The Role of Electric Dominance for Particle Injection in Relativistic Reconnection

Abstract

Magnetic reconnection in relativistic plasmas -- where the magnetization $\sigma \gg 1$ -- is regarded as an efficient particle accelerator, capable of explaining the most dramatic astrophysical flares. We employ two-dimensional (2D) particle-in-cell simulations of relativistic pair-plasma reconnection with vanishing guide field and outflow boundaries to quantify the impact of the energy gain occurring in regions of electric dominance ($E>B$) for the early stages of particle acceleration (i.e., the ``injection'' stage). We calculate the mean fractional contribution $\zeta ({ϵ}^{\ast },{ϵ}_{\mathrm{T}}$) by $E>B$ fields to particle energization up to the injection threshold energy, ${ϵ}^{\ast }=\sigma /4$; here, ${ϵ}_{\mathrm{T}}$ is the particle energy at time $T$. We find that $\zeta$ monotonically increases with $\sigma$ and ${ϵ}_{\mathrm{T}}$; for $\sigma \gtrsim 50$ and ${ϵ}_{\mathrm{T}}/\sigma \gtrsim 8$, we find that $\gtrsim 80\%$ of the energy gain obtained before reaching ${ϵ}^{\ast }=\sigma /4$ occurs in $E>B$ regions. We find that $\zeta$ is independent of simulation box size $L_x$, as long as ${ϵ}_{\mathrm{T}}$ is normalized to the maximum particle energy, which scales as ${ϵ}_{\max }\propto L_{\mathrm{x}}^{1/2}$ in 2D. The distribution of energy gains ${ϵ}_{\chi }$ acquired in $E>B$ regions can be modeled as $dN/d{ϵ}_{\chi }\propto {ϵ}_{\chi }^{-0.35}\exp [-({ϵ}_{\chi }/0.06\sigma {)}^{0.5}]$. Our results help assess the role of electric dominance in relativistic reconnection with vanishing guide fields, which may be realized in the magnetospheres of black holes and neutron stars.

Comments: 11 pages, 13 figures