Neutrinos and gamma rays from Seyfert galaxies constrain the properties of coronal turbulence

Abstract

The TeV neutrino signal observed by IceCube from the active galactic nucleus (AGN) NGC 1068 can probe its innermost coronal regions. If these neutrinos originate from hadrons accelerated within a magnetized turbulent corona, their intensity and spectrum depend on the turbulent magnetic field strength and turbulence coherence scale. The gamma rays accompanying neutrino production are absorbed in this optically thick environment, in a way that depends sensitively on the size of the corona. By a joint fit of the IceCube and Fermi-LAT observations, we translate the multimessenger signal from NGC 1068 and the tentative signal from NGC 7469 into quantitative constraints on coronal properties. NGC 1068, with a significant TeV neutrino excess, favors a compact, strongly magnetized corona with a large turbulence coherence length relative to the coronal size. NGC 7469, with two $\sim 100$ TeV neutrino events, points instead to a somewhat larger corona with much smaller coherence length and high magnetization, but a very small fraction of energy in non-thermal protons. We obtain the diffuse flux from a population of Seyfert galaxies identical to either NGC 1068 or NGC 7469. Finally, we consider a third scenario, motivated by the spectral break observed in the diffuse neutrino flux at tens of TeV, with coronal properties intermediate between the two point-source-inspired models. To enable detailed comparisons with the IceCube and electromagnetic observations, we release our model predictions in a GitHub repository.

Figures (9)

• Figure 1: Neutrino and electromagnetic emission from the coronal region, for NGC 1068 (left) and NGC 7469 (right). We show the $68\%$ and $95\%$ confidence level (C.L.) regions with different shadings, together with the predicted best-fit curves. Since the confidence regions are shown mostly as a visual guide, we use the $\chi^2$ threshold for the TS corresponding to one degree of freedom, so as to match the exclusion contours for the individual parameters shown in Figs. \ref{['fig:1068_param']} and \ref{['fig:7469_params']}. The assumed SED is shown in orange for both galaxies. We do not include extragalactic gamma-ray absorption, which would only affect very-high-energy gamma rays above hundreds of GeV. All Fermi-LAT measurements are used only as upper bounds to leave space for additional contributions from different emission regions.
• Figure 2: Constraints on coronal properties of NGC 1068 from the combined neutrino and gamma-ray signal. We highlight the regions of parameter space where $\beta_{\rm tur}<1$, signaling a strongly magnetized plasma, and $U_{p,\rm nth}/U_{B,\rm tur}<1$, signaling a negligible dynamical feedback of non-thermal protons on the turbulence.
• Figure 3: Constraints on coronal properties of NGC 7469 from the combined neutrino and gamma-ray signal. We highlight the regions of parameter space where $\beta_{\rm tur}<1$, signaling a strongly magnetized plasma, and $U_{p,\rm nth}/U_{B,\rm tur}<1$, signaling a negligible dynamical feedback of non-thermal protons on the turbulence.
• Figure 4: Predicted diffuse neutrino flux from a population of Seyfert galaxies with coronal parameters informed by our fit for NGC 1068 (left), NGC 7469 (center), and intermediate properties that fit by eye the spectral break of the diffuse flux (right). Different colors indicate the contribution of coronae of different $L_X^{2-10\,\rm keV}$, in $\rm erg\,s^{-1}$, as indicated in the legend. In all cases, we rescale the total flux by a factor $\rho<1$, to match the normalization of the diffuse flux observed by IceCube. Measured flux points are taken from IceCube:2025ewu.
• Figure 5: Neutrino luminosity and characteristic peak neutrino energy (defined as the average energy over the energy distribution) for varying $L_X^{2-10\,\rm keV}$.