TeV Neutrinos and Hard X-Rays from Relativistic Reconnection in the Corona of NGC 1068

Abstract

The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact “coronal” region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a large-scale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, X-ray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particle-in-cell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is dn p / dE p ∝ E p − 1 below the break and dn p / dE p ∝ E p − s above the break, with IceCube neutrino observations suggesting s ≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal X-rays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron–positron pairs that makes the corona moderately Compton thick.

High-Energy and Ultra-High-Energy Neutrino Astrophysics

“…The coronal region would be an ideal candidate, due to the dense X-ray photon field which renders it gamma-ray opaque via γγ processes. Various acceleration mechanisms have been proposed to explain the formation of a non-thermal proton population, including diffusive shock acceleration [132], stochastic acceleration in magnetic turbulence [122,133], and magnetic reconnection [124,134]. Independently of the acceleration model at work, the coronal region would efficiently lead to neutrino production either via pp collisions with the material in the coronal region or via pγ collisions with the coronal X-rays and possibly the OUV photons from the accretion disk.…”