KM3-230213A: Record-setting PeV Neutrino Event

• The PeV neutrino event KM3-230213A is defined as the highest-energy neutrino observed (~220 PeV), providing pivotal clues about UHE cosmic ray sources and hadronic processes.
• Its detection via a bright, through-going muon track with reconstructed energy around 120 PeV and precise directional containment effectively rules out atmospheric background origins.
• The event drives joint multimessenger analyses that challenge diffuse flux models, suggesting that rare transient or anisotropic sources may dominate ultra-high-energy neutrino production.

The PeV neutrino event KM3-230213A refers to an ultra-high-energy neutrino candidate detected by the KM3NeT/ARCA telescope, characterized by an estimated neutrino energy of approximately 220 PeV (90% CL: 72 PeV–2.6 EeV). This observation, the highest-energy neutrino detected to date, holds significant implications for astroparticle physics, including the nature of ultra-high-energy cosmic ray (UHECR) sources, hadronic acceleration processes, the extragalactic high-energy neutrino flux, and multimessenger constraints from gamma-ray and astrophysical neutrino observatories.

1. Detection, Reconstruction, and Event Significance

KM3-230213A was recorded on 13 February 2023 by the Mediterranean ARCA sub-array of KM3NeT (Collaboration et al., 12 Feb 2025, collaboration, 12 Feb 2025). The event signature is a bright, through-going muon track, consistent with a charged-current muon neutrino interaction. The deposited muon energy was reconstructed as $E_\mu = 120^{+110}_{-60}$ PeV. Simulations yield a parent neutrino energy likelihood peaked at $E_\nu \approx 220$ PeV, with a wide confidence interval, driven by stochastic muon energy loss.

The reconstructed arrival direction is RA = $94.3^\circ$, Dec = $-7.8^\circ$ (J2000), with a $1.5^\circ$ 68% containment and $3.0^\circ$ 99% containment radius (Collaboration et al., 12 Feb 2025). Event selection criteria exclude significant backgrounds: the probability of an atmospheric muon or neutrino origin is $<10^{-5}$ yr$^{-1}$.

No comparable events at such energies have been reported by IceCube or the Pierre Auger Observatory (PAO) in more than a decade of exposure, placing this single detection at $\gtrsim5\sigma$ significance as an astrophysical—or exotic physics—candidate (Collaboration et al., 12 Feb 2025).

2. Joint Neutrino Landscape and Tension with Diffuse Flux Limits

Comprehensive analyses fold the KM3-230213A event together with null results from IceCube (High-Energy Starting Events, EHE) and Auger to constrain the flux normalization for an isotropic $E^{-2}$ single-flavor spectrum (Collaboration et al., 12 Feb 2025, Neronov et al., 18 Feb 2025). The joint fit yields

$$E^2 \Phi^{\rm 1f}_{\nu + \bar \nu} = 7.5 \times 10^{-10}\ \mathrm{GeV\,cm^{-2}\,s^{-1}\,sr^{-1}}$$

within the KM3NeT energy window (72 PeV–2.6 EeV), more than an order of magnitude below the naive estimate ($5.8 \times 10^{-8}$), driven by non-observations elsewhere.

There is a persistent tension ($2.5\sigma$–$3\sigma$) between this event and the diffuse all-sky upper limits; BPL and SPL fits combining IceCube UHE and lower energy samples show either no statistically significant spectral break (unless relying on KM3NeT alone, which would grossly violate IceCube/PAO bounds), or only mild evidence for a PeV-scale hardening.

Thus, the event is difficult to accommodate as part of a homogenous diffuse flux unless the true astrophysical spectrum has an upturn or new component above tens of PeV, or unless the sources are rare/anisotropic (Collaboration et al., 12 Feb 2025, Neronov et al., 18 Feb 2025).

3. Astrophysical Production and Cascade Gamma-Ray Emission

3.1 Neutrino–Gamma-Ray Link

Theoretical frameworks posit that such a PeV-scale neutrino arises from the decay of charged pions produced via $p\gamma$ or $pp$ interactions at extreme energies. Because $\pi^0$ are generated at comparable rates, the production of a $220$ PeV neutrino predicts co-emission of $\sim440$ PeV gamma rays. Quantitatively, for a photohadronic source, the injected gamma-ray efficiency is related to the neutrino flux by

$$E_{\gamma,0} \frac{dN_\gamma}{dE_\gamma} \approx 4\, E_\nu \frac{dN_\nu}{dE_\nu}$$

(for $pp$, the factor is $2$) (Fang et al., 13 Feb 2025).

