Synchrotron-to-curvature transition regime of radiation of charged particles in a dipole magnetic field

TL;DR

This paper investigates the transition of charged particle radiation from synchrotron to curvature regimes in dipole magnetic fields by numerically solving equations of motion and analyzing the resulting spectra, with applications to astrophysical scenarios.

Contribution

It introduces a numerical method to study the transition between radiation regimes without prior assumptions, revealing the coexistence and relative contributions of different regimes in astrophysical environments.

Findings

Transition regimes depend on particle pitch angle damping.

Synchrotron, synchro-curvature, and curvature emissions can coexist.

Different astrophysical models show varying dominance of radiation regimes.

Abstract

The details of trajectories of charged particles become increasingly important for proper understanding of processes of formation of radiation in strong and curved magnetic fields. Because of damping of the perpendicular component of motion, the particle's pitch angle could be decreased by many orders of magnitude leading to the change of the radiation regime -- from synchrotron to the curvature mode. To explore the character of this transition, we solve numerically the equations of motion of a test particle in a dipole magnetic field, and calculate the energy spectrum of magnetic bremsstrahlung self-consistently, i.e. without a priori assumptions on the radiation regime. In this way we can trace the transitions between the synchrotron and curvature regimes, as well as study the third (intermediate or the so-called synchro-curvature) regime. We briefly discuss three interesting astrophysical scenarios, the radiation of electrons in the pulsar magnetosphere in the polar cap and outer gap models, as well as the radiation of ultrahigh energy protons in the magnetosphere of a massive black hole, and demonstrate that in these models the synchrotron, synchro-curvature and curvature regimes can be realized with quite different relative contributions to the total emission.