ULX Collimation by Outflows in Moderately Magnetized Neutron Stars

Abstract

We perform radiative magnetohydrodynamics simulations in general relativity of super-Eddington disk accretion onto neutron stars endowed with a magnetic dipole corresponding to surface strengths not exceeding 100 Giga-Gauss. Accretion is found to power strong outflows that collimate the emergent radiation of the accretion columns, leading to apparent radiative luminosities of ∼100 Eddington, when the true luminosity is a few Eddington units. Surprisingly, the collimation cone/angle widens with increasing magnetic field. Thus, in our simulations the apparent luminosity of the neutron star is substantially larger for the weaker magnetic fields (1010 G) than for the stronger ones (1011 G). We conclude that a super-Eddington accreting neutron star with dipole magnetic field on the order of 1010 G is the most likely source of ultraluminous X-rays.