Towards Modelling AR Sco: Generalised Particle Dynamics and Strong Radiation-Reaction Regimes

TL;DR

This paper develops advanced numerical methods for simulating relativistic particle dynamics with radiation reaction in extreme astrophysical environments, improving accuracy and efficiency for modeling sources like the white dwarf pulsar AR Sco.

Contribution

It introduces higher-order explicit integrators with adaptive time stepping for full particle dynamics including radiation reaction, demonstrating their superiority over standard methods in accuracy and computational efficiency.

Findings

Higher-order schemes outperform symplectic integrators in accuracy.

Adaptive time stepping significantly reduces computational time.

Particles reach radiation-reaction equilibrium consistent with analytic models.

Abstract

Numerical simulations of relativistic plasmas have become more feasible, popular, and crucial for various astrophysical sources with the availability of computational resources. The necessity for high-accuracy particle dynamics is especially highlighted in pulsar modelling due to the extreme associated electromagnetic fields and particle Lorentz factors. Including the radiation-reaction force in the particle dynamics adds even more complexity to the problem, but is crucial for such extreme astrophysical sources. We have also realised the need for such modelling concerning magnetic mirroring and particle injection models proposed for AR Sco, the first white dwarf pulsar. This paper demonstrates the benefits of using higher-order explicit numerical integrators with adaptive time step methods to solve the full particle dynamics with radiation-reaction forces included. We show that for standard test scenarios, namely various combinations of uniform $E$- and $B$-fields and a static dipole $B$-field, the schemes we use are equivalent to and in extreme field cases outperform standard symplectic integrators in accuracy. We show that the higher-order schemes have massive computational time improvements due to the adaptive time steps we implement, especially in non-uniform field scenarios and included radiation reaction where the particle gyro-radius rapidly changes. When balancing accuracy and computational time, we identified the adaptive Dormand-Prince eighth-order scheme to be ideal for our use cases. The schemes we use maintain accuracy and stability in describing the particle dynamics and we indicate how a charged particle enters radiation-reaction equilibrium and conforms to the analytic Aristotelian Electrodynamics expectations.