Pitch-angle Anisotropy Imprinted by Relativistic Magnetic Reconnection
Luca Comisso, Brian Jiang
Abstract
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 < < 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>min</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msub> </mml:math> controlled by B g / B 0 and σ 0 . The energy-dependent pitch-angle anisotropy, evaluated through the mean of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>sin</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mi>α</mml:mi> </mml:math> of particles at a given energy, exhibits power-law ranges with negative ( m < ) and positive ( m > ) slopes below and above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>min</mml:mi> <mml:mi>α</mml:mi> </mml:mrow> </mml:msub> </mml:math> , 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.