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Limit on high energy neutrino emission from Abell 119 using IceCube 10-year muon track data

Using 10 years of IceCube muon track data, researchers found no significant high-energy neutrino emission from galaxy cluster Abell 119, setting an upper limit that marginally rules out a hadronic origin for its observed GeV gamma-ray emission.

Original authors: Sri Devaki Meduri, Shantanu Desai

Published 2026-05-13

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CC BY 4.0

Original authors: Sri Devaki Meduri, Shantanu Desai

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Hunting for Cosmic Ghosts

Imagine the universe is a giant, dark ocean. Most of the time, it's quiet. But occasionally, massive explosions happen—like stars dying or black holes colliding—that send out invisible "ghosts" called neutrinos. These ghosts are so tiny and ghostly that they can pass through entire planets without hitting anything.

Scientists have been trying to catch these ghosts for years. They know that when high-energy particles crash into each other (a process called "hadronic interaction"), they should produce both gamma rays (a type of light) and neutrinos. It's like a fireworks display: if you see the light, you should also hear the boom.

The Mystery of Abell 119

Recently, astronomers looked at a massive group of galaxies called Abell 119. Using a telescope called Fermi-LAT, they saw a bright flash of gamma rays coming from there.

The Theory: Some scientists guessed that this flash was caused by a "hadronic" process. In simple terms, they thought protons (particles) were crashing into each other inside the galaxy cluster, creating the light.
The Prediction: If this theory is true, there must be a matching stream of neutrinos coming from the same spot. The scientists calculated exactly how many neutrinos they expected to find.

The Detective Work: IceCube's 10-Year Search

To solve the mystery, the authors of this paper acted like detectives. They used data from IceCube, a giant neutrino detector buried deep in the ice at the South Pole.

The Data: They looked at 10 years of records (from 2008 to 2018), checking over 1.1 million events (potential ghost sightings).
The Method: They focused specifically on the area of the sky where Abell 119 is located. They used a statistical tool (a "likelihood" test) to ask: "Is there a cluster of neutrinos here that looks like a signal, or is it just random noise?"

Think of it like listening to a crowded party. You are trying to hear one specific person's voice (the signal) over the background chatter (noise). You listen for 10 years, hoping to hear that voice clearly.

The Results: Silence in the Dark

After crunching the numbers, the result was disappointing for the "hadronic" theory:

No Excess: They found zero statistically significant signal. The number of neutrinos they saw was exactly what you would expect from random background noise.
The Verdict: There is no "boom" to match the "fireworks."

Setting the Limit: How Loud Can the Ghost Be?

Even though they didn't find the neutrinos, they didn't come away empty-handed. They calculated a limit.

The Analogy: Imagine you are trying to hear a whisper in a noisy room. You don't hear it, so you can't say exactly how loud the whisper is. But, you can say, "If that whisper were any louder than 50 decibels, I definitely would have heard it."
The Paper's Finding: The authors calculated that if the neutrino stream were as strong as the "hadronic" theory predicted, IceCube would have seen it. Since they didn't see it, the actual neutrino stream must be weaker than the prediction.
The Margin: Their limit is about 1.2 times lower than what the theory predicted. This means the theory is "marginally ruled out"—it's like the whisper being just a tiny bit too quiet to be the one they were looking for.

What's Next?

The paper concludes that while this specific theory for Abell 119 is looking shaky, we aren't done yet.

Future Detectors: New, bigger detectors (like IceCube-Gen2, P-ONE, and others) are being built. These will be like upgrading from a cheap microphone to a high-end recording studio.
The Promise: These future tools will be sensitive enough to definitively say whether the "hadronic" theory is completely wrong or if the neutrinos are just hiding very well.

Summary

The Clue: We saw gamma rays from galaxy cluster Abell 119.
The Guess: This light came from particle crashes, which should also make neutrinos.
The Search: We listened for 10 years with IceCube.
The Result: We heard nothing. The neutrinos weren't there (or were too weak).
The Conclusion: The idea that the gamma rays came from particle crashes is likely incorrect, but we need even better microphones (future detectors) to be 100% sure.

