The origins of extremely energetic particles, called cosmic rays, continue to puzzle astronomers. Some of the highest energy cosmic ray protons can reach one million billion electronvolts (PeV) in energy, but the sources of these protons, or PeVatrons, have been difficult to pin down.
Cosmic rays accelerated by PeVatrons produce pions when interacting with surrounding matter, which then decay into gamma rays and ghostlike particles called neutrinos, the subject of study for the IceCube Neutrino Observatory at the South Pole. TeV gamma ray sources are excellent candidates for galactic PeVatrons, but gamma rays could also originate from other processes. Thus, the simultaneous detection of neutrinos and gamma rays could confirm the presence of a galactic PeVatron.
In a multimessenger study submitted to The Astrophysical Journal, the IceCube Collaboration presents a joint analysis that combines data from IceCube and the High-Altitude Water Cherenkov (HAWC) Observatory located in Mexico that looks for TeV-energy-scale gamma rays. No significant neutrino emission was found from the direction of the gamma-ray sources.
Collaborators looked at 22 known gamma-ray sources from the third HAWC catalog and 14 years of IceCube data containing track-like events and jointly fitted the data to models for PeVatron candidates through a maximum likelihood analysis. A new likelihood analysis software called i3mla was developed for specific use in a common analysis framework called the multi-mission maximum likelihood framework (3ML), which is widely used by the gamma-ray astronomy community.
“The joint search using HAWC and IceCube data revealed that none of the candidates emit neutrinos that are strong enough to be detected by IceCube,” says Kwok Lung (Jason) Fan, a physics PhD student at the University of Maryland and study lead. “The hypothesis that all gamma-ray emission is created side by side with high-energy cosmic rays can be ruled out for several candidate sources.”
The study also showed that a few of the sources cannot be purely hadronic, suggesting other physical processes must also contribute to high-energy emission. The proposed IceCube-Gen2 detector is expected to be up to five times more sensitive to galactic neutrino sources compared to IceCube, which would improve understanding of the physical processes that make up TeV gamma-ray emitters.
“The new IceCube analysis software could enable future multimessenger analyses using IceCube data and data of different electromagnetic wavelengths on other potential source classes,” says Fan. “This could help us better understand the physics of these astrophysical objects.”
+ info “Search for joint multimessenger signals from potential Galactic PeVatrons with HAWC and IceCube,” IceCube Collaboration: R. Abbasi et al. Submitted to The Astrophysical Journal. arxiv.org/abs/2405.03817