An Improved Algorithm for Crossing Curved Light Surfaces: rapidly rotating pulsar magnetospheres in curved spacetime

TL;DR

This paper introduces a new coordinate method to improve numerical solutions of the Grad-Shafranov equation for pulsar magnetospheres in curved spacetime, revealing how general relativistic effects influence magnetic structures and luminosity.

Contribution

The authors develop a novel coordinate frame that simplifies the crossing of the light surface in curved spacetime, enabling more accurate modeling of pulsar magnetospheres under general relativity.

Findings

Curvature reduces open magnetic flux and luminosity.

Frame-dragging enhances spacelike current generation.

New coordinate method improves numerical stability.

Abstract

The structure of force-free, steady and axisymmetric magnetosphere of a neutron star (NS) is governed by the Grad-Shafranov (GS) equation, which is a second-order differential equation but degrades to first-order on the light surface (LS). The key to numerically solving the GS equation is to enable magnetic field lines smoothly cross the LS, and crossing a straight LS in flat spacetime has been a well-studied problem. But the numerical algorithm implementation becomes complicate in the presence of a bent LS, e.g. in curved spacetime, since there is no suitable computation grid adapted to it. We propose to circumvent this grid-LS mismatch problem by introducing a new coordinate frame designed such that the LS in it is a straight line. As an application, we investigate the general relativistic (GR) effect in magnetosphere structure of rapidly rotating pulsars in detail, where the LS is bent towards the central NS. We split the GR effect into two parts, curvature and frame-dragging; measure each of them and examine their dependence on the NS mass and the angular velocity for pulsars embedded in aligned dipole and multipole magnetic fields. Qualitatively speaking, we find that the curvature effect compactifies the magnetic field lines near the NS, therefore reduces the open magnetic flux and the Poynting luminosity, while the frame-dragging effect contributes a minor part in shaping the magnetosphere structure but plays a role in enhancing the spacelike current generation.