The spectrum of cosmic neutrinos can unmask the types of astrophysical sources that produce these and other high-energy particles. The IceCube Neutrino Observatory, whose detectors lie buried in Antarctic ice, has been measuring cosmic neutrinos since 2010. Early data releases suggested that the neutrino spectrum is a single falling power law, which is consistent with simple models relating cosmic neutrinos to cosmic rays. But now, after 14 years of observation, IceCube’s data show evidence for a break, or knee-like downward bend, in the spectrum at an energy of around 30 tera-electron-volts [1, 2]. Such a break could evince a mix of neutrino sources.

Cosmic neutrinos are predominantly generated whenever high-energy cosmic rays collide with other particles. The neutrino spectrum can therefore reveal information about how and where cosmic rays are accelerated. If the acceleration takes place exclusively in shock environments, where collisions produce neutrinos, the neutrino spectrum would be a single power law. However, the latest analysis of neutrino data by the IceCube Collaboration has uncovered a more complex spectrum. The researchers sifted through a decade’s worth of neutrino observations using improved models of both backgrounds and detector uncertainties. The results show a spectrum break with a statistical confidence of 4 sigma (where 5 sigma constitutes a bona fide detection).

The break could mean that neutrinos come from more than one source class, with each class having a different way of accelerating cosmic rays, says collaboration member Vedant Basu from the University of Utah. He also points out that the observed shape of the neutrino spectrum is consistent with predictions based on the properties of the diffuse gamma-ray background, supporting models that assume the two types of particles originate from the same sources.