The effect of inverse Compton losses on particle acceleration in three-dimensional relativistic reconnection

TL;DR

This study uses 3D PIC simulations to show that inverse Compton cooling affects the energy spectrum of trapped particles in relativistic magnetic reconnection, but not the overall reconnection process or free particle acceleration.

Contribution

It demonstrates that inverse Compton losses modify the trapped particle spectrum without impacting the reconnection rate or free acceleration, validating previous models.

Findings

Cooling does not alter reconnection rate or free phase physics.

Trapped electron spectrum steepens to γ^{-3} above cooling break.

No significant energization occurs during the trapped phase.

Abstract

Relativistic magnetic reconnection is a key mechanism for dissipating magnetic energy and accelerating particles in astrophysics. In the absence of radiative cooling, recent particle-in-cell (PIC) simulations have shown that high-energy particles gain most of their energy in the upstream region, during a short-lived "free phase" where they meander between the two sides of the layer; when they get captured/trapped by the downstream flux ropes, they undergo a "trapped phase", where no significant energization occurs. Here, we perform a suite of 3D PIC simulations of relativistic reconnection including inverse Compton (IC) losses in the weakly cooled regime in which the radiation-reaction-limited Lorentz factor $\gamma_{\rm rad}$ exceeds the magnetization $\sigma$. We show that electron cooling losses do not appreciably alter the reconnection rate, the structure of the layer, and the physics of particle acceleration in the free phase, so the spectrum of free electrons is $dN_{\rm free}/d\gamma \propto \gamma^{-1}$, as in the uncooled case. The spectrum of trapped electrons above the cooling break $\gamma_{\rm cool}$ (in the range $\gamma_{\rm cool}<\gamma<\gamma_{\rm rad}$) is $dN/d\gamma \propto \gamma^{-3}$, steeper than the scaling $dN/d\gamma \propto \gamma^{-2}$ of uncooled simulations. This confirms that no significant particle energization occurs during the trapped phase. Our results validate the model by arXiv:2302.12269 for particle acceleration in 3D relativistic reconnection, and imply that radiative emission models of reconnection-powered astrophysical sources should employ a two-zone structure, that differentiates between free, rapidly accelerating particles and trapped, passively cooling particles.