KM3-230213A: Record UHE Neutrino Event

• Ultra-high-energy neutrinos, exemplified by KM3-230213A, are rare astrophysical particles with energies over 50 PeV, marking a new sensitivity regime.
• The event’s energy reconstruction and spectral analyses using both KM3NeT data and non-detections from IceCube and Auger reveal a 2.5–3σ statistical tension with standard flux models.
• Multi-wavelength searches and transient source hypotheses, including potential blazar flares, support an extragalactic origin for this unprecedented neutrino event.

The ultra-high-energy (UHE) neutrino event KM3-230213A is a singular astrophysical neutrino candidate detected by the KM3NeT/ARCA telescope in February 2023. With a reconstructed energy around 220 PeV and a 90% confidence interval of 72 PeV to 2.6 EeV, KM3-230213A stands as the most energetic neutrino event ever observed. Its detection, significance, and subsequent multi-instrument and theoretical analyses have broad implications for astroparticle physics, high-energy astrophysics, and tests of fundamental symmetries.

1. Detection and Energy Characterization

KM3-230213A was observed as a bright, track-like muon event in the partially operational KM3NeT/ARCA detector using only 19–21 of 230 planned detection lines, yielding an active exposure of 335 days. The reconstructed neutrino energy $E_\nu$ is $220^{+570}_{-110}$ PeV, placing it deep in the UHE neutrino regime (≳50 PeV). The atmospheric background at these energies is negligibly small ($\sim10^{-5}$/year for such tracks), supporting its astrophysical—rather than atmospheric—origin (Collaboration et al., 12 Feb 2025, Adriani et al., 12 Feb 2025).

Comparison with observatories such as IceCube and the Pierre Auger Observatory indicates that neither has detected any neutrino events above tens of PeV despite higher exposures in some configurations. Essential observational parameters are:

Detector	Energy Range	Activity at UHE	Exposure Notes
KM3NeT/ARCA	100 TeV–100 EeV	1 event (KM3-230213A)	Partial detector, 19–21 lines
IceCube	To ~10 EeV (EHE sample)	0 events above ~60 PeV	Decade-long monitoring
Pierre Auger	To 100 EeV	0 UHE neutrino events	Integrated cosmic-ray search

The KM3NeT exposure for the relevant analysis is defined as $$\mathcal{E}^{\rm KM3}(E) = 4\pi T^{\rm KM3} A_{\rm eff}^{\rm KM3}(E)$$, where $T^{\rm KM3}$ is live time and $A_{\rm eff}$ is the all-sky, all-flavor effective area averaged over the observed UHE interval.

2. Spectral Modeling, Flux Normalisation, and Joint Analyses

The astrophysical UHE neutrino flux is modeled under both single-power-law (SPL) and broken-power-law (BPL) hypotheses. The SPL per-flavor flux parametrization is: $$\Phi^{\rm 1f}_{\nu+\bar\nu}(E) = \phi \left(\frac{E}{100\,\rm{TeV}}\right)^{-\gamma_1}$$ where $\phi$ is the flux normalization and $\gamma_1$ is the spectral index.

A fit with only the KM3NeT event (over its 90% energy containment) returns: $$E^2\Phi^{\rm 1f}_{\nu+\bar\nu} \approx 5.8^{+10.1}_{-3.7} \times 10^{-8}\;\rm{GeV\,cm^{-2}\,s^{-1}\,sr^{-1}}$$ When incorporating IceCube and Auger non-detections with a joint likelihood (using Poisson probabilities for the zero-event outcome), the best-fit flux is: $$E^2\Phi^{\rm 1f}_{\nu+\bar\nu} = 7.5^{+13.1}_{-4.7} \times 10^{-10}\;\rm{GeV\,cm^{-2}\,s^{-1}\,sr^{-1}}$$ within the UHE region (approximately 72 PeV – 2.6 EeV) (Collaboration et al., 12 Feb 2025).

