PeV Diffuse Neutrinos from Magnetically Arrested Disks in Low-Luminosity Active Galactic Nuclei

Abstract

Radiatively inefficient accretion flows (RIAFs) in low-luminosity active galactic nuclei (LLAGNs) are promising candidates for the PeV astrophysical diffuse neutrino background (DNB), but how the magnetically arrested disk (MAD) state affects their neutrino output and what fraction of LLAGNs are in this state remain unclear. We present a comprehensive study of diffuse neutrino emission from the inner accretion flows of LLAGNs in the MAD state, including all three flavors and both hadronuclear ($pp$) and photohadronic ($pγ$) production channels. Assuming that $\sim 5\%$ of the accretion power is transferred to nonthermal protons, current IceCube data allow at most $2.3\times10^{-3}$ of LLAGNs to have their inner accretion flows in a MAD state that can produce high-energy neutrinos. A lower fraction of the accretion power transferred to nonthermal protons would allow a proportionally larger fraction of such LLAGNs. Assuming that the comoving number density of low-excitation radio galaxies (LERGs) approximately matches that of LLAGNs in the MAD state, we also estimate a MAD fraction of $\simeq 1.6\times10^{-3}$. The corresponding diffuse intensity is compatible with current IceCube data and testable by IceCube-Gen2. For this MAD fraction, the all-flavor diffuse intensity peaks at $E_ν\simeq1.5\,{\rm PeV}$ with $E_ν^2Φ_ν\simeq2.6\times10^{-9}\,{\rm GeV\,cm^{-2}\,s^{-1}\,sr^{-1}}$. Current IceCube data are beginning to challenge several predictions from previous LLAGN RIAF models, and the MAD component can become the dominant contribution from LLAGN accretion flows beyond the cutoffs of most earlier RIAF spectra considered here.%%%This constraint suggests that only a small fraction of LLAGNs are supplied with sufficient coherent net poloidal magnetic flux.