A Global Hunt for the Sources of Cosmic Neutrinos

A Global Hunt for the Sources of Cosmic Neutrinos
Astronomy has firmly entered the era of "multi-messenger" astrophysics. To understand the most violent phenomena in our Universe, we can no longer rely on light alone; we must combine our view of the electromagnetic spectrum with gravitational waves and neutrinos.
I am excited to share the results of a massive, multi-year project I have had the privilege of leading on behalf of the H.E.S.S. collaboration. In a paper recently accepted by the Astrophysical Journal, over 800 researchers from the world’s leading gamma-ray and neutrino observatories have joined forces to hunt for the origins of cosmic rays.

The Smoking Gun High-energy neutrinos are ghost-like particles that travel unimpeded across the cosmos. They are unique signatures of hadronic interactions, making them the perfect indicators of where cosmic rays—high-energy protons and atomic nuclei—are accelerated. However, because they interact so weakly, pinpointing their exact source is incredibly difficult.
The solution lies in teamwork. Identifying a high-energy gamma-ray counterpart to a neutrino event would provide the "smoking gun" needed to identify these cosmic particle accelerators.

A Unique Collaboration This project represents a unique effort combining data from facilities that are often competitors. We brought together the four major Imaging Atmospheric Cherenkov Telescope (IACT) arrays--FACT, H.E.S.S., MAGIC, and VERITAS—along with the Fermi-LAT satellite and the IceCube Neutrino Observatory.

Skymap in equatorial coordinates showing IceCube alert positions in the period from 2017 September to 2021 January. Alerts followed up by IACTs are shown in color.

By analyzing follow-up observations of IceCube neutrino alerts between 2017 and 2021, we created a comprehensive "legacy dataset". This global network allowed us to cover the entire sky, with H.E.S.S. monitoring the Southern Hemisphere and our partners covering the North.
Key Results While we did not find significant gamma-ray associations for most neutrino events, this non-detection is scientifically valuable. It allowed us to derive combined upper limits on the gamma-ray flux that are far more constraining than any single instrument could achieve alone, helping to refine theoretical models of neutrino emission.

One highlight was the detection of Very High Energy (VHE) gamma-rays from the blazar 1ES 1312-423 by H.E.S.S., triggered by a cluster of high-energy neutrinos in March 2019. While this proves our ability to react rapidly, standard analyses suggest the gamma-ray activity might not be directly linked to the neutrinos in this specific case.

Looking Ahead This work demonstrates that the current generation of telescopes can operate effectively as a global reaction network. The lessons we have learned are vital for the future Cherenkov Telescope Array Observatory (CTAO) and the next generation of neutrino detectors like KM3NeT. The hunt continues!

Read the full paper: FACT, H.E.S.S., MAGIC, VERITAS, Fermi-LAT, and IceCube Collaborations, "Prompt Searches for Very-High-Energy Gamma-Ray Counterparts to IceCube Astrophysical Neutrino Alerts," J. Abhir et al 2026 ApJ 997 141.