Diffuse neutrino flux from relativistic reconnection in AGN coronae

Abstract

IceCube observations point to Active Galactic Nuclei (AGN) as promising contributors to the observed astrophysical neutrino flux. Close to the central black hole, protons can be accelerated through magnetic reconnection to very high energies and subsequently interact with abundant X-ray photons in the source, leading to neutrino production. We investigate whether the diffuse neutrino flux observed by IceCube can originate, via proton acceleration, in reconnection-powered coronae of non-jetted AGN. We create a library of neutrino spectral templates, over a large grid of values for the three key model parameters: the proton plasma magnetization of the corona $\sigma_{\rm p}$, the X-ray coronal luminosity, and the black hole mass. Synchrotron cooling of pions and muons plays a significant role due to the large coronal magnetic fields. We couple the single-source model with a mock AGN catalog, consistent with the observed X-ray and mid-infrared AGN samples at redshifts $z=0-4$, to infer the diffuse neutrino flux. Coronal emission satisfactorily explains the most recent IceCube measurements of the diffuse neutrino flux up to energies of $\sim 1$~PeV, provided that $\sim$10\% of the AGN coronae have $\sigma_{\rm p} \sim 10^5$, while the rest are distributed over a range of lower magnetizations. Coronal emission is suppressed at higher energies by pion and muon cooling, so that another population is required, with jetted AGN being strong candidates.