Radiative reconnection-powered TeV flares from the black hole magnetosphere in M87

Active Galactic Nuclei (AGN) in general, and the supermassive black hole in M87 in particular, regularly show bright and rapid gamma-ray flares up to energies > 100 GeV. Luminosities of these flares are non-negligible compared to the total jet power of the AGN, and the variation timescales are comparable to the dynamical time of the event horizon. However, the emission mechanism for these TeV flares is not well understood. Recent high-resolution GRMHD simulations show clear indications of episodic magnetic reconnection events occurring close to 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. We show that abundant pair production is expected in the vicinity of the reconnecting sheet, to the extent that the produced secondary pair-plasma dominates the reconnection dynamics. Using analytic estimates backed by 2D particle-in-cell simulations we demonstrate that even in the presence of strong synchrotron cooling, reconnection can still produce a hard power-law distribution of pair plasma imprinted in the outgoing synchrotron (up to few tens of MeV) and the inverse-Compton signal (up to TeV). Our findings provide a viable explanation for the observed TeV flaring activity of M87, and lay a strong basis to predict the lower-energy counterparts (micron-to-keV), as well as the polarization of the flare.