On the origin of ~ 100 TeV neutrinos from the Seyfert galaxy NGC 7469

TL;DR

This work investigates the origin of IceCube-detected neutrinos near 100 TeV from the Seyfert galaxy NGC 7469 by linking neutrino production to coronal particle acceleration. It combines Fermi-LAT GeV gamma-ray upper limits with IceCube neutrino data to assess whether coronal turbulence or magnetic reconnection can accelerate protons to PeV energies and produce the observed neutrino signals without violating gamma-ray constraints. The results indicate that both turbulence (with magnetization σ ~ 1) and relativistic reconnection (σ ~ 10) in a predominantly pair-dominated corona can yield maximum proton energies near or above 2 PeV and hard neutrino spectra; gamma-ray cascades constrain the emitting region to Rc ≲ 150 Rg in some cases. The study highlights how neutrino observations probe AGN coronal physics and explains differences with NGC 1068 as arising from different magnetization and corona conditions, offering a path to discriminate acceleration mechanisms in AGN cores.

Abstract

The origin of TeV-PeV neutrinos detected by IceCube remains largely unknown. The most significant individual neutrino source is the close-by Seyfert galaxy NGC 1068 at 4.2$σ$ level with a soft spectral index. Another notable candidate is the Seyfert galaxy NGC 7469, which has been recently proposed as a potential neutrino emitter. The likelihood fit of the IceCube data for this source returned a very hard spectral index of ~ 1.9 and the excess is dominated by two high-energy events, issued as two neutrino alerts IC220424A and IC230416A. The energies of the two neutrinos are estimated to be 100-200 TeV, implying a maximum proton energy > 2 PeV, significantly higher than that in NGC 1068. The lack of lower-energy neutrinos from NGC 7469 also suggests a neutrino spectrum harder than that of NGC 1068. In this paper, we analyze the Fermi-LAT observations of NGC 7469, which yield non-detection. By requiring the cascade flux accompanying neutrino production not to exceed the upper limit of the GeV flux, the size of the neutrino-emitting region can be constrained when the neutrino flux takes a high value of the allowed range. We suggest that protons are accelerated to PeV energies via turbulence or magnetic reconnection in the corona of NGC 7469 and interact with OUV photons from the accretion disk and X-rays from the corona through the $pγ$ process, producing neutrinos with energy of 100-200 TeV. In the turbulence acceleration scenario, the required maximum proton energy can be achieved with a magnetization parameter close to unity ($σ\sim 1$), while in the reconnection scenario, a magnetization parameter with $σ\sim 10$ is needed. In both scenarios, a pair dominated composition for the corona is preferred. The difference in the neutrino spectrum between NGC 7469 and NGC 1068 could be due to a different magnetization despite that they belong to the same type of AGN.

Figures (4)

• Figure 1: Timescales of proton acceleration, escape, and cooling in the corona of NGC 7469 in turbulence scenario. The upper panel and lower panel represent the case 1 and case 2 in turbulence scenario, respectively. The timescale of turbulence acceleration is shown in black dashed line and the thin solid colored line refers to the energy-loss timescale. The shared parameters for the two cases are $R_c \approx 10 R_{\rm g}$, $\beta_p\simeq 0.1$, $\ell_c = R_{\rm g}$. In case 1, $n_{p} \simeq10^{10}\,{\rm cm^{-3}}$, and the magnetic field is $B = 1.1\times 10^4\,{\rm G}$; In case 2, $n_{p} \simeq 10^{9}\,{\rm cm^{-3}}$, and the magnetic field is $B = 3.4\times 10^3\,{\rm G}$.
• Figure 2: Multi-messenger emissions from NGC 7469 in the turbulence acceleration scenario. The upper panel illustrates the case 1 corresponding to a high neutrino flux of $10^{-12}\, {\rm erg \,cm^{-2}\,s^{-1}}$, while the lower panel shows the case 2 corresponding to a lower neutrino flux of $10^{-13} \, {\rm erg \,cm^{-2}\,s^{-1}}$. For both cases, the purple shaded region indicates the neutrino flux reported by IceCube IceCube2025arXiv251013403A. The blue arrows indicate the upper limits for GeV gamma-ray emission imposed by Fermi-LAT. The black solid curve represents the neutrino spectrum whereas the blue curves show the cascade gamma-ray emission, We normalize the proton spectrum by Eq.\ref{['eq:up']}. The parameters are adopted as $\beta_p \simeq 0.1$ ($\sigma \simeq 1$) with the Alfvén speed of $v_A\simeq c$ and $E_{p,{\rm max}}\simeq2 \,{\rm PeV}$.
• Figure 3: Same as Fig.\ref{['Fig:Timescale_Turbulence']} but for the reconnection scenario. The parameter values used are $R_c\approx10R_{\rm g}$, $\sigma = 10$, $B = 7.5\times 10^{3}\,{\rm G}$ and $n_p = 3\times 10^8\,{\rm cm^{-3}}$
• Figure 4: Same as Fig.\ref{['Fig:Neutrino spectrum turbulence']} but for the reconnection scenario. The upper panel illustrated the case 1 with the high neutrino flux of $10^{-12}\, {\rm erg \,cm^{-2}\,s^{-1}}$ corresponding to $\eta_p\simeq 0.2$, while the lower panel shows the case 2 with a low neutrino flux of $10^{-13} \, {\rm erg \,cm^{-2}\,s^{-1}}$, corresponding to $\eta_p\simeq 0.02$. The parameter values used are $\sigma = 10$, $n_p\simeq 3\times 10^8\, {\rm cm^{-3}}$, and $E_{p,{\rm max}}\simeq60 \,{\rm PeV}$.