GRMHD simulations of accreting neutron stars I: nonrotating dipoles

TL;DR

This paper uses 2D general relativistic magnetohydrodynamic simulations to study how magnetic fields influence accretion onto nonrotating neutron stars, revealing the relationship between magnetic strength, accretion rate, and torque fluctuations.

Contribution

It provides the first detailed GRMHD simulation analysis of accretion onto nonrotating neutron stars with varying magnetic fields, connecting magnetic field strength to the magnetospheric radius and torque behavior.

Findings

Magnetospheric radius scales as B^{4/7} consistent with Newtonian theory.

Material torque is linearly related to accretion rate but fluctuates rapidly.

Strong magnetic fields cause significant torque fluctuations, possibly explaining X-ray pulsar spin variations.

Abstract

We study the general-relativistic dynamics of matter being accreted onto and ejected by a magnetised and nonrotating neutron star. The dynamics is followed in the framework of fully general relativistic magnetohydrodynamics (GRMHD) within the ideal-MHD limit and in two spatial dimensions. More specifically, making use of the numerical code BHAC, we follow the evolution of a geometrically thick matter torus driven into accretion by the development of a magnetorotational instability. By making use of a number of simulations in which we vary the strength of the stellar dipolar magnetic field, we can determine self-consistently the location of the magnetospheric (or Alfv\'en) radius $r_{\rm msph}$ and study how it depends on the magnetic moment $\mu$ and on the accretion rate. Overall, we recover the analytic Newtonian scaling relation, i.e. $r_{\rm msph} \propto B^{4/7}$, but also find that the dependence on the accretion rate is very weak. Furthermore, we find that the material torque correlates linearly with the mass-accretion rate, although both of them exhibit rapid fluctuations. Interestingly, the total torque fluctuates drastically in strong magnetic field simulations and these unsteady torques observed in the simulations could be associated with the spin fluctuations observed in X-ray pulsars.