The Single Source

The diffuse astrophysical neutrino flux — the cumulative signal from all neutrino sources in the universe — has a spectral break around 30 TeV. Below the break, one power law. Above it, another. The break is usually treated as a feature requiring multiple source populations or multiple production mechanisms.

arXiv:2603.15754 asks: can a single source class explain the entire spectrum, break included?

If photo-pion interactions are the dominant neutrino production process, the target photon temperature determines the neutrino energy peak. A multi-parameter fit favors target photon temperatures of 0.1 to 1 keV — consistent with X-ray environments like AGN cores. The spectral break at 30 TeV emerges naturally from the kinematics of photo-pion production on thermal targets at these temperatures.

Three mechanisms can limit the maximum neutrino energy: soft injection spectra that fall off before reaching high energies, a maximum proton energy in the PeV range, or magnetic field effects on the secondary mesons. The magnetic option requires fields of a few tens of kilogauss — again consistent with AGN core environments.

The paper predicts that future measurements can discriminate between these scenarios. The neutrino flavor composition and the neutrino-antineutrino ratio (accessible via the Glashow resonance at 6.3 PeV) differ between the three limiting mechanisms. The spectral break alone cannot distinguish them, but the flavors can.

The interpretive principle: before invoking multiple populations to explain a complex spectrum, check whether a single population with the right internal physics can produce the complexity alone. The break is not evidence of heterogeneity — it is the natural shape of photo-pion production on a thermal target.