Technical Summary: Limit on High Energy Neutrino Emission from Abell 119 using IceCube 10-year Muon Track Data

Problem and Motivation
The origin of the diffuse astrophysical neutrino flux detected by IceCube in the TeV-PeV range remains largely unidentified, with only a few point sources (TXS 0506+056 and NGC 1068) showing statistically significant excesses. Recently, Harale and Paul [21] reported a $4\sigma$ detection of GeV gamma-ray emission from the galaxy cluster Abell 119 using 14 years of Fermi-LAT data. This emission was interpreted as consistent with $\pi^0$ decay within the intra-cluster medium (ICM), suggesting a hadronic origin driven by cosmic-ray proton interactions. If this hadronic interpretation is correct, the cluster should simultaneously emit high-energy neutrinos with a predicted energy flux of $E^2\phi_\nu \approx 3 \times 10^{-10} \text{ GeV cm}^{-2} \text{ s}^{-1} \text{ sr}^{-1}$ . While the IceCube Collaboration has previously conducted stacked analyses of galaxy clusters, no specific search for neutrino emission from Abell 119 had been performed prior to this work.

Methodology
The authors conducted a time-integrated search for high-energy muon neutrinos from Abell 119 using the publicly available 10-year muon track dataset from the IceCube Collaboration (April 2008 – July 2018). The dataset, comprising 1,134,431 events, was processed using a modified version that removes duplicated events found in the original release.

The analysis employed an unbinned maximum likelihood ratio method. The selection criteria included neutrino events with declinations within $\pm 5^\circ$ of the cluster's coordinates (RA=14.0357 $^\circ$ , DEC=−1.20021 $^\circ$ ).

Signal Model: The signal probability density function (PDF) was modeled as a two-dimensional Gaussian centered on the source position, weighted by the angular uncertainty of each event.
Background Model: Background events were assumed to be uniformly distributed in right ascension within the selected declination band.
Test Statistic (TS): The TS was defined as $2 \log [L(\hat{n}_s) / L(0)]$ , where $\hat{n}_s$ is the number of signal events maximizing the likelihood. Under the null hypothesis, the TS follows a half- $\chi^2$ distribution with one degree of freedom.
Flux Limit Calculation: Upper limits were derived assuming a power-law neutrino spectrum $\phi(E) = \phi_0 (E/E_0)^{-\Gamma}$ with a spectral index $\Gamma = 2.2$ , matching the inferred gamma-ray spectral index. The pivot energy was set to $E_0 = 10^5$ GeV. The effective area ( $A_{eff}$ ) included contributions from charged and neutral current interactions.

Key Results

Null Detection: The analysis yielded a best-fit test statistic of $TS = 0.034$, corresponding to $\hat{n}_s = 1.14$ signal events. This value is consistent with a background fluctuation, indicating no statistically significant excess of neutrinos from Abell 119.
Upper Limits: At a 95% confidence level, the upper limit on the number of signal events was determined to be $n_s^{95} = 14.38$ .
Flux Constraint: Translating this into a differential muon neutrino energy flux, the authors obtained an upper limit of $E^2\Phi_\nu = 2.42 \times 10^{-10} \text{ GeV cm}^{-2} \text{ s}^{-1} \text{ sr}^{-1}$ at 100 TeV.
Comparison with Theory: This observed limit is approximately 1.2 times lower than the predicted flux ( $3 \times 10^{-10} \text{ GeV cm}^{-2} \text{ s}^{-1} \text{ sr}^{-1}$ ) required to explain the gamma-ray emission via a hadronic mechanism. Consequently, the hadronic origin scenario for the gamma-ray emission in Abell 119 is found to be "marginally in tension" with the current neutrino data.

Significance and Future Outlook
The paper concludes that while galaxy clusters as a class have already been constrained as dominant contributors to the diffuse IceCube background, individual cluster limits provide crucial constraints on specific hadronic gamma-ray emission models. The current result marginally challenges the hadronic interpretation of the Abell 119 gamma-ray signal.

The authors project that future neutrino detectors, specifically IceCube-Gen2, P-ONE, TRIDENT, and HUNT, will possess sufficient sensitivity to definitively rule out the hadronic mechanism for Abell 119's gamma-ray emission. Assuming a 10-year livetime, these next-generation instruments are expected to improve current limits by an order of magnitude, thereby providing a conclusive test of the hadronic origin hypothesis for this cluster.