Sensitivity of JEM-EUSO to GRB Neutrinos

JEM-EUSO is a mission to study ultra-high-energy cosmic rays (UHECRs) by measuring the fluorescence light from giant air showers at the altitude of the International Space Station. In the tilted mode, JEM-EUSO will become very sensitive to the \v{C}erenkov light from the earth skimming tau neutrinos at the energy range of $10^{16-18}$eV. In this paper we will discuss high-energy tau neutrinos from nearby gamma-ray bursts (GRBs). From simulations of cascade in GRB photon fields including various hadronic/leptonic processes, we estimate the neutrino flux from GRBs. Our results show that both muons and pions are dominant sources of neutrinos at the energy range of $10^{16-18}$ eV. We discuss the possibility of detecting the \v{C}erenkov light of upward going showers from Earth skimming tau neutrinos coming from some closest GRBs.

💡 Research Summary

The paper evaluates the capability of the JEM‑EUSO (Extreme Universe Space Observatory) mission to detect ultra‑high‑energy (UHE) tau neutrinos originating from nearby gamma‑ray bursts (GRBs). JEM‑EUSO will be mounted on the Japanese Experiment Module of the International Space Station at an altitude of about 400 km. In its standard nadir‑looking mode it observes fluorescence light from extensive air showers (EAS) produced by cosmic rays with energies above ~10¹⁹ eV. When the telescope is tilted, the field of view (±30°) covers a ground area of roughly 250 km radius, and the effective target mass for upward‑going neutrino‑induced showers increases from ~10¹² tons to >10¹³ tons. This enlarged volume, together with the longer path length of tau leptons emerging from the Earth’s crust, enhances the sensitivity to Earth‑skimming tau neutrinos in the 10¹⁶–10¹⁸ eV range.

To predict the neutrino flux, the authors employ a Monte‑Carlo cascade code (Asano et al. 2009) that simulates particle acceleration and interactions inside the internal shock region of a GRB. Both electrons and protons are injected with power‑law spectra (∝ E⁻ᵖ exp(−E/E_max)), with a proton index p_p = 2 (required for the GRB‑UHECR scenario) and electron index p_e = 2. The code includes synchrotron radiation, inverse‑Compton scattering (including Klein‑Nishina effects), γγ pair production, photomeson production (π, K), photopair production, and the decays of pions, muons, and kaons. The authors explore two representative energy partition cases: (i) equipartition between protons and electrons (ε_p/ε_e = 1) and (ii) a proton‑dominated scenario (ε_p/ε_e = 30), motivated by recent estimates that GRBs may need 10–100 times more energy in protons than in electrons to account for the observed UHECR flux.

The simulated photon spectra reproduce typical GRB features: a synchrotron peak around a few hundred keV, a self‑absorption break near 100 eV, and a high‑energy cutoff due to γγ opacity. In the proton‑dominated case the cascade boosts the gamma‑ray output by a factor of ~5 and produces a hard component peaking near 100 MeV, similar to some observed GRBs with high‑energy excesses.

Neutrino spectra derived from the same simulations show that, in the 10¹⁶–10¹⁸ eV band, muon‑neutrinos from charged‑pion decay (π± → μ → ν) dominate, while muon decay (μ → ν) contributes mainly below 10¹⁷ eV. At energies above 10¹⁹ eV, kaon decay (K → ν) becomes important because kaons suffer less cooling before decay. The overall neutrino flux scales roughly with the proton energy budget; the proton‑dominated model yields a neutrino fluence several times larger than the equipartition case.

For detection sensitivity, the authors adopt a neutrino‑nucleon cross‑section σ_νN ≈ 2 × 10⁻³³ (ε_ν/10¹⁶ eV)^0.363 cm², a tau decay length L_τ ≈ 5 × 10⁴ (E_τ/10¹⁶ eV) m, and a Cherenkov cone half‑angle of 1.4°. Using these parameters they compute the minimum detectable neutrino energy as a function of the nadir angle (the angle between the incoming neutrino direction and the vertical). In the nadir direction (<30°) Earth opacity suppresses the flux, making detection unlikely. As the nadir angle increases, the effective target volume grows because tau leptons can travel longer distances in the crust before emerging, and the larger Cherenkov footprint increases the observable area. Consequently, the sensitivity improves up to nadir angles of ~65°–70°, where the threshold energy rises but the volume gain outweighs the loss. The authors present preliminary sensitivity curves (Fig. 2) and note that detailed Monte‑Carlo air‑shower simulations are still in preparation.

Considering the local GRB rate (0.2–1 Gpc⁻³ yr⁻¹) and assuming a tilt angle of 35°, the solid angle of the field of view that includes nadir angles of 65°–70° is roughly 0.1 rad. This translates into a few‑percent chance per year of observing a nearby (≤ 1 Gpc) bright GRB with sufficient neutrino fluence. In the optimistic proton‑dominated scenario, even GRBs at redshift z ≈ 1 (typical of the observed population) could be detectable if the telescope is tilted to larger angles, albeit with reduced probability.

The paper concludes that JEM‑EUSO, by exploiting its tilted mode and the large atmospheric target mass, offers a unique opportunity to detect UHE tau neutrinos from GRBs. While the expected detection rate is modest, a single observation would provide decisive evidence for proton acceleration in GRBs and support the hypothesis that GRBs contribute to the observed ultra‑high‑energy cosmic‑ray flux. Further work, including full air‑shower Monte‑Carlo studies and refined background estimates, is needed to solidify the projected sensitivities.