Supernova Neutrinos

A core-collapse supernova converts about $3\times 10^{53}$ erg of gravitational binding energy into neutrinos of all flavors over roughly ten seconds. This is approximately 99% of the total energy release, with kinetic energy of the ejecta at 1% and the electromagnetic signal at just 0.01%. Supernova neutrinos are therefore both the primary channel of energy transport and the earliest observable signal from the explosion.

Phases of emission

The neutrino luminosity evolves through distinct phases.

Neutronization burst (~10 ms). When the collapsing core density exceeds ~$10^{14}$ g/cm³, electron capture on protons converts protons to neutrons and releases a short, intense pulse of ${\nu }_e$.

Accretion phase (~100 ms). Matter accretes onto the proto-neutron star, heating it and producing thermal neutrinos. All flavors are present but ${\nu }_e$ and ${\bar{\nu }}_e$ dominate energetically.

Cooling phase (~10 s). The proto-neutron star cools by thermal neutrino emission. Typical energies are 10–15 MeV for ${\nu }_e$ and ${\bar{\nu }}_e$, slightly higher for ${\nu }_x$ ($={\nu }_{\mu },{\nu }_{\tau }$ and their antiparticles) which escape from deeper, hotter regions.

SN 1987A

On 23 February 1987, the supernova designated SN 1987A exploded in the Large Magellanic Cloud, 51 kpc away. Three neutrino detectors registered a burst:

• Kamiokande-II in Japan: 12 events over 13 seconds
• IMB in Ohio: 8 events
• Baksan in the Soviet Caucasus: 5 events

Twenty-five total events — tiny in absolute terms, but the first direct confirmation of the core-collapse neutrino mechanism. The energies and timing were consistent with the predicted thermal cooling spectrum. Masatoshi Koshiba shared the 2002 Nobel Prize for this observation.

From SN 1987A, bounds were placed on neutrino mass, lifetime, magnetic moment, and electric charge, as well as on exotic physics (axions, majorons, Lorentz violation). No more than four hours separated the neutrino signal from the optical flash — consistent with neutrinos being massless or very light, and with them moving essentially at the speed of light over 170,000 years.

Galactic rate

The Milky Way hosts roughly one to three core-collapse supernovae per century. The next galactic supernova — at an expected distance of 8 kpc — would produce roughly:

• Super-Kamiokande: ~7,000 events
• IceCube: ~500,000 detected photons from a correlated noise increase
• DUNE (40 kt LAr): ~3,000 events
• JUNO: ~5,000 events
• A future Hyper-Kamiokande: ~50,000 events

Multiple detectors observing the same event provide triangulation and spectral complementarity. The SuperNova Early Warning System (SNEWS) coordinates real-time alerts across the detector community.

Physics from a galactic burst

A well-measured supernova neutrino burst would provide information on:

• Mass ordering, through the ${\nu }_e$ and ${\bar{\nu }}_e$ spectra reshaped by MSW resonances in the stellar envelope
• The explosion mechanism, through the accretion-phase luminosity time profile
• Neutrino self-interaction effects, including collective flavor oscillations at high densities
• Neutron-star formation or black-hole formation, through the presence or abrupt truncation of the cooling tail
• Absolute neutrino mass, through arrival time spread as a function of energy (a mass of 1 eV would introduce a few-millisecond spread over the galactic distance)
• New physics: anomalous energy-loss channels, axion-like emission, neutrino decay

The scientific yield per event is enormous; the limiting factor is purely the supernova rate.

Diffuse supernova neutrino background (DSNB)

Integrated over cosmic history, all past core-collapse supernovae produce a steady, isotropic flux of neutrinos known as the diffuse supernova neutrino background. The predicted ${\bar{\nu }}_e$ flux is approximately 2–5 per cm² per second in the 10–30 MeV range — comparable to atmospheric neutrinos but cleanly separable through timing and angular distribution.

Super-Kamiokande has recently been loaded with gadolinium, enabling neutron-tagged IBD events and substantially improving sensitivity to the DSNB. A first detection is expected within this decade; it will anchor the cosmic supernova rate as a function of redshift and provide an independent test of the Standard Solar Model and stellar-evolution physics.

Burst time profile

Schematic luminosity of the three temporal phases of a core-collapse supernova burst.