The Interplay of Magnetically Dominated Turbulence and Magnetic Reconnection in Producing Nonthermal Particles

Abstract

Magnetized turbulence and magnetic reconnection are often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully-kinetic 2D and 3D particle-in-cell simulations, we investigate the interplay between turbulence and reconnection in generating nonthermal particles in magnetically-dominated (or, equivalently, "relativistic") pair plasmas. A generic by-product of the turbulence evolution is the generation of a nonthermal particle spectrum with a power-law energy range. The power-law slope p is harder for larger magnetizations and stronger turbulence fluctuations, and it can be as hard as p 2. The Larmor radius of particles at the high-energy cutoff is comparable to the size l of the largest turbulent eddies. Plasmoid-mediated reconnection, which self-consistently occurs in the turbulent plasma, controls the physics of particle injection. Then, particles are further accelerated by stochastic scattering off turbulent fluctuations. The work done by parallel electric fields -naturally expected in reconnection layers -is responsible for most of the initial energy increase, and is proportional to the magnetization σ of the system, while the subsequent energy gain, which dominates the overall energization of high-energy particles, is powered by the perpendicular electric fields of turbulent fluctuations. The two-stage acceleration process leaves an imprint in the particle pitch-angle distribution: low-energy particles are aligned with the field, while the highest energy particles move preferentially orthogonal to it. The energy diffusion coefficient of stochastic acceleration scales as D γ ∼ 0.1σ(c/l)γ 2 , where γ is the particle Lorentz factor. This results in fast acceleration timescales t acc ∼ (3/σ) l/c. Our findings have important implications for understanding the generation of nonthermal particles in high-energy astrophysical sources.

High-energy neutrinos from fallback accretion of binary neutron star merger remnants

“…[48,49]. These works indicate that the equatorial outflow can be Poynting-flux dominated and hence enable reconnection processes [50][51][52].…”

A Nonthermal Bomb Explains the Near-infrared Superflare of Sgr A*

“…particle-in-cell (PIC) simulations-are showing that: (i) Magnetic reconnection events with high magnetizations 1 of σ 10 lead to particles following power-law energy distributions with an index p ranging from 1 to 2 (e.g. Sironi & Spitkovsky 2014;Guo et al 2014), (ii) the presence of relativistic 2 turbulence acceleration leads to a powerlaw index closer to 2 (Comisso & Sironi 2019) and (iii) reconnection can deposit a large fraction (up to about 50%) of the dissipated energy in nonthermal electrons.…”

Generalized Fermi Acceleration

“…Furthermore, because |q u − 2| generically is a small quantity, and because t s generally is a mild (increasing) function of the momentum, the above suggests that D pp ∼ p 2 k min δ u 2 , in a reasonable approximation. Interestingly, this scaling has been observed in recent particle-in-cell simulations of relativistic turbulence [14,15].…”

“…The idea is to split the cascade into two regions, the small-scale part with k t s , and the large-scale part with k t s , then to use the corresponding expressions derived above, for each. First, one decomposes the four-velocity fluctuations as 15) with |δ u k | 2 k ∝ k 1−q u , where q u represents the 1D power spectrum index of the velocity field, and k min the smallest wavenumber of the cascade (i.e., the inverse of the outer scale). Then, the various contributions of compressive modes, acceleration, shear and vorticity on a scale k are written, e.g.…”