Proton Acceleration by Collisionless Shocks in Supermassive Black Hole Coronae: Implications for High-energy Neutrinos

Abstract Recent observations by the IceCube Neutrino Observatory have revealed a significant excess of high-energy neutrinos from nearby Seyfert galaxies, such as NGC 1068, without a corresponding flux of high-energy gamma-rays. This suggests that neutrinos are produced via hadronic interactions in a region opaque to gamma-rays, likely a hot corona surrounding the central supermassive black hole. However, the mechanism responsible for accelerating the parent protons to the required energies (∼100 TeV) remains an open question. In this study, we investigate diffusive shock acceleration (DSA) in active galactic nucleus (AGN) coronae using a suite of one-dimensional particle-in-cell simulations spanning a broad range of plasma parameters. We find that DSA is a robust and efficient mechanism for proton acceleration, consistently channeling approximately 10% of the shock’s kinetic energy into nonthermal ions, even for shocks with sonic Mach numbers as low as M s ≈ 2. In contrast, the efficiency of electron acceleration is highly variable and less efficient (<1%) in our parameter survey. These findings provide strong, first-principles support for the hadronic models of neutrino production in AGN, and offer quantitative constraints that can explain the observed gamma-ray deficit.