Constraining the contribution of Seyfert galaxies to the diffuse neutrino flux in light of point source observations

Abstract

Recently, the IceCube Collaboration reported evidence for TeV neutrino emission from several nearby Seyfert galaxies, with the highest significance found for NGC 1068. Assuming stochastic proton acceleration in magnetized turbulence inside the corona, we model the neutrino emission of Seyfert galaxies as a function of their X-ray luminosity. Applying our model to NGC 1068, we obtain a good fit to the public IceCube data and constrain the coronal radius to $\lesssim 5 R_S$ by comparing our MeV $γ$-ray predictions to Fermi-LAT observations. Extending to the full Seyfert population, we estimate their diffuse neutrino contribution and find that they can explain a significant fraction of the observed flux below $10\,\mathrm{TeV}$. However, scenarios with highly turbulent coronae and high cosmic-ray pressure across the population are ruled out. In particular, if all sources shared the best-fit parameters obtained for NGC 1068, their cumulative neutrino emission would exceed current upper limits at TeV energies by $3.8σ$. Our results, informed by both neutrino and $γ$-ray data, show that those Seyfert galaxies that emerge as neutrino point sources must be exceptionally efficient neutrino emitters and are not representative of the broader population.

Figures (14)

• Figure 1: Neutrino spectra for different X-ray luminosities computed for $\eta = 10$ (top panel) and $\eta = 150$ (bottom panel). The neutrino spectra are normalized so that $P_\mathrm{CR}/P_\mathrm{th} = 0.5$ and the dimensionless coronal radius is set to $r_c = 5$. $E_\nu L_{E_\nu} \equiv E_\nu^2 \,\mathrm{d}N_\nu/\mathrm{d}E_\nu\mathrm{d}t$ denotes the differential all-flavor neutrino luminosity.
• Figure 2: Cascaded $\gamma$-ray spectra for different emission radii calculated for the X-ray luminosity of NGC 1068 ($L_X = 4.17\times 10^{43}\,\mathrm{erg}/\mathrm{s}$) and $\eta = 20$. Here, the underlying proton spectra are normalized so that $L_p/L_X = 0.5$. $E_\gamma\, L_{E_\gamma} \equiv E_\gamma^2 \,\mathrm{d}N_\gamma/\mathrm{d}E_\gamma\mathrm{d}t$ denotes the differential $\gamma$-ray luminosity.
• Figure 3: Log-likelihood ratio as a function of $n_s$ and $\eta$. The best-fit parameters obtained for NGC 1068 are indicated by a red cross and the gray contours represent the $1\sigma$ and $2\sigma$ confidence regions according to Wilks' theorem Wilks1938. The region above the red line corresponds to $P_\mathrm{CR}/P_\mathrm{th} > 0.5$ and is thus excluded.
• Figure 4: Best-fit all-flavor neutrino spectrum of NGC 1068 (red) together with the corresponding cascaded $\gamma$-ray spectrum (blue). The shaded bands indicate the $1\sigma$ and $2\sigma$ uncertainty regions of the flux and, for the neutrino spectrum, the central 68% energy range. The latest Fermi-LAT data of NGC 1068 Ajello2023 is shown for comparison.
• Figure 5: 2--10 keV X-ray luminosity functions of AGNs from Ref. Ueda2014 (U14) and Ref. Buchner2015 (B15). The red shaded bands correspond to the $1\sigma$ and $2\sigma$ uncertainty regions of the U14 XLF derived from the statistical errors of the model parameters assuming that all parameters are uncorrelated, i.e. these bands indicate upper limits on the size of the actual uncertainty regions. The blue and green shaded bands indicate the 68% and 95% credible regions of the B15 XLFs for the constant-slope and constant-value prior, respectively.