Diffuse neutrino flux from failed supernovae

I study the diffuse flux of electron antineutrinos from stellar collapses with direct black hole formation (failed supernovae). This flux is more energetic than that from successful supernovae, and therefore it might contribute substantially to the total diffuse flux above realistic detection thresholds. The total flux might be considerably higher than previously thought, and approach the sensitivity of SuperKamiokande. For more conservative values of the parameters, the flux from failed supernovae dominates for antineutrino energies above 30-45 MeV, with potential to give an observable spectral distortion at Megaton detectors.

💡 Research Summary

This paper investigates the contribution of electron‑antineutrinos (ν̄_e) emitted by “failed supernovae,” i.e., massive stellar collapses that form a black hole directly without a successful explosion, to the diffuse supernova neutrino background (DSNB). The author begins by contrasting the conventional picture of core‑collapse supernovae, where the neutrino burst is relatively soft (average energies ≈10–15 MeV) and quickly fades after the shock revival, with the theoretical expectations for failed collapses. In a failed event the proto‑neutron star never launches a successful shock; instead, the core continues to contract until an event horizon forms. During this prolonged accretion phase the matter remains at extreme densities (∼10¹⁴ g cm⁻³) and temperatures (∼10 MeV), keeping neutrinos trapped and allowing a sustained, hotter neutrino emission. Consequently the emitted ν̄_e spectrum is significantly harder, with average energies in the 20–30 MeV range and a non‑negligible high‑energy tail extending beyond 40 MeV.

To quantify the diffuse flux, the author adopts a parametric model for a single failed supernova: core temperature T_c, radius R_c, total emitted neutrino number N_ν, and spectral shape (approximated by a pinched Fermi‑Dirac distribution). The model is calibrated against recent numerical simulations that follow the collapse up to black‑hole formation. The cosmological contribution is then obtained by integrating over redshift, using a star‑formation history ψ(z) and an initial‑mass function to estimate the rate of black‑hole‑forming collapses, R_f(z). The fraction of massive stars that end as failed supernovae, f_BH, is treated as a key uncertain parameter; plausible values range from 10 % to 30 % of all core‑collapse events.

Two representative scenarios are explored. In the “conservative” case (f_BH≈0.1, average ν̄_e energy ⟨E⟩≈20 MeV), the diffuse flux from failed supernovae overtakes that from successful explosions for neutrino energies above ≈30 MeV. In the “optimistic” case (f_BH≈0.3, ⟨E⟩≈30 MeV), the total ν̄_e flux approaches the current Super‑Kamiokande detection limit (∼5 cm⁻² s⁻¹ for E>10 MeV) and could already be marginally present in existing data. Importantly, the high‑energy portion (E ≳ 45 MeV) of the DSNB would be dominated by the failed‑supernova component, producing a characteristic spectral distortion that is absent in models that include only successful explosions.

The paper discusses the observational implications for present and future water‑Cherenkov detectors (Super‑Kamiokande, Hyper‑Kamiokande) and liquid‑argon experiments (DUNE). Because the inverse‑beta‑decay cross‑section rises sharply with energy, the hard ν̄_e tail yields a disproportionately large event rate in the 30–60 MeV window, where backgrounds from atmospheric neutrinos and reactor antineutrinos are relatively low. A megaton‑scale detector could therefore resolve the predicted bump and directly measure the fraction of failed supernovae, providing a novel probe of black‑hole formation channels in the universe.

The author also addresses several sources of uncertainty. The exact failed‑supernova rate depends on metallicity‑dependent stellar evolution and the high‑mass end of the initial‑mass function. Neutrino flavor conversion (including collective effects and matter‑enhanced MSW resonances) could reshape the observable ν̄_e spectrum, especially at high energies where self‑interaction potentials are large. Finally, rotation, magnetic fields, and possible fallback accretion after black‑hole formation might modify the emission characteristics, calling for more sophisticated multi‑dimensional simulations.

In summary, the study demonstrates that failed supernovae are likely to contribute a substantial, and potentially dominant, component to the high‑energy tail of the diffuse supernova neutrino background. This contribution brings the total DSNB flux close to current experimental sensitivities and predicts a measurable spectral feature that future megaton‑scale detectors should be able to detect, opening a new window on the demographics of stellar death and black‑hole birth in the cosmos.