Neutrino and pair creation in reconnection-powered coronae of accreting black holes

TL;DR

This work develops a two-parameter model for reconnection-powered AGN coronae, where protons accelerated in magnetospheric current sheets drive both secondary-pair production and high-energy neutrinos. Analytical estimates link neutrino luminosity and peak energy to the proton magnetization $σ_p$ and the coronal Eddington ratio $λ_{X, m Edd}$, while numerical simulations with the ATHEνA code reveal how γγ annihilation dominates secondary-pair production and how the neutrino spectrum shifts with $σ_p$. The model applied to Seyfert galaxies, including NGC 1068, shows that IceCube neutrino observations can be accommodated for plausible $σ_p$ values, and it makes concrete predictions for the stacked neutrino flux from non-blazar AGN that respect current observational limits. The framework provides a robust, two-parameter tool for predicting coronal neutrinos and lepton densities across the AGN population, with implications for the diffuse neutrino background and multimessenger studies.

Abstract

A ubiquitous feature of accreting black hole systems is their hard X-ray emission which is thought to be produced through Comptonization of soft photons by electrons and positrons in the vicinity of the black hole, in a region with optical depth of order unity. The origin and composition of this Comptonizing region, known as the corona, is a matter open for debate. In this paper we investigate the role of relativistic protons accelerated in black-hole magnetospheric current sheets for the pair enrichment and neutrino emission of AGN coronae. Our model has two free parameters, namely the proton plasma magnetization $σ_{\rm p}$, which controls the peak energy of the neutrino spectrum, and the Eddington ratio $λ_{\rm X, Edd}$ (defined as the ratio between X-ray luminosity $L_{\rm X}$ and Eddington luminosity $L_{\rm Edd}$), which controls the amount of energy transferred to secondary particles. For sources with $λ_{\rm X, Edd} \gtrsim λ_{\rm Edd, crit}$ (where $λ_{\rm Edd, crit} \sim 10^{-1}$ for $σ_{\rm p}=10^5$ or $\sim 10^{-2}$ for $σ_{\rm p}=10^7$), proton-photon interactions and $ γγ$ annihilation produce enough secondary pairs to achieve Thomson optical depths $τ_{\rm T} \sim 0.1-10$. In the opposite case of $λ_{\rm X, Edd} \lesssim λ_{\rm Edd, crit}$, the coronal pairs cannot originate only from hadronic interactions. Additionally, we find that the neutrino luminosity scales as $L^2_{\rm X}/L_{\rm Edd}$ for $λ_{\rm X, Edd} \lesssim λ_{\rm Edd, crit}$, while it is proportional to $L_{\rm X}$ for higher $λ_{\rm X, Edd}$ values. We apply our model to four Seyfert galaxies, including NGC 1068, and discuss our results in light of recent IceCube observations.

Figures (14)

• Figure 1: AGN sketch showing the central massive black hole surrounded by an accretion disk. Perpendicularly to the disk a relativistic jet can be formed. A corona can be shown close to the central object. The numbers in the sketch refer to: (1) magnetospheric current sheet, (2) circumnuclear medium, (3) turbulent accretion disk (4) jet and (5) jet boundary current sheet.
• Figure 2: Decomposition of the spectral energy distribution (SED) of the hadronic cascade emission (black solid line) for $\sigma_{\rm p}=10^5$ (left panel) and $\sigma_{\rm p}=10^7$ (right panel). The black dashed line represents the all-flavor neutrino energy distribution. The dash-dotted grey line indicates the corona X-ray spectrum. Other parameter used here are: $s_{\rm p}=2$, $L_{\rm X}=10^{43}$ erg s$^{-1}$ and $M_{\rm bh}=10^7 M_\odot$ ($\lambda_{\rm X, Edd}=10^{-2}$).
• Figure 3: Characteristic timescales as a function of the proton Lorentz factor for the parameters of figure \ref{['fig:components']}. Solid lines represent the timescales for the acceleration process (black), proton synchrotron cooling (magenta), photomeson production (orange), and Bethe-Heitler pair production (blue). The X-ray photons of the corona are considered as targets for the two latter processes. The dotted black line represents the Hillas-limited Lorentz factor, while magenta and orange dotted lines represent the proton Lorentz factor at which the acceleration timescale is equal to the synchrotron and photomeson ones, respectively.
• Figure 4: Left panel: Energy distribution of protons at injection (transparent solid lines) and at steady state (bold solid lines). Steady state neutron energy distributions are also shown (transparent dashed lines). The color bar refers to the proton magnetization value, which determines the peak of the injected proton energy distribution (see Eq. \ref{['eq:dNdEdt']}). Right panel: Spectral energy distribution of cascade photons (solid lines) and neutrinos of all flavors (dashed lines) emitted from the coronal region. Other parameters used: $L_{\rm X}=10^{43}$ erg s$^{-1}$ and $M_{\rm bh}=10^{7}M_{\rm \odot}$ ($\lambda_{\rm X, Edd}=10^{-2}$).
• Figure 5: Fraction of the proton (solid dark blue line), photon (dash-dotted orange line), neutrino (dashed light blue line), and neutron (dotted red line) bolometric energy density over the total energy density of all four species as a function of $\sigma_{\rm p}$. The corona luminosity in all panels is $L_{\rm X} = 10^{43}$ erg s$^{-1}$ but the Eddington ratio changes from $10^{-1}$ in panel (a) to $10^{-4}$ in panel (d) in decrements of 10.