3.2 Electromagnetic Cascades on the Extragalactic Background

Once injected, $\gtrsim100$ PeV $\gamma$-rays interact with the extragalactic background light (EBL) and cosmic microwave background (CMB) via $\gamma\gamma\rightarrow e^+e^-$, initiating electromagnetic cascades. Secondary $e^\pm$ rapidly lose energy in the CMB/EBL via inverse-Compton scattering, resulting in a spectrum of cascade photons at GeV–TeV energies. The evolution of the cascade is governed by coupled Boltzmann equations for $n_\gamma(E,x)$ and $n_e(E,x)$, incorporating pair production and IC upscattering (Fang et al., 13 Feb 2025).

The observationally relevant cascade flux depends critically on:

• EBL photon density: $n_{\rm EBL}(\epsilon,z)$ [Domínguez et al. 2011 model].
• IGM magnetic field strength $B$; large $B$ causes angular/time smearing, diminishing detectability.
• Internal opacity within the source (parameterized by optical depth $\tau_{\rm int}$).

The flux at Earth after including all attenuation and cascade effects, for monochromatic injection at $E_0 \approx 440$ PeV, can be approximated for energies $E \lesssim E_{\rm th}$ by

$$E^2\Phi_\gamma(E) \sim \frac{E_0^2 (dN_\gamma/dE_0)}{4\pi d_L^2}\frac{1}{\ln(E_0/E_{\rm th})} \begin{cases} (E/E_{\rm th})^{1/2} & E < E_{\rm th} \ 1 & E_{\rm th} < E < \min[E_0/(1+z),E_c] \end{cases}$$

where $E_{\rm th}\sim 0.1$–$1$ TeV, set by EBL/CMB photon energies, and $E_c$ is the high-energy absorption cutoff (Fang et al., 13 Feb 2025).

3.3 Observational Prospects: Existing and Future Gamma-Ray Observatories

Monte Carlo cascade simulations for various source distances and IGMF strengths show:

• At $z=0.1$, $B=3\times 10^{-14}$ G: broad peak $E^2\Phi_{\gamma}\sim10^{-12}$–$10^{-13}$ erg cm$^{-2}$ s$^{-1}$ at 0.1–1 TeV, within the sensitivity of VERITAS, H.E.S.S., MAGIC, LHAASO, and HAWC.
• For $B\gtrsim 3\times10^{-13}$ G, the cascade is delayed and suppressed below IACT sensitivity, but may be recovered by future facilities such as CTAO or SWGO (Fang et al., 13 Feb 2025).

The non-detection of a TeV flare in the direction of KM3-230213A would therefore suggest either:

• High $B$ ($\gtrsim$ few$\,\times\,10^{-13}$ G); or
• High internal opacity ($\tau_{\rm int} \gg 1$), implying a radio-loud source at low frequencies (e.g., in blazar jets) (Fang et al., 13 Feb 2025).

4. Source Scenarios: Transient and Steady-State Models

4.1 Transient Flaring Origin

The alternative to a steady, isotropic UHE neutrino flux is a transient outburst of duration $T\lesssim2$ years with a flux normalization

$$F \approx 3\times 10^{-10}(1\,\mathrm{yr}/T)\ \mathrm{erg\,cm^{-2}\,s^{-1}}$$

that satisfies both the ARCA detection and IceCube/PAO null results (Neronov et al., 18 Feb 2025). Such a population must be rare ($R \lesssim 0.4$ yr$^{-1}$ sky$^{-1}$), require hard spectra at $E_p\gtrsim 10^{19}$ eV, or production by photohadronic interactions on IR photons ($\epsilon_\gamma\sim0.2$ eV for $E_\nu \approx 220$ PeV).

If no GeV–TeV gamma-ray transient is observed in coincidence, explanations may invoke:

• Extremely collimated neutrino emission ($\Theta_\nu \ll \Theta_\gamma$);
• High internal $\gamma\gamma$ opacity with suppressed electromagnetic cascade at Earth;
• Multi-year cascade time delay exceeding the transient duration (Neronov et al., 18 Feb 2025).

All-sky rate constraints restrict such flares to rare but highly energetic sources, e.g., powerful AGN, tidal disruption events, or jet–IR-torus interactions.

4.2 Cosmogenic Scenarios

Cosmogenic neutrinos arise from UHECR (proton) interactions ($p\gamma$) with the CMB/EBL. To match the observed KM3-230213A event rate under existing UHECR and ν constraints, the required parameter space involves:

• A local proton fraction $f_p \sim 5$–$10$\% at $E\gtrsim10^{19}$ eV;
• Strong positive source evolution (e.g., $\propto(1+z)^{3-5}$) up to $z \sim 6$ (collaboration, 12 Feb 2025, Cermenati et al., 16 Jul 2025);
• An injection spectral index $\gamma \approx 2.1$–$2.3$ and maximum rigidity $R_{\max}\gtrsim10^{20}$ V.

