The contribution of turbulent AGN coronae to the diffuse neutrino flux

TL;DR

The paper investigates whether magnetized-turbulence acceleration in AGN coronae can account for the IceCube high-energy neutrinos, extending the successful NGC 1068 interpretation to a population and the diffuse background in the $1$–$100$ TeV range. It develops a self-consistent coronal model where non-thermal protons are energized by turbulence with $L_p=\mathcal{F}_p L_B$ and radiative losses set the photohadronic targets via a Marconi 2004 SED, then computes per-source neutrino spectra from $pp$ and $p\gamma$ interactions and the resulting diffuse flux by integrating over the LDDE luminosity function. The results show local Seyferts with $L_X$ in the $10^{42}-10^{44}$ erg s$^{-1}$ band can explain the IceCube $1$–$100$ TeV diffuse flux, with a sub-linear $L_\nu\propto L_X^{0.8}$ scaling and peak energies at tens of TeV; a proton-loading factor of a few percent suffices in most cases. The study thus provides a coherent link between individual Seyfert neutrino excesses and the ambient extragalactic background within the coronal magnetized-turbulence framework, while highlighting the limitations at higher energies and the dependence on model assumptions such as the standard-candle coronal properties.

Abstract

Active galactic nuclei (AGN) can accelerate protons to energies of $\sim$10-100 TeV, with secondary production of high-energy neutrinos. If the acceleration is driven by magnetized turbulence, the main properties of the resulting proton and neutrino spectra can be deduced based on insights from particle-in-cell simulations of magnetized turbulence. We have previously shown that these properties are consistent with the TeV neutrino signal observed from the nearby active galaxy NGC 1068. In this work, we extend this result to a population study. We show that the produced neutrino flux depends mainly on the energetics of the corona - the relative fraction of X-ray, magnetic, and non-thermal proton energy - and on the spectral energy distribution of the AGN. We find that coronae with similar properties can explain neutrinos from the candidate AGN for which IceCube has reported an excess, albeit less significant than NGC 1068. Building on this framework, we show how the neutrino signal evolves with the AGN luminosity, and use this AGN sequence to predict the diffuse neutrino flux from the extragalactic population, showing that it can account for the diffuse neutrino signal observed by IceCube in the $\sim$1-100 TeV energy range.

Figures (7)

• Figure 1: Spectral energy distribution of the AGN photons for varying luminosity. The legend shows the values of $L_X$ for the different curves. The photon spectrum is obtained from the prescription in Marconi_2004_SED.
• Figure 2: Impact of coronal parameters on the neutrino production from NGC 1068. In each panel, we independently vary each of the parameters $\mathcal{F}_X$, $R$, and $\eta$. The observed flux has been converted to intrinsic source luminosity using a luminosity distance for the source of $d_L=10.1$ Mpc Padovani:2024ibi.
• Figure 3: Neutrino production from Seyfert galaxies from which IceCube has reported a neutrino excess IceCube24_Seyfert. We show the neutrino spectra for our baseline choice of parameters reported in the figure. The AGN luminosity and black hole mass adopted for each galaxy are summarized in Table \ref{['tab:astrophysical_params']}. We also show results for the model which includes the hydrodynamical turbulent escape, as discussed in the main text. The flux bands are extracted from the recent IceCube results IceCube24_Seyfert, while for NGC 1068 we also show the results from the 2022 study IceCube-NGC1068.
• Figure 4: (Left) Neutrino flux for varying X-ray luminosities; in the legend, we report the 2-10 keV luminosity $L_X$. The chosen values are the same as for the AGN SED in Fig. \ref{['fig:sed']}. (Right) Neutrino luminosity and characteristic peak neutrino energy as a function of the total X-ray luminosity $L_{X,\rm tot}$. We choose to use $L_{X,\rm tot}$, rather than $L_X$, to highlight more clearly the fraction of the total X-ray power that is emitted in neutrinos. We show with a dashed line the approximate fit in Eq. \ref{['eq:fit_luminosity']} for the $L_\nu-L_{X,\rm tot}$ relation.
• Figure 5: Predicted diffuse neutrino flux (black) from the AGN population. We show the reported flux data points from the IceCube collaboration Naab:2023xcz. We also show the separate contributions from different bands of $L_X$ with colored curves.