Cosmic Ray Acceleration by Turbulence-Driven Magnetic Reconnection and the Origin of the Neutrinos in NGC 1068

TL;DR

The paper tackles the origin of IceCube’s high-energy neutrinos from the obscured AGN NGC 1068, where a significance of $4.2\sigma$ and $79^{+22}_{-20}$ events are reported without a TeV gamma-ray counterpart. It proposes turbulence-driven magnetic reconnection in the inner disk–corona as an efficient first-order Fermi accelerator capable of boosting protons to $E_p \gtrsim 10^{14}$ eV within a large-scale current sheet. A one-zone coronal-disk model incorporates disk and coronal photon fields and strong $\gamma\gamma$ absorption to compute hadronic and photo-hadronic cascades, reproducing the IceCube neutrino signal while remaining below MAGIC TeV limits; intrinsic X-ray variability can reconcile GeV data (Fermi-LAT) with the model. The results demonstrate a viable multi-messenger production channel in obscured AGN, though general-relativistic curvature effects are not included and are left for future work. $E_p \gtrsim 10^{14}$ eV, $4.2\sigma$, and $79^{+22}_{-20}$ events are key numerical anchors in the narrative.

Abstract

The Seyfert Type II galaxy NGC 1068 has been identified as a potential neutrino source by IceCube, with a 4.2$σ$ significance detection of a 79$^{+22}_{-20}$ neutrino excess from 2011 to 2020, despite the absence of a gamma-ray counterpart. The observed high-energy neutrino emission indicates the presence of a hadronic component, along with strong gamma-ray absorption, likely via pair production, and efficient particle acceleration. In this work, we investigate turbulence-driven magnetic reconnection as a mechanism for particle acceleration in the coronal accretion flow surrounding the central black hole. We develop a one-zone model for both acceleration and emission, following the framework of de Gouveia Dal Pino and Lazarian (2005) and Kadowaki et al. (2015) to explore how fast magnetic reconnection in the inner coronal disk region accelerates protons and electrons, shaping the spectral energy distribution (SED). Our model incorporates strong pair production attenuation and interactions with optical, ultraviolet (OUV), and X-ray photon fields in the corona, which serve as effective targets for proton interactions. Unlike recent studies, we find that particle acceleration to the extreme energies required to explain observations is primarily driven by first-order Fermi acceleration within the turbulent reconnection layers in a large scale current sheet, rather than by drift acceleration. Additionally, we demonstrate that accelerated protons primarily lose energy through photopion interactions with the OUV background, subject to important constraints from the coronal X-ray emission.

Figures (3)

• Figure 1: Schematic representation of our model. Field lines tied to the BH horizon meet oppositely oriented disk lines in the corona, forming a large-scale current sheet embedded in turbulence. The main features include the inner disk radius, $R_X$, and the turbulent reconnection region, characterized by the current sheet (neutral zone) width, $\Delta R_X$, and height of the reconnection region, $L_X$. Particles are accelerated and emit within this region. $L_X$ is constrained by the coronal extension $L$ (see text for more details). Adapted from dalpino_lazarian_2005kadowaki_etal_15.
• Figure 2: Hadronic acceleration and cooling timescales in the NGC 1068 corona. This figure shows the proton energy-independent acceleration timescale due to first-order Fermi and the cooling processes whithin the turbulent reconnection layer in the corona. The solid blue curve represents the acceleration timescale, which exhibits a cutoff right after $10^{18}$ eV, corresponding to the energy where the proton's Larmor radius becomes comparable to the reconnection sheet width ($\Delta R_X$). Beyond this energy, drift acceleration would become the dominant mechanism. Energy loss timescales are shown for synchrotron radiation (solid gray), proton-proton (p-p) interactions (light blue, dot-dashed line), and photo-hadronic interactions (photo-pion, p$\gamma$, in solid orange; Bethe-Heitler pair production in dashed orange). The photo-hadronic processes account for both disk blackbody and X-ray photon fields (orange curves). Bohm diffusion (solid black) is included for comparison. The total energy loss timescale is represented by the solid red curve. The intersection of the acceleration and total loss curves determines the maximum proton energy achievable $\simeq 10^{14}$ eV.
• Figure 3: Two possible Spectral Energy Distributions (SEDs) of the source NGC1068. In both panels it is shown IceCube neutrinos (shaded blue area), MAGIC constraints on $\gamma$-rays (orange upper limits) along with the Archival Data (gray points), from IceCube_2022. The dashed orange line represents the modeled blackbody radiation from the accretion disk, while the solid green line shows the combination of the disk and coronal X-ray photon fields. The dark red points show Fermi-LAT detections in the GeV band, presenting variability due to the modeling of X-rays (Bauer2015 or Marinucci2016). The solid black curve represents the total modeled emission from a photo-hadronic cascade (sum of dashed and dotted-dashed cascading lines and physical processes), and the solid blue curve shows the total neutrino emission, with its normalization scaled to represent only the muon neutrinos. The model does not account for Extragalactic Background Light (EBL) gamma-ray attenuation.