The Magnetosphere of Oscillating Neutron Stars in General Relativity

TL;DR

This paper develops a general relativistic model of neutron star magnetospheres driven by stellar oscillations, revealing that relativistic effects reduce energy losses compared to Newtonian predictions, with implications for magnetar activity.

Contribution

It provides the first general relativistic analysis of oscillation-induced neutron star magnetospheres, extending previous Newtonian work and deriving analytical solutions for small-amplitude oscillations.

Findings

Relativistic effects shrink the polar cap size.

Energy losses are smaller in GR than in Newtonian models.

The low current density approximation is valid for many oscillation modes.

Abstract

Just as a rotating magnetised neutron star has material pulled away from its surface to populate a magnetosphere, a similar process can occur as a result of neutron-star pulsations rather than rotation. This is of interest in connection with the overall study of neutron star oscillation modes but with a particular focus on the situation for magnetars. Following a previous Newtonian analysis of the production of a force-free magnetosphere in this way Timokhin et al. (2000), we present here a corresponding general-relativistic analysis. We give a derivation of the general relativistic Maxwell equations for small-amplitude arbitrary oscillations of a non-rotating neutron star with a generic magnetic field and show that these can be solved analytically under the assumption of low current density in the magnetosphere. We apply our formalism to toroidal oscillations of a neutron star with a dipole magnetic field and find that the low current density approximation is valid for at least half of the oscillation modes, similarly to the Newtonian case. Using an improved formula for the determination of the last closed field line, we calculate the energy losses resulting from toroidal stellar oscillations for all of the modes for which the size of the polar cap is small. We find that general relativistic effects lead to shrinking of the size of the polar cap and an increase in the energy density of the outflowing plasma. These effects act in opposite directions but the net result is that the energy loss from the neutron star is significantly smaller than suggested by the Newtonian treatment.