The BPL spectrum, motivated by a possible new neutrino production channel (e.g. cosmogenic origin), reads: $$\Phi^{\rm 1f}_{\nu+\bar\nu}(E) = \begin{cases} \phi \left(\frac{E}{100\,\text{TeV}}\right)^{-\gamma_1}, & E \leq E_b\ \phi \left(\frac{E_b}{100\,\text{TeV}}\right)^{-\gamma_1} \left(\frac{E}{E_b}\right)^{-\gamma_2}, & E > E_b \end{cases}$$ with $E_b$ denoting the break energy.

When combining lower-energy IceCube data (High-Energy Starting Events, cascades, etc.), fits may hint at a peaking in the BPL scenario at $E_b\sim 1$–$10$ PeV and a post-break hardening ($\gamma_2 \sim 1$–2.3). However, BPL fits that match the single KM3NeT event typically overpredict UHE neutrino counts in IceCube/Auger, thus conflicting with null UHE detections in those experiments.

3. Statistical Tension and Spectral Implications

A central result is the persistent tension—quantified between $2.5\sigma$ and $3\sigma$—between the flux suggested by the KM3NeT detection and the absence of UHE (>50 PeV) neutrinos in IceCube and Auger (Collaboration et al., 12 Feb 2025, Neronov et al., 18 Feb 2025, Collaboration et al., 12 Feb 2025). This is evaluated using both frequentist (parameter goodness-of-fit, Wilks' theorem) and Bayesian (posterior predictive check) methods. For example: $$\overline{\chi}^2_{\rm in}(\phi, \gamma_1) = \chi^2_{\rm in}(\phi, \gamma_1) - \sum_d \hat{\chi}^2_{d,\rm in}$$ where $d$ indexes datasets. The result is a best one-tailed z-score of about $2.5\sigma$.

This tension means that, if the UHE neutrino flux were as high as implied by KM3-230213A (under conventional isotropic, SPL/BPL models), multiple events should have appeared in the years-long IceCube and Auger exposures. The fact that only KM3NeT reports a detection at these energies suggests either a statistical upward fluctuation, a much lower average flux, or an unexpectedly complex spectrum (possibly involving transient sources or new production physics) (Collaboration et al., 12 Feb 2025, Neronov et al., 18 Feb 2025, Collaboration et al., 12 Feb 2025).

4. Origin Scenarios: Extragalactic Dominance

Multiple analyses converge to argue strongly against a Galactic origin for KM3-230213A. The principal evidence includes:

• Extreme event energy ($\gtrsim 72$ PeV), implying parent cosmic ray energies in the hundreds of PeV regime.
• The event's equatorial and Galactic coordinates are well removed from the Galactic Center and Plane and lack nearby known powerful cosmic-ray accelerators in the region (within systematic uncertainties) (Adriani et al., 12 Feb 2025).
• Theoretical limits on achievable energy in Galactic accelerators (e.g. pulsar wind nebulae) are orders of magnitude too low, even with maximally efficient parameters:

$$E_{\max} [\rm PeV] \simeq 2 \eta_e \eta_B^{1/2} (\dot{E}_{36})^{1/2}$$

where $\dot{E}_{36}$ is the spin-down power in $10^{36}\,$erg/s units—far below the requirements for observed pulsars in the region.

• Diffuse Galactic emission models and gamma-ray upper limits from HAWC and LHAASO are many orders of magnitude below that needed to explain the event's inferred flux (Adriani et al., 12 Feb 2025).

Consequently, the astrophysical origin is robustly extragalactic—consistent with the need for more extreme environments, such as blazars or active galactic nuclei (AGN), where efficient particle acceleration and low target density constraints can be satisfied.

