Since high-energy astrophysical neutrinos were first observed in 2013, the IceCube Neutrino Observatory at the South Pole has continued searching for their elusive sources. So far, evidence of high-energy neutrino emission has been found from the blazar TXS 0506+056, the active galaxy NGC 1068, and most recently, the Milky Way. Still, neutrino emission from these sources alone do not account for the total astrophysical neutrino flux. As such, researchers are on the hunt for more cosmic sources of astrophysical neutrinos.
In a recent paper, the IceCube Collaboration uses a new strategy to look for point sources in the southern sky. For IceCube, located at the South Pole, the southern sky is the sky right above the detector. This is a challenging region because IceCube is constantly bombarded by atmospheric muons and neutrinos created in cosmic-ray interactions in Earth’s atmosphere, producing a significant background that obscures astrophysical neutrinos.
For this new analysis, IceCube uses a specific morphology of neutrino events—the light patterns created by neutrinos interacting in or around the IceCube detector—to isolate the astrophysical neutrino signal.
The IceCube Collaboration presents the first search for neutrino sources in the Galactic plane using a 10-year dataset consisting of starting track events. They did not find any statistically significant evidence of neutrino sources, but set constraining limits on the hadronic emission from TeV gamma-ray galactic plane objects and models of the diffuse galactic plane neutrino flux. Their results were included in a paper submitted to The Astrophysical Journal.
Starting track events, signals that start inside the detector instead of outside the detector, were selected to capture the neutrino events with interactions inside the detector. The enhanced starting track event selection (ESTES) focuses on neutrinos with energies of 1-500 TeV, which not only removes the atmospheric muon background but also the atmospheric neutrino background in the southern sky. This is due to the fact that the atmospheric neutrinos in the southern sky are likely accompanied by muons produced in the same air shower, at energies above 1 TeV, and therefore appear like incoming track events instead of starting track events due to the light deposited by the muons.
The rejection of atmospheric neutrinos yields a sample with a higher astrophysical neutrino purity than the other IceCube event selections, particularly for neutrinos with energies below 100 TeV in the southern sky. Additionally, the starting track selection results in a better energy estimate than other track samples due to the observation of particles produced from neutrino interactions at their point of production inside the detector.
For ESTES, researchers developed a tool that evaluates, on an event by event basis, the likelihood for a starting event versus an incoming event by reconstructing the neutrino interaction vertex, taking advantage of the enhanced pointing resolution from track events generated by muon neutrinos.
“Using the reconstructed vertex and the direction of the event, we can calculate the probability that the muon really started inside the detector and was thus created by a neutrino,” says Sarah Mancina, an IceCube collaborator and lead on the study while a PhD student at the University of Wisconsin–Madison (UW–Madison). Mancina is currently a postdoctoral researcher at the Università di Padova.
“This probability can take into account how far inside the detector the reconstructed vertex lies as well as where the event passed through the detector, which is important due to IceCube’s hexagonal shape leading to ‘alleyways’ where muons can sneak into the detector.”
Researchers performed four different searches to look for various sources of neutrinos: 1) an all-sky scan, 2) a search for neutrino emission from the locations of known gamma-ray bright sources, 3) catalog searches looking for correlations between neutrinos and four catalogs of galactic plane source classes, and 4) a search for neutrino emission across the galactic plane by testing models of the diffuse neutrino flux generated by cosmic rays interacting with the interstellar medium.
While the ESTES sample alone was not able to produce a statistically significant result, it consisted of mostly unique IceCube events that were previously unselected by other samples. For future studies, this sample can be combined with other IceCube event selections to produce a more complete sample that takes advantage of the pros and cons of the various selections within IceCube.
“Because the ESTES sample has a high astrophysical purity, it is an ideal candidate for producing new high-energy neutrino alerts for the multimessenger community,” says Mancina. “By adding ESTES to the neutrino alert streams, IceCube would be able to send neutrino alert events for neutrinos with energies in the single to tens of TeV, when previously we were limited to events with energies around or above 100 TeV.”
“The classic idea of neutrino telescopes was to look for upgoing muons, because only neutrinos could travel through Earth and create such muons. The neutrino self-veto applied in this analysis allows us to reject a trillion downgoing muons and extract an almost pure cosmic neutrino sample in a large part of the southern sky. It is like a new telescope was switched on in IceCube to look in the opposite direction: south”, says UW–Madison professor Albrecht Karle.
+ info “Time-Integrated Southern-Sky Neutrino Source Searches with 10 Years of IceCube Starting-Track Events at Energies Down to 1 TeV,” IceCube Collaboration: R. Abbasi et al. Submitted to The Astrophysical Journal. arxiv.org/abs/2501.16440