Jet-red giant interactions as a source of extragalactic neutrinos: Insights from KM3-230213A

TL;DR

The paper investigates the origin of the PeV neutrino KM3-230213A in the context of jet–red-giant interactions in AGN jets. By deriving the required proton distribution under shock acceleration and photohadronic production, it links the neutrino energy to the proton energy scale and estimates a proton luminosity around $L_p \sim 10^{47} \ \mathrm{erg\,s^{-1}}$ with a pronounced departure from equipartition ($u_p/u_B \sim 10^3$). Among 17 blazars in the error region, PMN J0606-0724 is identified as the most plausible electromagnetic counterpart due to a coincident radio flare and energetics compatible with a transient, proton-loaded jet scenario; the model invokes a fast internal shock interacting with a red giant embedded in the jet, producing a month-to-year-long neutrino burst via enhanced $p\gamma$ interactions, while gamma rays are absorbed and cascaded to the MeV–GeV band. The framework predicts rare but detectable neutrino bursts in aligned radio-loud jets and outlines observational signatures and testable predictions for PMN J0606-0724 and similar systems with current and next-generation km$^3$ detectors.

Abstract

The production sites of high-energy astrophysical neutrinos remain uncertain, though growing evidence suggests a connection to relativistic jets in active galactic nuclei (AGN). We present a detailed analysis of the recent PeV neutrino event KM3-230213A reported by the KM3NeT collaboration, aiming to constrain the physical conditions of its source. Assuming proton acceleration at shocks, we derive the properties of the proton distribution and the energetics required to explain the neutrino emission. Using contemporaneous multiwavelength observations of three AGN flaring candidates within the error region, we examine the plausibility of each of them as the possible counterpart. Our results favor PMN J0606-0724, which exhibits a prominent radio flare coincident with the neutrino arrival. In this framework, the red-giant interaction remains the key driver of baryon injection and shock acceleration, while the dominant external photon field sets the neutrino energy scale: photospheric photons from the red giant yield ~1-10 PeV neutrinos, whereas the ~220 PeV event KM3-230213A is more naturally produced through interactions with colder infrared photons from the dusty torus.

Figures (3)

• Figure 1: Total proton luminosity as observed in the lab-frame as a function of the Doppler factor. Three different redshifts are used for the three sources of interest (see text). We also show the radio luminosity (OVRO, at $15~\rm{GHz}$) for PMN J606$-$0724, the $\gamma$-ray luminosity at $1~\rm{GeV}$ (Fermi-LAT) for 0605$-$085, and the X-ray luminosity integrated between $0.2 - 2.3~\rm{keV}$ (eROSITA) for MRC 0614$-$083.
• Figure 2: Ratio of proton energy density $u_{\rm p}$ over the magnetic one $u_{\rm b}$, as a function of the magnetic field strength $B$ and the size of the emission region $R$, for a proton luminosity of $L_{\rm p} = 10^{47}~\rm{erg}\cdot\rm{s}^{-1}$. White contour lines represent ratio values of $1$ (equipartition) and $10^3$. Vertical colored lines indicate estimates of the emitting region sizes of the three selected sources.
• Figure 3: Expected peak neutrino energy $E_\nu$ from $p\gamma$ interactions as a function of the temperature $T_\gamma$ of the thermal target photon field (see Eq. \ref{['eq: Enu_T']}). The dashed blue line displays the estimated energy for the neutrino event KM3$-$210213A within its $90\%$ confidence area (blue shade), and the dashed vertical lines represent typical ranges of temperatures of external photon fields (dusty torus (DT), red giant (RG), and broad line region (BLR)).