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

A search for high-energy muon neutrino emission from Abell 119 was conducted using 10 years of IceCube muon track data to test whether this galaxy cluster could be a source of neutrinos that might explain recent GeV gamma-ray detections, which hinted at a hadronic origin. The study found no statistically significant excess, setting an upper limit on the differential neutrino energy flux from the cluster.

The Gist

The search for neutrino emission from Abell 119 using the IceCube 10-year muon track data yielded no statistically significant excess, resulting in a 95% confidence level upper limit on the differential muon neutrino energy flux at 100 TeV of approximately 2.42 × 10−10 GeV cm−2 s−1 sr−1.

Motivation and Context

The research was motivated by a recent detection of GeV gamma rays from Abell 119 using the Fermi-LAT telescope, which suggested a hadronic origin for the emission. Previous work found 4σ evidence for gamma-ray emission from Abell 119 in the energy range of 100 MeV to 1 TeV using Fermi-LAT data. These neutral pions are believed to be remnants of hadronic interactions of cosmic-ray protons in the intra-cluster medium (ICM). The expected neutrino energy flux based on this hadronic origin was estimated to be E2ϕν ≈ 3 × 10−10 GeV cm−2 s−1 sr−1.

Data and Methodology

The analysis utilized the publicly available 10-year muon track dataset provided by the IceCube Collaboration, which contains a total of 1,134,431 events recorded between April 2008 (IC-40 configuration) and July 2018 (IC86-VII). A modified version of this dataset was employed where duplicated events present in the original IceCube release have been removed. The analysis employed the unbinned maximum likelihood emission method, following a framework originally proposed in [44].

The selection criteria involved selecting neutrino events with declinations lying within ±5◦ of the galaxy cluster. The signal PDF was modeled as a two-dimensional Gaussian:

Si = 1 / (2πσ2i) exp − θi − θs2 / 2σ2i, where θi−θs represents the angular separation between the reconstructed neutrino direction and the source position, and σi is the angular uncertainty associated with the event. The background PDF was assumed to be uniformly distributed in right ascension within the selected declination band.

Results and Limits

The best-fit test statistic value obtained for Abell 119 was T S = 0.034, which is consistent with a background fluctuation. Consequently, the study concluded that there is no detection of neutrinos from this cluster. The upper limit on the number of signal events at 95% confidence level was determined to be n95s = 14.38.

Translating this into a flux limit, assuming a power-law form for the neutrino spectrum, the 95% confidence level upper limit on the differential (muon) neutrino energy flux was found to be 2.42 × 10−10 GeV cm−2 s−1 sr−1 at E = 100 TeV. This limit is about 1.2 times lower than the predicted neutrino flux required to explain the hadronic origin of the galaxy cluster emission.

Future Prospects

The researchers calculated the projected sensitivities of future neutrino telescopes, including IceCubeGen2, P-ONE, TRIDENT, and HUNT, using a lifetime of 10 years. The results suggest that future detectors should soon be able to rule out the hadronic mechanism for gamma-ray emission. This demonstrates the synergy of neutrino astrophysics with gamma-ray astrophysics in constraining models of hadronic gamma-ray emission within clusters. The current limit is in slight tension with the prediction in [21], viz., 3 × 10−10 GeV cm−2 s−1 sr−1.

How it works

The analysis employs the unbinned maximum likelihood ratio method, which is based on a framework originally proposed in [44]. The signal PDF is modeled as a two-dimensional Gaussian: Si = 1 / (2πσ2i) exp − θi − θs2 / 2σ2i, where θi−θs represents the angular separation between the reconstructed neutrino direction and the source position, and σi is the angular uncertainty associated with the event. The likelihood function for the full dataset is then constructed as L(ns) = Y N i=1 Pi, where Si and Bi denote the signal and background PDFs, respectively.

Here are the specific improvements for an AI system, derived from the findings and methodology presented in this scientific paper:

1. Development of a specialized neutrino flux simulation module: The paper details how to calculate expected neutrino event counts based on a power-law spectrum, effective area, and integration over energy bins (Eqs. 6-8).

2. Implementation of the Unbinned Maximum Likelihood Emission Search Algorithm: The core analysis relies on maximizing a likelihood function (Eq. 3) to determine if observed events are consistent with a signal versus background.

3. Integration of Astrophysical Constraints for Source Modeling: The system should incorporate external data constraints, such as the predicted neutrino flux from hadronic interactions in Abell 119 (the reference value of 3 × 10−10 GeV cm−2 s−1 sr−1), to provide a benchmark against which observed limits are compared.

4. Automated Upper Limit Calculation Framework: The system must be capable of calculating confidence-level upper limits (e.g., the 95% c.l. limit of 2.42 × 10−10 GeV cm−2 s−1 sr−1 at 100 TeV) by mapping test statistic thresholds (TS > 25 for a potential detection) to required signal event counts, then inverting the flux equation (Eq. 8).

5. Projection of Future Detector Sensitivity: The system can use the derived limits and re-scale them based on the projected effective exposure of next-generation detectors (IceCubeGen2, P-ONE, TRIDENT, HUNT) to predict future observational capabilities for ruling out specific astrophysical origins (like hadronic emission).

The improved AI system can perform the following actions:

1. Identify and search for potential high-energy neutrino sources within large astronomical structures (like galaxy clusters), specifically focusing on environments hinted at by co-detected electromagnetic signatures (e.g., GeV gamma rays).

2. Quantify whether observed neutrino event distributions from data archives are statistically consistent with a known background (null hypothesis) or indicate a potential new physical process (signal).

3. Determine stringent, quantifiable upper limits on the differential neutrino energy flux from specific astrophysical objects at high energies (e.g., 100 TeV), providing crucial constraints for particle acceleration models in those environments.

4. Test theoretical hadronic models of gamma-ray emission against observational data by calculating the expected neutrino flux predicted by those models and comparing it to experimental upper limits, thereby definitively ruling out or marginalizing specific physical scenarios (as demonstrated in the paper's conclusion).

We carry out a search for high energy muon neutrino emission from the galaxy cluster Abell 119, motivated by a recent tentative detection of GeV gamma-ray emission from this cluster using the Fermi-LAT telescope, which hinted at a hadronic origin. For this purpose, we used the 10-year muon track data from 2008-2018, provided by the IceCube Collaboration and implement the unbinned maximum likelihood method. We do not find any statistically significant excess and the observed value of the test statistic is consistent with a null result. We then obtain upper limits (at 90% confidence level) on the differential muon neutrino energy flux from this cluster, whose value is equal to 1.2 times 10-9 GeV cm-2 s-1 at a pivot energy of 100 TeV, after assuming a neutrino spectral index of 2.0. The neutrino flux expected under the hadronic interpretation is, however, approximately an order of magnitude smaller than our flux limit, and hence the hadronic model cannot be ruled out based on our upper limit. Among the next generation neutrino detectors, IceCube-Gen2 and TRIDENT neutrino detector should be able to confirm or rule out a hadronic origin for the gamma-ray emission in Abell 119 with 10 years of exposure.