The joint multimessenger models indicate that only UHECR source classes with a subdominant proton composition, hard spectrum, and strong evolution can accommodate KM3-230213A without saturating the Fermi-LAT EGB with associated cascaded $\gamma$-rays (Cermenati et al., 16 Jul 2025).

5. Source Counterpart Studies and Blazar Candidates

Seventeen blazar-like AGN candidates with multiwavelength activity are located in the KM3-230213A error region (Collaboration et al., 12 Feb 2025). Notable cases include:

• PMN J0606–0724 ($z=1.277$): prominent radio flare coincident within 5 days of the neutrino, with pre-trial chance probability 0.26%. No significant contemporaneous gamma-ray detection. Derived energetics ($L_p\sim10^{47}$ erg/s) are consistent with shock-acceleration scenarios for neutrino production at $\sim220$ PeV via interactions with IR photons from a dusty torus (Clairfontaine et al., 3 Nov 2025).
• MRC 0614–083: rising X-ray flux with $\sim2\sigma$ significance, although redshift is unknown; its X-ray/neutrino luminosity ratio disfavors it as a strong hadronic counterpart.
• PMN J0605–085: gamma-ray flaring activity, but temporal offset relative to neutrino, weakens the plausibility.

Hadronic acceleration models require proton energies $E_p\gtrsim20\,E_\nu\sim4.4$ EeV and favor transient jet–obstacle scenarios (e.g., red giant interactions) with baryon-loaded shocks and dominant external IR photon fields (Clairfontaine et al., 3 Nov 2025). The absence of GeV–TeV $\gamma$-ray flares in candidate blazars is consistent with strong internal $\gamma\gamma$ absorption in dense jet or torus environments.

6. Constraints from Gamma-Ray and Multimessenger Observations

Contemporaneous follow-up with VERITAS and other IACT facilities (fields covering the $3^\circ$ localization region) found no statistically significant gamma-ray excess above $E_{\text{th}}=550$ GeV, imposing a $99\%$ CL upper limit on the integral flux at $E>550$ GeV: $$\Phi_{UL}(E > 550\ \mathrm{GeV}) = 9.49 \times 10^{-13}\ {\rm cm}^{-2}\ {\rm s}^{-1}$$ (1.81% of the steady Crab Nebula flux at the same threshold) (Mooney, 29 Sep 2025). This result constrains models with low source opacity and low IGMF, requiring either significant in-source absorption or extreme distances ($z \gtrsim 0.2$) to evade joint neutrino–gamma-ray observability.

The full upper limit profile is set by the effective area and exposure, with the Rolke method for $99\%$ CL intervals: $$\Phi_{UL}(E) = \frac{N_{UL}}{A_{\rm eff}(E) T_{\rm obs}}$$ where $N_{UL}$ is derived from the combined ON/OFF count statistics.

Notably, for modest $B$, the predicted cascade GeV–TeV “afterglow” should be visible to current gamma-ray survey instruments as a point source coincident with the KM3NeT direction (Fang et al., 13 Feb 2025). Non-detection further restricts the allowed astrophysics of the source and the structure of the intervening LSS magnetic field.

7. Implications and Future Outlook

The observation of KM3-230213A at $\sim220$ PeV marks an inflection point in UHE neutrino astronomy. Its interpretation is constrained by a complex array of multimessenger data:

• Diffuse, isotropic UHE astrophysical neutrino origin remains disfavored by joint IceCube, KM3NeT, and PAO fits unless there is a spectrum break or upturn above $\sim$PeV.
• Cosmogenic scenarios can match the rate with a subdominant hard proton fraction and strong cosmic evolution, but are tightly restricted by $\gamma$-ray bounds and UHECR composition data.
• Transient, rare extragalactic sources—particularly energetic AGN flares involving photohadronic interactions with IR photon fields—remain viable, given the extreme required luminosity, short duration, and suppressed electromagnetic “twin” signatures.
• Present and future gamma-ray observations (CTA, LHAASO, SWGO, continued IACT surveys) are essential for constraining joint source models.
• Improved exposure from KM3NeT (full ARCA), IceCube-Gen2, and radio-based arrays (GRAND, RNO-G) will clarify the spectral shape at the highest energies and determine whether such events are statistical outliers or herald a new neutrino component.

The strong multimessenger constraints on $\gamma$-ray and neutrino fluxes sharpen the focus on the most powerful cosmic accelerators and the structure of the intergalactic medium. The detection or non-detection of gamma-ray afterglows coincident with future UHE neutrino events will decisively test the origin scenarios outlined in current models (Fang et al., 13 Feb 2025, Neronov et al., 18 Feb 2025, Collaboration et al., 12 Feb 2025, Mooney, 29 Sep 2025).