Diffuse astrophysical neutrino sources

IceCube records high-energy neutrinos, particles arriving from across the sky, by detecting light from their interactions in Antarctic ice. Which kinds of cosmic objects supply this widespread signal? Events clustered near a galaxy’s direction can support an association, but a single arrival from that direction can also be background.[2]

High-energy neutrinos arrive from across the sky as a diffuse flux, a spread-out signal. Which source populations supply the full signal, and can their combined emission explain its energies and sky distribution?

In one IceCube analysis, a neutrino interaction makes a muon, a charged particle whose light traces a track through the ice. Its reconstructed direction and energy are compared with background expectations. An excess near NGC 1068 and ensemble evidence for X-ray-bright active galaxies establish parts of the picture. A separate search compares neutrino directions and times with stars torn apart by black holes, individually and in a stacked search that combines them; its nondetection constrains those candidate sources.[1] [2]

Known sources do not yet explain the entire astrophysical flux.[1] [2]

They do not uniquely fix each population’s total contribution or explain the full measured spectrum. A recent tidal-disruption-event search is submitted research, not a settled population census.[1] [2]

These particles reveal energetic activity hidden from ordinary light.[2]

Energetic protons colliding with matter or radiation can make neutrinos and gamma rays. In NGC 1068 the measured signals differ greatly, so gamma-ray brightness alone is a poor guide to its neutrino activity. Accounting for source populations would show how broadly this difference matters beyond identified emitters.[2]