Hidden Cores of Active Galactic Nuclei as the Origin of Medium-Energy Neutrinos: Critical Tests with the MeV Gamma-Ray Connection

Mysteries about the origin of high-energy cosmic neutrinos have deepened by the recent IceCube measurement of a large diffuse flux in the 10–100 TeV range. Based on the standard disk-corona picture of active galactic nuclei (AGN), we present a phenomenological model enabling us to systematically calculate the spectral sequence of multimessenger emission from the AGN coronae. We show that protons in the coronal plasma can be stochastically accelerated up to PeV energies by plasma turbulence, and find that the model explains the large diffuse flux of medium-energy neutrinos if the cosmic rays carry only a few percent of the thermal energy. We find that the Bethe-Heitler process plays a crucial role in connecting these neutrinos and cascaded MeV gamma rays, and point out that the gamma-ray flux can even be enhanced by the reacceleration of secondary pairs. Critical tests of the model are given by its prediction that a significant fraction of the MeV gamma-ray background correlates with $\sim 10\mathrm{TeV}$ neutrinos, and nearby Seyfert galaxies including NGC 1068 are promising targets for IceCube, KM3Net, IceCube-Gen2, and future MeV gamma-ray telescopes.

Figure 1

Schematic picture of the AGN disk-corona scenario. Protons are accelerated by plasma turbulence generated in the coronae, and produce high-energy neutrinos and cascaded gamma rays via interactions with matter and radiation.

Figure 2

Disk-corona SEDs used in this work, for $L_X={10}^{42}$, ${10}^{43}$, ${10}^{44}$, ${10}^{45}$, and ${10}^{46}\mathrm{erg}{\mathrm{s}}^{-1}$ (from bottom to top). See text for details.

Figure 3

EGB and ENB spectra in our AGN corona model. The data are taken from Swift BAT [15] (green), Nagoya balloon [100] (blue), SMM [101] (purple), COMPTEL [102] (gray), Fermi LAT [103] (orange), and IceCube shower events (black) [5] (consistent with the global fit [4]). Solid thick (thin) curves are for $\beta =1$ and $q=5/3$ ($\beta =3$ and $q=3/2$ with the reacceleration contribution), respectively.

Figure 4

Point source fluxes of all flavor neutrinos and gamma rays from a nearby AGN, NGC 1068. The ten-year IceCube data [111] and the Fermi gamma-ray data [112] are shown. For eASTROGAM [113] and AMEGO [114] sensitivities, the observation time of ${10}^6\mathrm{s}$ is assumed. Solid thick (thin) curves are for $\eta =10$ and $P_{\mathrm{CR}}/P_{\mathrm{th}}=0.7\%$ ($\eta =70$ and $P_{\mathrm{CR}}/P_{\mathrm{th}}=30\%$), respectively. For comparison, a neutrino flux in the starburst scenario of Murase and Waxman [108] is overlaid.