Neutrino flare from supernova SN 2021foa: first robust signal of high-energy neutrinos from an interacting supernova ✦
Context
High-energy neutrino astronomy is experiencing rapid development since IceCube's discovery of the diffuse astrophysical neutrino flux. Yet, identifying individual sources remains a challenge: only a few objects, such as the blazar TXS 0506+056 and the active galaxy NGC 1068, have been associated with neutrino signals. Collapsing supernovae have long been considered promising sources: shock waves in dense circumstellar material (cosmic dust and gas) can accelerate protons, producing neutrinos. However, previous searches yielded no statistically compelling results. Type IIn supernovae, with their exceptionally dense circumstellar shells, are particularly interesting for such searches, and SN 2021foa—the nearest and brightest among them—became an ideal target. The idea that collapsing supernovae can generate neutrinos dates back to pioneering work by Fritz Zwicky and Subrahmanyan Chandrasekhar, who laid the foundations of modern stellar evolution theory.
Methods
The study used the second data release of IceCube muon tracks (DR2) spanning 11 years of the full 86-string configuration. An unbinned maximum likelihood method was applied, with a likelihood function incorporating spatial, energy, and temporal components. The spatial model is a Gaussian around the source, the energy model is a power-law spectrum with index γ, and the temporal model is a Gaussian flare centered at T0 with width σ. The analysis was performed using the SkyLLH package. To assess significance, 15,000 Monte Carlo simulations were conducted with random positions and times in the southern sky (declination < -10°), where the supernova is located.
Results
An excess of neutrino events was detected near the optical peak of SN 2021foa. Best-fit parameters: central flare time—approximately 19 days after the discovery of the supernova, coinciding with the brightness peak in the r-band; Gaussian flare width σ≈2.3 days; spectral index γ=4.9±1.6; number of signal events ns=3.86±2.0. The test statistic TS=24.7 (and up to 28.2 in a positional scan). Four neutrinos with energies of 15–30 TeV are clustered within 1° of the source. The probability of a chance clustering of this many events within 6 days, based on 100,000 simulations, is less than 10⁻⁵, and the global significance for TS=24.7 reached 4.0σ (p≈6.7×10⁻⁵). The TS map confirms compatibility with the optical position within a 90% confidence region. The estimated total (all-flavor) isotropic-equivalent neutrino energy for the flare is Eν,iso≈4.7×10⁵² erg (in the 20 TeV–1 PeV range), with large uncertainties due to imprecision in γ. Because the source is located in the southern sky, IceCube detects only the high-energy tail of the spectrum, so the total energy could be an order of magnitude higher, reaching ~5×10⁵³ erg. This value significantly exceeds the optical (~10⁵⁰ erg) and kinetic energy of the ejecta (~3×10⁵¹ erg), ruling out an explanation solely by shock interaction with the shell.
Implications
This result for the first time with high significance links a collapsing supernova with high-energy neutrino emission. The soft spectrum (γ~4.9) indicates strong cooling of accelerated particles in a dense environment, similar to NGC 1068, and possibly the operation of a "choked" jet from a central engine—a black hole or magnetar. This calls for a reassessment of the role of supernovae in forming the diffuse neutrino background and opens a new channel for studying processes in the cores of massive stars. Moreover, for the first time for a supernova, agreement is observed across three channels: neutrinos, photons, and spectral evolution (spectroscopy), marking a transition to full-fledged multi-messenger astrophysics.
Future development
Future research can proceed along several lines: detailed modeling of neutrino production in choked jets, accounting for the complex structure of the circumstellar medium, including layers of hydrogen and helium; searches for similar signals from other type IIn supernovae in IceCube data, as well as in upcoming neutrino telescopes KM3NeT and IceCube-Gen2; and combination with photometry data to refine the time delay. Of particular interest is comparison with non-thermal emission in other wavebands (gamma-ray, radio) to fully unravel the energetics and particle acceleration mechanisms.
Impact
The discovery will impact multi-messenger astronomy, cosmic ray physics, and the theory of massive star evolution. In particular, it provides an independent method for probing hidden central engines of supernovae and conditions in their circumstellar dust.
Next steps
It is necessary to perform a deeper analysis accounting for systematic uncertainties, as well as to search for neutrino signals from other nearby IIn supernovae, such as SN 2017hcd, and to test the choked jet hypothesis through numerical hydrodynamic simulations.
Key open problems
This work sheds light on two fundamental problems: the origin of high-energy cosmic neutrinos and the explosion mechanism of massive stars. The connection of neutrinos with type supernovae IIn bridges particle astrophysics and transient physics, and the choked jet model unifies the physics of black holes and neutron stars in the context of neutrino generation.
🎯 SN 2021foa is unique among supernovae: its spectrum switched multiple times between hydrogen-rich (IIn) and helium-rich (Ibn) types in just 50 days, earning it the nickname "flip-flop." This behavior points to an extreme mass-loss history of the progenitor, possibly a transition from a luminous blue variable to a Wolf-Rayet star.
Key numbers
- Distance to SN 2021foa: 34.9 Mpc
- Statistical significance: 4.0σ
- Isotropic-equivalent neutrino energy: ~4.7×10⁵² erg
- Number of registered neutrinos: 4
- Peak apparent magnitude: 15.4m