The role of electric dominance for particle injection in relativistic reconnection

Journal Article · 12 February 2025 · Monthly Notices of the Royal Astronomical Society

DOI: https://doi.org/10.1093/mnras/staf270 · OSTI ID:2526653

Gupta, Sanya ; Sridhar, Navin ; Sironi, Lorenzo

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 ($$E\gt B$$) for the early stages of particle acceleration (i.e. the ‘injection’ stage). Given an injection threshold energy $$\epsilon ^\ast =\sigma /4$$ for the particles that eventually attain energy $$\epsilon _{\rm T}$$ by time T, we calculate the mean fractional contribution $$\zeta (\epsilon ^\ast ,\epsilon _{\rm T})$$ by $$E\gt B$$ fields to particle energization at the time when the threshold $$\epsilon ^\ast$$ is reached. We find that $$\zeta$$ monotonically increases with $$\sigma$$ and $$\epsilon _{\rm T}$$; for $$\sigma \gtrsim 50$$ and $$\epsilon _{\rm T}/\sigma \gtrsim 8$$, we find that $$\gtrsim 80~{{\ \rm per\ cent}}$$ of the energy gain obtained before reaching $$\epsilon ^\ast =\sigma /4$$ occurs in $$E\gt B$$ regions. We find that $$\zeta$$ is independent of simulation box size $$L_x$$, as long as $$\epsilon _{\rm T}$$ is normalized to the maximum particle energy, which scales as $$\epsilon _{\rm max}\propto L_{\rm x}^{1/2}$$ in 2D. The distribution of energy gains $$\epsilon _{\chi }$$ acquired in $$E\gt B$$ regions can be modelled as $$\mathrm{ d}N/\mathrm{ d}\epsilon _{\chi }\propto \epsilon _{\chi }^{-0.35}\exp [-(\epsilon _{\chi }/0.06\, \sigma)^{0.5}]$$. Our results help assess the role of electric dominance in relativistic reconnection with vanishing guide fields, which is realized in the magnetospheres of black holes and neutron stars.

• Sponsoring Organization: USDOE
• Grant/Contract Number: SC0023015
• OSTI ID: 2526653
• Journal Information: Monthly Notices of the Royal Astronomical Society, Journal Name: Monthly Notices of the Royal Astronomical Society Journal Issue: 1 Vol. 538; ISSN 0035-8711
• Publisher: Oxford University PressCopyright Statement
• Country of Publication: United Kingdom
• Language: English