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Active galactic nuclei in general, and the supermassive black hole in M87 in particular, show bright and rapid gamma-ray flares up to energies of 100 GeV and above. For M87, the flares show multiwavelength components, and the variability timescale is comparable to the dynamical time of the event horizon, suggesting that the emission may come from a compact region near the nucleus. However, the emission mechanism for these flares is not well understood. Recent high-resolution general-relativistic magnetohydrodynamic simulations show the occurrence of episodic magnetic reconnection events that can power flares near the black hole event horizon. In this work, we analyze the radiative properties of the reconnecting current layer under the extreme plasma conditions applicable to the black hole in M87 from first principles. We show that abundant pair production is expected in the vicinity of the reconnection layer, to the extent that the produced secondary pair plasma dominates the reconnection dynamics. Using analytic estimates backed by two-dimensional particle-in-cell simulations, we demonstrate that in the presence of strong synchrotron cooling, reconnection can produce a hard power-law distribution of pair plasma imprinted in the outgoing synchrotron (up to a few tens of MeV) and the inverse-Compton signal (up to TeV). We produce synthetic radiation spectra from our simulations, which can be directly compared with the results of future multiwavelength observations of M87* flares.

We present high-resolution 2D and 3D simulations of magnetized decaying turbulence in relativistic, resistive magnetohydrodynamics. The simulations show dynamic formation of large-scale intermittent long-lived current sheets being disrupted into plasmoid chains by the tearing instability. These current sheets are locations of enhanced magnetic-field dissipation and heating of the plasma. We find magnetic energy spectra ∝k^−3/2, together with strongly pronounced dynamic alignment of Elsässer fields and of velocity and magnetic fields, for strong guide-field turbulence, whereas we retrieve spectra ∝k^−5/3for the case of a weak guide-field.