Pitch-angle Anisotropy Imprinted by Relativistic Magnetic Reconnection

Abstract

Radiation emitted by nonthermal particles accelerated during relativistic magnetic reconnection is critical for understanding the nonthermal emission in a variety of astrophysical systems, including blazar jets, black hole coronae, pulsars, and magnetars. By means of fully kinetic particle-in-cell simulations, we demonstrate that reconnection-driven particle acceleration imprints an energy-dependent pitch-angle anisotropy and gives rise to broken power laws in both the particle energy spectrum and the pitch-angle anisotropy. The particle distributions depend on the relative strength of the non-reconnecting (guide field) versus the reconnecting component of the magnetic field ( B _g / B _0 ) and the lepton magnetization ( σ _0 ). Below the break Lorentz factor γ _0 (injection), the particle energy spectrum is ultra-hard ( p _ is highly sensitive to B _g / B _0 . Particles’ velocities align with the magnetic field, reaching minimum pitch angle α at a Lorentz factor ${\gamma }_{\min \alpha }$ controlled by B _g / B _0 and σ _0 . The energy-dependent pitch-angle anisotropy, evaluated through the mean of ${\sin }^{2}\alpha $ of particles at a given energy, exhibits power-law ranges with negative ( m _ ) slopes below and above ${\gamma }_{\min \alpha }$ , becoming steeper as B _g / B _0 increases. The generation of anisotropic pitch-angle distributions has important astrophysical implications. We address their effects on regulating synchrotron luminosity, spectral energy distribution, polarization, particle cooling, the synchrotron burn-off limit, emission beaming, and temperature anisotropy.

Keywords

• High energy astrophysics
• Plasma physics
• Magnetic fields
• Non-thermal radiation sources
• Particle astrophysics