Global Magnetohydrodynamic Simulations of Monster Shocks in Neutron Star Magnetospheres

TL;DR

This paper uses global relativistic MHD simulations to study ultra-relativistic magnetized shocks in neutron star magnetospheres, revealing their formation, behavior, and effects of background perturbations, with implications for high-energy astrophysical phenomena.

Contribution

It provides the first comprehensive simulations of monster shocks in neutron star magnetospheres, including oblique shocks and mode conversion effects, advancing understanding of their dynamics and observational signatures.

Findings

Confirmed analytical predictions for equatorial shocks

Demonstrated shock fragmentation and Lorentz factor enhancements due to perturbations

Showed the generic nature of monster shock formation via mode conversion

Abstract

Waves launched from the neutron star surface or inner magnetosphere propagate through the magnetosphere as small perturbations, but can grow relative to the background magnetic field and steepen into ``monster shocks'' -- ultra-relativistic magnetized shocks which can power high-energy emission from magnetars, neutron star mergers and collapse. They occur in magnetically dominated plasma and are described by relativistic magnetohydrodynamics (MHD). We present global relativistic MHD simulations of monster shocks in unperturbed and perturbed (``wrinkled'') backgrounds with a global dipolar geometry. Our simulations confirm analytical predictions for equatorial shocks and provide new insight into the behavior of oblique shocks off the equator. Simulations where the shock is formed through Alfv\'{e}n mode to fast mode conversion are also presented, demonstrating the generic nature of the monster shock mechanism. We explore how the presence of additional modes in the magnetosphere modifies the shock behavior. Modes of comparable amplitude can fragment the shock front, substantially reduce the magnetization, produce localized enhancements in the Lorentz factor relative to an unperturbed dipole background, and intermittently generate additional shocks along a line of sight.