Pitch-angle Anisotropy Imprinted by Relativistic Magnetic Reconnection

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 < < 1), while above γ 0 , the spectral index 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 γ min α controlled by B g / B 0 and σ 0 . The energy-dependent pitch-angle anisotropy, evaluated through the mean of sin 2 α of particles at a given energy, exhibits power-law ranges with negative ( m < ) and positive ( m > ) slopes below and above γ min α , 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.