Gamma Rays and Ghost Particles: Investigating IceCube Alerts with HAWC

In this episode, we dive into the cutting-edge realm of multi-messenger astronomy to explore how scientists are attempting to link high-energy neutrinos with gamma-ray emissions to uncover the origins of ultra-high-energy cosmic rays. We discuss a recent study by the HAWC collaboration, which cross-referenced 368 public astrophysical neutrino alerts from the IceCube observatory with archival gamma-ray data from the HAWC observatory in Mexico.

We break down the unique capabilities of both observatories and how researchers utilized a Bayesian Block algorithm to search for spatial and temporal coincidences (flares) between the two datasets. Tune in to hear why the active galactic nuclei (AGN) Markarian 421 and Markarian 501 appeared as matches in the data, and learn why researchers ultimately suspect these exciting detections are likely false positives. We'll explain the hadronic physics behind neutrino production (like pion decay), how the data disfavors these simple models, and what this means for the future of detecting multi-messenger transient events.

Key Takeaways:

* The Multi-Messenger Approach: How observing both TeV gamma-rays and neutrinos can confirm if a source is accelerating cosmic rays through hadronic interactions.

* The Observatories: A look at IceCube, a cubic-kilometer neutrino detector buried in Antarctic ice, and HAWC, a high-altitude water Cherenkov gamma-ray detector in Mexico.

* The Findings: The study found a roughly 5% coincident detection rate between the 368 IceCube alerts and HAWC data, which matches the expected background false-positive rate.

* The Markarian Mystery: While AGNs Markarian 421 and 501 were found within the containment radii of two neutrino alerts, poor spectral fit constraints and the low astrophysical probability of the alerts suggest they are false positives rather than confirmed neutrino sources.

Reference:

Alfaro, R., et al. (The HAWC collaboration). "Investigating IceCube Neutrino Alerts with the HAWC $\gamma$-Ray$ Observatory." Draft version February 20, 2026. *arXiv:2602.16818v1*.

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: J. Goodman, HAWC Collaboration