The steady growth of the high-energy spectral cut-off in relativistic magnetic reconnection

Journal Article · Fri Oct 05 00:00:00 EDT 2018 · Monthly Notices of the Royal Astronomical Society

DOI:https://doi.org/10.1093/mnras/sty2702· OSTI ID:1543972

Petropoulou, Maria  ^[1]; Sironi, Lorenzo ^[2]

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1. Princeton Univ., NJ (United States)
2. Columbia Univ., New York, NY (United States)

Magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation. With large-scale two-dimensional particle-in-cell simulations of relativistic reconnection (i.e. with magnetization σ >> 1) in pair plasmas, we study the long-term evolution of the power-law slope and high-energy cut-off of the spectrum of accelerated particles. We find that the high-energy spectral cut-off does not saturate at γcut ~ 4σ, as claimed by earlier studies, but it steadily grows with time as long as the reconnection process stays active. At late times, the cut-off scales approximately as γ_cut ∝√t, regardless of the flow magnetization and initial temperature. We show that the particles dominating the high-energy spectral cut-off reside in plasmoids, and in particular in a strongly magnetized ring around the plasmoid core. The growth of their energy is driven by the increase in the local field strength, coupled with the conservation of the first adiabatic invariant. We also find that the power-law slope of the spectrum (p = -d log N/d log γ) evolves with time. For σ ≳ 10, the spectrum is hard at early times (p ≲ 2), but it tends to asymptote to p ~ 2; the steepening of the power-law slope allows the spectral cut-off to extend to higher and higher energies, without violating the fixed energy budget of the system. Our results demonstrate that relativistic reconnection is a viable candidate for accelerating the high-energy particles emitting in relativistic astrophysical sources.

• Research Organization: Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Columbia Univ., New York, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: SC0016542
• OSTI ID: 1543972
• Journal Information: Monthly Notices of the Royal Astronomical Society, Vol. 481, Issue 4; ISSN 0035-8711
• Publisher: Royal Astronomical Society
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
Astronomy & Astrophysics