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The Galactic Neutrino Sky: Predictions from Gamma-ray Source Populations

This paper demonstrates that the observed high-energy neutrino excess from the Galactic plane, particularly toward the inner Galaxy, can be naturally explained by the spatial distribution of hadronic gamma-ray sources without requiring additional renormalization of emission models.

Original authors: Leo Seen, Ke Fang

Published 2026-08-11

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CC BY 4.0

Original authors: Leo Seen, Ke Fang

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

Imagine the universe as a giant, noisy cosmic party. For decades, scientists have been trying to figure out where the loudest music is coming from. They've been listening for "neutrinos," which are like invisible, ghostly particles that zip through everything without bumping into it. Because they are so hard to catch, finding them is like trying to spot a specific snowflake in a blizzard. But recently, a massive detector called IceCube, buried deep in the ice of Antarctica, finally heard a distinct hum coming from the center of our own galaxy, the Milky Way.

The big mystery is: What is making this noise? Is it a diffuse fog of particles drifting everywhere, or is it a crowd of specific, bright "speakers" (individual cosmic sources) shouting from the center? To solve this, scientists often look at gamma rays, which are high-energy light particles. Think of gamma rays and neutrinos as siblings; they are often born from the same chaotic events involving cosmic rays crashing into gas. If you see a bright gamma-ray source, there's a good chance a neutrino is hiding nearby, too. The question is whether the "fog" or the "speakers" are responsible for the specific pattern of neutrinos IceCube is seeing.

This paper, titled "The Galactic Neutrino Sky," takes a fresh look at that pattern. The authors, Leo W. Seen and Ke Fang, decided to stop guessing and start building a map. They gathered a massive list of known gamma-ray sources in our galaxy—like a "who's who" of cosmic fireworks ranging from low-energy to incredibly high-energy bursts. They filtered out the "noise" (like pulsars, which are spinning neutron stars that usually play a different tune) to focus on the candidates most likely to be the neutrino generators. They created two different versions of this map, which they call "ReGal-γ" and a "CTA template," to see if the pattern of these known sources could explain the neutrino signal.

Here is what they found: The old idea that the neutrino signal is just a smooth, even fog of particles doesn't quite fit the data. The fog is there, but it's too spread out. However, when the authors added the "speakers"—the specific, resolved gamma-ray sources they found in their catalogs—the picture changed. The combined model, which mixes the diffuse fog with these specific sources, perfectly matches the neutrino signal IceCube detected.

Crucially, the paper suggests that while the "fog" (diffuse emission) might be the biggest contributor to the total neutrino count across the whole sky, the intense "peak" of neutrinos coming from the inner Galaxy is likely driven by these specific sources. It's as if the fog provides the background chatter of the party, but the loud, concentrated music in the center is coming from a specific group of bands. The authors tested this using two different lists of sources and found the result holds up: the spatial distribution of these gamma-ray sources naturally explains why the neutrino signal is so strong in the center of the galaxy.

They also checked their work against other models and found that they didn't need to "fake" the numbers (renormalize) to make the math work; the real, observed sources were enough to explain the signal. While they can't say for sure exactly which specific stars or gas clouds are the culprits yet, their study strongly suggests that the "inner Galaxy excess" is a signpost pointing to a population of hadronic emitters (sources powered by particle collisions) rather than just a general glow. The paper concludes that as we get more data from IceCube and other detectors like KM3NeT, we will be able to map these sources even more precisely, turning the blurry "fog" into a clear constellation of cosmic accelerators.

Technical Summary: The Galactic Neutrino Sky: Predictions from Gamma-ray Source Populations

Problem Statement
High-energy neutrino emission from the Galactic plane (GP) has been detected by IceCube with a significance of 5.7 $\sigma$ , exhibiting a prominent excess toward the inner Galaxy. While the total flux is broadly consistent with multi-messenger predictions derived from Galactic diffuse $\gamma$ -ray emission (GDGE), the spatial morphology of the signal suggests a potential contribution from discrete sources. Previous analyses primarily utilized spatial templates designed to trace diffuse neutrino emission (GDNE), such as the Fermi- $\pi^0$ and KRA $\gamma$ models. However, these diffuse-only models often require renormalization to fit the observed inner-Galaxy excess. The central problem addressed is whether the observed neutrino intensity profile, particularly the peak toward the inner Galaxy, can be explained by the spatial distribution of resolved and unresolved hadronic $\gamma$ -ray sources without ad-hoc renormalization of the diffuse component.

Methodology
The authors construct a comprehensive Galactic neutrino emission model by integrating three components: resolved $\gamma$ -ray sources, diffuse emission, and unresolved source populations.