5. Candidate Counterparts, Multi-Wavelength, and Cascade Constraints

Blazar counterparts are natural extragalactic candidates and have been systematically searched within the $\sim 3^\circ$ error region of the KM3-230213A direction (Collaboration et al., 12 Feb 2025). Seventeen candidate blazars (identified using X-ray, radio, infrared, VLBI, 5BZCAT, and Fermi-LAT catalogs) were examined. Notable activity includes:

• Source #8 exhibited a major radio flare peaking $\sim 5$ days after the neutrino event (mean field chance coincidence $\sim 0.26\%$).
• Source #1 showed a rising X-ray trend, and Source #6 an extended gamma-ray flare.

While these associations are suggestive, the statistical weight is modest and not sufficient to claim an unambiguous counterpart identification. The evidence supports the broader blazar–UHE neutrino paradigm but does not isolate a unique source (Collaboration et al., 12 Feb 2025).

Hadronic models predict UHE gamma-ray production, followed by electromagnetic cascades on extragalactic background light (EBL). Cascade gamma rays should be detectable in the GeV–TeV range by air Cherenkov and air shower observatories for IGMF $B\lesssim 3\times 10^{-13}$ G (Fang et al., 13 Feb 2025). Non-detections in Fermi-LAT scans in both persistent and transient windows strongly constrain either the IGMF strength, the source distance, or require the gamma rays produced in the source to be fully absorbed internally and cascaded to lower energies (Crnogorčević et al., 20 Mar 2025). In such a scenario, gamma-ray-dark, radio-loud sources are favored.

6. Transient Source and Spectrum Hardness Hypotheses

Several studies examine the hypothesis that the event resulted from a short ($T\lesssim2$ yr), powerful flare—either from a blazar, AGN, or other explosive astrophysical transient (Neronov et al., 18 Feb 2025, Das et al., 15 Apr 2025). In this scenario:

• The required muon neutrino flux is $F \simeq 3\times 10^{-10}(1\,\rm{yr}/T)$ erg cm$^{-2}$ s$^{-1}$ over the flare duration.
• Spectrum constraints demand an extremely hard injection ($\Gamma\lesssim 1$), favoring nearly monoenergetic UHECRs or neutrino production via photohadronic interactions with mid-infrared photons.
• The all-sky rate of such flares is restricted to $R\lesssim 0.4\,\rm{yr}^{-1}$ by IceCube non-detection statistics, consistent with rare, energetically extreme sources.

The joint analysis including cosmic-ray and gamma-ray backgrounds constrains transient and diffuse production scenarios further, generally favoring a model involving either rare UHE cosmic-ray sources with hard spectra or rare, intense transient phenomena that avoid overproducing the diffuse gamma-ray background (Das et al., 15 Apr 2025, Cermenati et al., 16 Jul 2025).

7. Implications, Future Prospects, and Theoretical Consequences

The detection of KM3-230213A provides a reference point for UHE neutrino searches, flux modeling, and multi-messenger modeling. Critical implications are:

• The energetic scale reaches the regime expected for cosmogenic neutrinos, the so-called "GZK" process.
• There is a current $\sim2.5$–$3\sigma$ tension between the detection and steady, isotropic models given global null results at similar energies. This tension cannot be fully alleviated by simple broken-power-law extensions to standard spectra.
• The extragalactic origin is strongly preferred over Galactic scenarios, both theoretically and from the absence of identifying evidence for local UHE accelerators or sufficient diffuse Galactic emission.
• Interpretation as a transient or rare flare is consistent with the data and circumnavigates the constraints imposed by null multi-year exposures. The required energetics are extreme but within reach for rare AGN events or powerful blazar flares.
• Ongoing and next-generation detectors (full KM3NeT, extended IceCube-Gen2, radio UHE neutrino arrays) are vital for increasing the UHE event sample and testing the spectrum, isotropy, and transient hypotheses.
• Systematic multi-wavelength and wide-field gamma-ray searches remain key in either confirming or ruling out multi-messenger associations and in constraining the broader UHE astrophysical neutrino background.

In summary, KM3-230213A inaugurates a new sensitivity regime in UHE neutrino astrophysics and compels an overview of transient, multi-messenger, and extragalactic modeling to decode the physical processes and sources underlying the most extreme neutrino events observed to date.