Gravitational Radiation from Accreting Millisecond Pulsars

TL;DR

This paper explores how accreting millisecond pulsars can emit detectable gravitational waves due to magnetic mountains formed by accreted matter, supported by MHD simulations showing stable, distorted configurations with significant quadrupole moments.

Contribution

It demonstrates that magnetic mountains in accreting neutron stars produce detectable gravitational waves and presents new 3D MHD simulation results of their stable configurations.

Findings

Magnetic mountains can generate gravitational waves detectable by current and future interferometers.

Simulations show the initial axisymmetric state relaxes into a nearly axisymmetric but distorted configuration.

A substantial quadrupole moment persists in the final state, supporting gravitational wave emission.

Abstract

It is widely assumed that the observed reduction of the magnetic field of millisecond pulsars can be connected to the accretion phase during which the pulsar is spun up by mass accretion from a companion. A wide variety of reduction mechanisms have been proposed, including the burial of the field by a magnetic mountain, formed when the accreted matter is confined to the poles by the tension of the stellar magnetic field. A magnetic mountain effectively screens the magnetic dipole moment. On the other hand, observational data suggests that accreting neutron stars are sources of gravitational waves, and magnetic mountains are a natural source of a time-dependent quadrupole moment. We show that the emission is sufficiently strong to be detectable by current and next generation long-baseline interferometers. Preliminary results from fully three-dimensional magnetohydrodynamic (MHD) simulations are presented. We find that the initial axisymmetric state relaxes into a nearly axisymmetric configuration via toroidal magnetic modes. A substantial quadrupole moment is still present in the final state, which is stable (in ideal MHD) yet highly distorted.