Cosmogenic Origin of KM3-230213A: Delayed Gamma-Ray Emission from A Cosmic-Ray Transient

TL;DR

The paper investigates whether the KM3-230213A event can arise from cosmogenic neutrinos produced by UHECRs from a transient extragalactic source rather than at-source interactions. Using 3-D CRPropa3 simulations with a turbulent extragalactic magnetic field, it models proton injection up to $ ext{sim} imes 10$ EeV, propagation with photopion and Bethe–Heitler interactions, and the resulting electromagnetic cascades that yield delayed cosmogenic gamma rays arriving after $ ext{sim} imes 10^{4}$--$ ext{sim} imes 10^{5}$ years. The study shows gamma-ray spectra peaking near $ ext{sim} imes 10$ TeV for nearby sources ($z\,\lesssim\,0.2$) and explores how detectability with CTA, LHAASO, and SWGO depends on source distance and $B_{ m RMS}$, offering a novel way to constrain past transients and the EGMF. If no contemporary electromagnetic counterparts are observed, the results imply a lower limit on source distance (\gtrsim 500 Mpc) and highlight how future sub-EeV neutrino detections could help distinguish cosmogenic from astrophysical origins, opening a new window into past cosmological transients.

Abstract

The highest-energy cosmic neutrino detected by the ARCA detector of KM3NeT has reignited the quest to pinpoint the sources of ultrahigh-energy cosmic rays (UHECRs; $E\gtrsim 0.1$ EeV). By uncovering the associated multimessenger signals, we investigate the origin of the 220 PeV $ν_μ$ event KM3-230213A from an unknown transient that accelerated cosmic rays to $\sim 10$ EeV. Unlike an astrophysical origin, where the $ν_μ$ is produced inside the source, here we consider UHECR protons that escape the source interact with the cosmic background radiation, producing a PeV-EeV cosmogenic neutrino spectrum. The secondary $e^\pm$ and $γ$-rays initiate an electromagnetic cascade, resulting in a cosmogenic $γ$-ray spectrum. The latter peaks at a delayed time of $\gtrsim 10^4$ years compared to the light travel time from the transient to observer, due to deflection of charged particles in the extragalactic magnetic field (EGMF). Our results shed light on the nature of the UHECR source for the $ν_μ$ event and provide crucial insights into the detection of multi-TeV $γ$-rays of cosmogenic origin from similar past cosmological transients. Using the $γ$-ray sensitivity of currently operating and next-generation imaging atmospheric Cherenkov telescopes, the flux and time-delay distribution can constrain the source distance. We further show that the detection of such a $γ$-ray signal above the background depends on the EGMF strength. Together with the non-detection of coincident spatial or temporal photon counterparts at the current epoch, this detection is the first compelling candidate for a sub-EeV cosmogenic neutrino.

Figures (7)

• Figure 1: Schematic view showing source position and observer. We use an observer sphere to collect secondary events with a maximum deflection of $1^\circ$ in our 3-D simulations.
• Figure 2: Time delay of neutrinos for different source redshifts. The dashed lines show the arrival times of the fastest $\gamma$-rays from different redshifts, denoted by different colour codes.
• Figure 3: Left: Observed neutrino and photon fraction as a function of time delay with respect to the source distance, for a source at $d_c = 133$ Mpc ($z\approx0.01$). The statistics are binned in various energy ranges. For neutrinos, we show the observed fraction corresponding to the central 90% energy range of the KM3NeT neutrino event. The initial neutrino burst is not included in this case, since all events arrive at $\Delta t = 0$. Right: The corresponding $\gamma$-ray spectra for different time delay intervals are shown and compared with the flux sensitivities of $\gamma$-ray telescopes, indicated by dashed curves. The Fermi-LAT sensitivity bruel2018fermilatimprovedpass8event corresponds to all-sky performance of the instrument. The CTA North and South CTAConsortium:2017dvg assume $50$ hours of exposure, while those for SWGO Albert:2019afb and LHAASO LHAASO:2019qtb correspond to $5$ year and $1$ year of observation, respectively.
• Figure 4: Same as Fig. \ref{['fig:d133']} but for comoving source distance $d=435$ Mpc ($z\approx 0.1$) of the transient point source.
• Figure 5: Same as Fig. \ref{['fig:d133']} but for comoving source distance $d=1$ Gpc ($z\approx0.24$) of the transient point source.