Magnetic Energy Buildup for Relativistic Magnetar Giant Flares

TL;DR

This paper models the magnetosphere of magnetars using general relativity to understand how magnetic energy can build up sufficiently to power giant flares, considering constraints like the Aly-Sturrock energy threshold and relativistic effects.

Contribution

It develops a relativistic model of magnetar magnetospheres, derives a generalized virial theorem, and investigates how GR effects influence magnetic energy storage and the Aly-Sturrock constraint.

Findings

GR effects increase the Aly-Sturrock energy threshold for magnetar eruptions.

More massive neutron stars have higher energy thresholds, making eruptions less likely.

Cross-field currents can help exceed the energy threshold in typical neutron stars.

Abstract

Motivated by coronal mass ejection studies, we construct general relativistic models of a magnetar magnetosphere endowed with strong magnetic fields. The equilibrium states of the stationary, axisymmetric magnetic fields in the magnetar magnetosphere are obtained as solutions of the Grad-Shafranov equation in a Schwarzschild spacetime. To understand the magnetic energy buildup in the magnetar magnetosphere, a generalized magnetic virial theorem in the Schwarzschild metric is newly derived. We carefully address the question whether the magnetar magnetospheric magnetic field can build up sufficient magnetic energy to account for the work required to open up the magnetic field during magnetar giant flares. We point out the importance of the Aly-Sturrock constraint, which has been widely studied in solar corona mass ejections, as a reference state in understanding magnetar energy storage processes. We examine how the magnetic field can possess enough energy to overcome the Aly-Sturrock energy constraint and open up. In particular, general relativistic (GR) effects on the Aly-Sturrock energy constraint in the Schwarzschild spacetime are carefully investigated. It is found that, for magnetar outbursts, the Aly-Sturrock constraint is more stringent, i.e., the Aly-Sturrock energy threshold is enhanced due to the GR effects. In addition, neutron stars with greater mass have a higher Aly-Sturrock energy threshold and are more difficult to erupt. This indicates that magnetars are probably not neutron stars with extreme mass. For a typical neutron star with mass of $1-2 M_{\odot}$, we further explore the effects of cross-field current effects, caused by the mass loading, on the possibility of stored magnetic field energy exceeding the Aly-Sturrock threshold.