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Magnetic reconnection’s ability to produce extreme particle‑dominated regions in M87

Establish whether magnetic reconnection can account for the extremely high ratio of non‑thermal electron energy density to magnetic energy density (Ue/UB ≈ 5.8 × 10^4) inferred for the VHE flare component in M87, reconciling this requirement with current reconnection models that favor near‑equipartition conditions.

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Background

To explain the hard VHE spectrum during the 2018 flare, the authors’ Model B invokes a fast‑moving component that implies a very weakly magnetized plasma, yielding Ue/UB values far above equipartition.

Current particle‑in‑cell simulations of relativistic magnetic reconnection generally produce conditions near equipartition, creating tension with the extreme particle dominance suggested by the model. Whether reconnection can reach such regimes remains unresolved.

References

The extent to which this could be attributed to magnetic reconnection is not yet clear as current reconnection models tend to suggest that the emission region is likely to be near equipartition (e.g., Petropoulou et al. 2016).

— Broadband Multi-wavelength Properties of M87 during the 2018 EHT Campaign including a Very High Energy Flaring Episode (2404.17623 - Algaba et al., 24 Apr 2024) in Section 4.3 High‑energy oriented model (Model B)