We perform particleincell simulations to elucidate the microphysics of relativistic weakly magnetized shocks loaded with electronpositron pairs. Various external magnetizations
We present
 NSFPAR ID:
 10408747
 Publisher / Repository:
 DOI PREFIX: 10.3847
 Date Published:
 Journal Name:
 Research Notes of the AAS
 Volume:
 7
 Issue:
 4
 ISSN:
 25155172
 Format(s):
 Medium: X Size: Article No. 77
 Size(s):
 Article No. 77
 Sponsoring Org:
 National Science Foundation
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Abstract σ ≲ 10^{−4}and pairloading factorsZ _{±}≲ 10 are studied, whereZ _{±}is the number of loaded electrons and positrons per ion. We find the following: (1) The shock becomes mediated by the ion Larmor gyration in the mean field whenσ exceeds a critical valueσ _{L}that decreases withZ _{±}. Atσ ≲σ _{L}the shock is mediated by particle scattering in the selfgenerated microturbulent fields, the strength and scale of which decrease withZ _{±}, leading to lowerσ _{L}. (2) The energy fraction carried by the postshock pairs is robustly in the range between 20% and 50% of the upstream ion energy. The mean energy per postshock electron scales as . (3) Pair loading suppresses nonthermal ion acceleration at magnetizations as low as ${\overline{E}}_{\mathrm{e}}\propto {\left({Z}_{\pm}+1\right)}^{1}$σ ≈ 5 × 10^{−6}. The ions then become essentially thermal with mean energy , while electrons form a nonthermal tail, extending from ${\overline{E}}_{\mathrm{i}}$ to $E\sim {\left({Z}_{\pm}+1\right)}^{1}{\overline{E}}_{\mathrm{i}}$ . When ${\overline{E}}_{\mathrm{i}}$σ = 0, particle acceleration is enhanced by the formation of intense magnetic cavities that populate the precursor during the late stages of shock evolution. Here, the maximum energy of the nonthermal ions and electrons keeps growing over the duration of the simulation. Alongside the simulations, we develop theoretical estimates consistent with the numerical results. Our findings have important implications for models of early gammaray burst afterglows. 
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