Bondi-Hoyle Accretion onto Magnetized Neutron Star

TL;DR

This study uses axisymmetric MHD simulations to explore how a magnetized neutron star accretes matter from the interstellar medium, revealing reduced accretion rates and complex magnetic interactions influenced by the star's magnetic field strength.

Contribution

It provides new insights into the effects of magnetic fields on accretion processes onto neutron stars, including oscillations and magnetic field stretching.

Findings

Magnetized neutron stars have lower accretion rates than non-magnetized ones.

Accretion occurs mainly along magnetic poles, with flow patterns influenced by magnetic field strength.

Magnetosphere oscillations lead to significant variability in accretion rates.

Abstract

Axisymmetric MHD simulations are used to investigate the Bondi-Hoyle accretion onto an isolated magnetized neutron star moving supersonically (with Mach number of 3) through the interstellar medium. The star is assumed to have a dipole magnetic field aligned with its motion and a magnetospheric radius R_m less then the accretion radius R_BH, so that the gravitational focusing is important. We find that the accretion rate to a magnetized star is smaller than that to a non-magnetized star for the parameters considered. Close to the star the accreting matter falls to the star's surface along the magnetic poles with a larger mass flow to the leeward pole of the star. In the case of a relatively large stellar magnetic field, the star's magnetic field is stretched in the direction of the matter flow outside of R_m (towards the windward side of the star). For weaker magnetic fields we observed oscillations of the closed magnetosphere frontward and backward. These are accompanied by strong oscillations of the mass accretion rate which varies by factors ~ 3. Old slowly rotating neutron stars with no radio emission may be visible in the X-ray band due to accretion of interstellar matter. In general, the star's velocity, magnetic moment, and angular velocity vectors may all be in different directions so that the accretion luminosity will be modulated at the star's rotation rate.