.

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Galina Lipunova, Pavel Abolmasov. 2026-09-23. Shock-free super-Eddington accretion onto magnetized neutron star. https://arxiv.org/abs/2609.28092

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