Multimessenger Constraints on Production Sites of High-Energy Neutrinos from NGC 1068

Abstract

The detection of high-energy neutrino signals from the nearby Seyfert galaxy NGC 1068 provides us with a unique opportunity to explore nonthermal processes near the center of supermassive black holes. Using the IceCube and Fermi-LAT data, we present general multimessenger constraints on the energetics of cosmic rays and the compactness of the neutrino emission region (${\mathcal R}$), considering not only $pγ$ but also $pp$ processes. Compared to the photohadronic scenario, the hadronuclear scenario can alleviate constraints on the emission region, yielding ${\mathcal R}\lesssim30-70$ for low-$β$ plasma and ${\mathcal R}\lesssim5-50$ for high-$β$ plasma. While our results support the previous conclusion that the photohadronic scenario favors a compact corona with ${\mathcal R}\sim3-10$, these suggest the relevance of further investigations into $pp$ neutrino contributions. When the cosmic-ray spectrum is extended from 1 GeV, we find that the requred cosmic-ray luminosity exceeds the X-ray luminosity for a spectral index of $s_{\rm CR}\gtrsim2$, which challenges some shock acceleration models. We also show that the beta decay scenario is unlikely even if the magnetic field is as strong as the maximum allowed by the Eddington luminosity. Given that NGC 1068 can be established as a neutrino source, our results will provide evidence for the standard hadronic scenario, including magnetically powered corona models having hard spectra with $s_{\rm CR}\lesssim2$.

Figures (6)

• Figure 1: Optical depth for the two-photon annihilation ($\tau_{\gamma\gamma \rm,e^{+}e^{-}}$) and effective optical depths for photomeson production ($f_{p\gamma}$), Bethe-Heitler pair production ($f_{\rm BH}$), and inelastic proton-proton collisions ($f_{pp}$) for two values of $\tau_T$. The emission radius is set to $R = 10 R_S$. For the photomeson production, the Bethe-Heitler process, and proton-proton collisions, the proton escape timescale is set to $t_* = 100 R/c$.
• Figure 2: Cascaded photon spectra (colored lines) and all-flavor neutrino spectra (dashed black lines) for different values of the emission radius $R$ for different parameter sets explored in this work. Among the photon spectra, solid lines correspond to $\xi_B=1$ while dashed lines correspond to $\xi_B=0.01$. All panels are made for injected cosmic-ray protons with $s_{\rm CR}=2$ and $\varepsilon_{p}^{\rm max}=30$ TeV. The solid black lines correspond to the 95 percent contour lines and the best-fit line from the IceCube data IceCube:2022der. Gamma-ray data from the Fermi-LAT Ajello:2023hkh and MAGIC MAGIC:2019fvw observations are also shown.
• Figure 3: Required minimum cosmic-ray proton luminosity, $L_\text{CR}$, as a function of the emission radius $R$ and the power-law index $s_{\rm CR}$ for $\varepsilon_p^{\rm min}=1$ GeV and $\varepsilon_p^{\rm max}=30$ TeV in the hadronic scenario. The blue contour corresponds to $L_{\rm corona}$, which is the total coronal X-ray luminosity. The red and orange lines correspond to the upper limits from gamma-ray observations for $\xi_B=1$ and $\xi_B=0.01$, respectively.
• Figure 4: Required minimum cosmic-ray proton luminosity, $L_\text{CR}$, as a function of the emission radius $R$ and power-law index $s_{\rm CR}$ for $\varepsilon_p^{\rm min}=10$ TeV and $\varepsilon_p^{\rm max}=30$ TeV in the minimum hadronic scenario for different values of $\tau_T$. The blue and orange lines have the same meaning as in Figure \ref{['fig:contour-1gev']}. The red hatching represents the excluded region for $\xi_B=1$.
• Figure 5: Cascaded gamma-ray spectra (colored lines) and all-flavor neutrino spectra (black lines) for different values of $R$ and magnetic field strength in the decay scenario. The magnetic field strengths used are computed in terms of $B_{\rm max}$ as defined in Equation \ref{['eq:Bmax']}. The solid lines correspond to cascades computed with the photomeson contribution and the dashed lines correspond to cascades neglecting the photomeson contribution. Attenuation due to CMB and EBL Gilmore:2011ks is considered. All panels are made using $s_{\rm CR}$ = 3.2, $\varepsilon_{A}^{\rm min}$ =5 PeV and $\varepsilon_{A}^{\rm max}$=15 PeV for the injected helium nuclei spectrum.