Resolved Source Templates (ReGal- $\gamma$ and CTA):
- ReGal- $\gamma$ : A primary template constructed by combining four major $\gamma$ -ray catalogs spanning GeV to PeV energies: Fermi-LAT 4FGL-DR4, H.E.S.S. Galactic Plane Survey (HGPS), LHAASO 1LHAASO, and HAWC 4HWC. The authors filter out extragalactic sources, pulsars (PSRs), pulsar wind nebulae (PWNe), and TeV halos, assuming their $\gamma$ -ray emission is predominantly leptonic.
- CTA Template: An independent template derived from catalogs used for Cherenkov Telescope Array Observatory (CTAO) and Southern Wide-field Gamma-ray Observatory (SWGO) simulations (gamma-cat, 3FHL, 2HWC, HGPS), also filtered to exclude PSRs, PWNe, and high-latitude sources.
- Conversion to Neutrinos: For each source, the $\gamma$ -ray flux is converted to a neutrino flux assuming a hadronic fraction $\chi$ (the fraction of $\gamma$ -rays originating from hadronic interactions). The neutrino spectrum is derived using the relation $E_\nu^2 dN_\nu/dE_\nu \approx \frac{\chi}{2} (E_\gamma^2 dN_\gamma/dE_\gamma)|_{E_\gamma=2E_\nu}$ . The parameter $\chi$ is treated as a free parameter, with $\chi=1$ used as a baseline for testing.
Diffuse and Unresolved Components:
- Diffuse Emission: The CRINGE model is adopted as the default GDNE template, derived from solving the Galactic cosmic-ray transport equation and fitted to local data.
- Unresolved Sources: Two models are tested to account for sources below current detection thresholds:
  - Model A: A synthetic population based on Abe et al. (2024), modeling SNRs, PWNe, and $\gamma$ -ray binaries, with PWNe excluded.
  - Model B: Based on Schwefer et al. (2023), rescaled to 50% of the total flux to approximate the removal of PWN-like contributions.
Comparison with Observations:
The predicted neutrino fluxes are convolved with IceCube's effective area ( $A_{eff}$ ) and livetime to generate expected event counts. The analysis compares the longitudinal and latitudinal profiles of the combined model (Resolved + Unresolved + Diffuse) against IceCube's 12-year cascade and track data, specifically focusing on the inner Galaxy ( $|l| \le 20^\circ, |b| \le 15^\circ$ ).

Key Contributions

ReGal- $\gamma$ Construction: The creation of a unified, multi-wavelength template of resolved Galactic $\gamma$ -ray sources (GeV–PeV) specifically tailored to identify candidate hadronic neutrino emitters.
Spatial Morphology Analysis: Demonstrating that the spatial distribution of neutrino sources, inferred from $\gamma$ -ray catalogs, provides a distinct morphological signature that differs from standard diffuse emission models.
Robustness Check: Validating the results using two independent source templates (ReGal- $\gamma$ and CTA) and two different unresolved source models to ensure conclusions are not artifacts of specific catalog choices.

Results

Spatial Concentration: The ReGal- $\gamma$ template predicts a neutrino intensity that is more strongly concentrated toward the inner Galaxy compared to diffuse-only models (CRINGE, Fermi- $\pi^0$ , KRA $\gamma$ ). This is because the density of resolved hadronic sources is higher in the inner Galaxy.
Spectral and Longitudinal Fit: When combining the unrenormalized CRINGE diffuse model with the resolved (ReGal- $\gamma$ or CTA) and unresolved source populations, the model successfully reproduces both the spectral energy distribution (SED) and the longitudinal count profile reported by IceCube.
No Renormalization Required: Unlike fits relying solely on diffuse templates, the combined model explains the observed inner-Galaxy excess without requiring additional renormalization of the emission models. The data is consistent with an average hadronic fraction of $\chi \sim 0.5–1$ .
Energy Dependence: At high energies (e.g., 100 TeV), the ReGal- $\gamma$ template reflects the scarcity of southern-sky sources due to the lack of air-shower observatories in the Southern Hemisphere, whereas the CTA template, relying on spectral extrapolations, carries larger uncertainties.

Significance
The paper argues that the pronounced peak in neutrino intensity toward the inner Galaxy is likely driven by a population of discrete hadronic sources rather than solely by diffuse cosmic-ray interactions. The findings suggest that the "excess" observed in previous diffuse-only analyses may be a morphological mismatch rather than a normalization issue. By establishing that resolved and unresolved $\gamma$ -ray sources can account for the observed neutrino signal, the study provides a self-consistent multi-messenger framework for the Galactic neutrino sky. The authors conclude that future measurements of energy-dependent longitudinal and latitudinal neutrino distributions will be critical for further constraining these models and determining the true spatial distribution of Galactic neutrino sources.

Technical Summary: The Galactic Neutrino Sky: Predictions from Gamma-ray Source Populations

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