High-energy neutrino emission from the Milky Way

The Milky Way is home to powerful astrophysical engines. These engines accelerate cosmic rays to energies far beyond terrestrial limits. For decades, scientists have asked: where are these engines located? How do these cosmic rays move through our Galaxy? We can see the electromagnetic glow of our Galaxy. However, neutrinos offer a cleaner view. These ghostly particles rarely interact with matter. Scientists using the IceCube detector at the South Pole have found evidence of high-energy neutrinos from our own Galaxy. This discovery establishes the Milky Way as the first confirmed astrophysical source of high-energy neutrinos. It opens a new window into our cosmic neighborhood.

The invisible landscape of the Galactic plane

Locating cosmic ray sources is difficult. Most messengers, like light or protons, are easily blocked or deflected. Protons are charged particles. As they travel, complex magnetic fields bend their paths. This erases the memory of their origin. Electromagnetic observations, such as gamma rays, provide a clearer picture. Yet, interstellar dust can obscure them.

Historically, neutrino astronomy focused on the extragalactic sky. A decade ago, IceCube discovered a diffuse flux of high-energy neutrinos. This signal appeared isotropic (arriving uniformly from all directions). There was no clear contribution from the Milky Way. In fact, distant extragalactic sources outshine our Galaxy by roughly an order of magnitude. Searching for a Galactic signal meant finding a faint, extended glow. This signal was hidden beneath a massive, isotropic background. Previous attempts failed to reach the 5 $\sigma$ discovery threshold. This threshold is the gold standard for scientific discovery. It means the chance of a statistical fluke is less than one in 3.5 million.

A unified approach to all-flavour detection

To find this faint signal, the IceCube Collaboration used a unified, all-flavour likelihood analysis. This approach is vital because of flavour oscillations. This is a quantum process where neutrinos transform between three types ($\nu_e$, $\nu_\mu$, and $\nu_\tau$) during travel. By the time they reach Earth, they arrive as a predictable mixture.

The researchers combined three distinct event morphologies (the shapes of light deposited in the detector) to boost sensitivity:

1. Shower-dominated events: These are compact bursts of light from $\nu_e$ or $\nu_\tau$ interactions. They have lower angular resolution (the ability to pinpoint direction). However, their high statistics drive the sensitivity for extended sources.
2. Starting tracks: These involve $\nu_\mu$ interactions where the neutrino starts inside the detector. They offer better angular resolution than showers.
3. Through-going tracks: These come from muons created outside the detector. They provide high statistics, especially in the northern sky.

The team also overhauled their "ice modelling." This is the mathematical description of light travel through South Pole ice. They included effects like ice birefringence (light traveling at different speeds based on polarization). They also added ice-layer undulations (irregularities in ice layers). Neglecting these factors previously caused biases in arrival direction. These updates improved the median angular resolution of showers by 1.5 to 2 times [Extended Data Fig. 3].

Establishing the 5.7 $\sigma$ discovery

The 12-year analysis marks a major shift in neutrino astronomy. The authors report high-energy neutrino emission from the Galactic plane at 5.7 $\sigma$ post-trial significance. This moves the Milky Way from a candidate to a confirmed source.

The signal is not uniform. It is concentrated in the inner region of our Galaxy. The researchers examined the central longitude bin ($|\ell| < 20^\circ$). In this region, they saw 217 shower events with energy above 5 TeV. They expected only $154.4 \pm 4.1$ background events. This excess is clearly visible in .

The background-subtracted residuals show a spike at the Galactic Center.

Signal strength depends on the theoretical model used. These models are called "templates." They predict how cosmic rays move through the Galaxy. The authors tested four templates. The Fermi-LAT $\pi^0$ template yielded the highest significance. However, this model underestimates the observed flux. It requires a normalization factor of $4.99^{+0.90}{-0.86}$ to match the data. This means the actual neutrino flux is about five times higher than this model predicts. Conversely, the CRINGE template aligns well with its predictions. It requires a normalization of $0.91^{+0.18}$, which is close to unity.

Limits of the current neutrino map

The map is not yet complete. The findings face two main limitations regarding the nature of the emission.

First, the data cannot yet distinguish between "diffuse" emission and "point" sources. It is unclear if neutrinos come from cosmic rays hitting interstellar gas. Alternatively, they might come from many individual, unresolved objects. These could be star clusters or supernova remnants. Second, the data cannot yet rule out exotic physics. The authors note the data cannot uniquely separate interstellar emission from dark-matter-induced neutrinos.

The analysis also has a mathematical trade-off. Researchers factorized the directional and energy probability density functions (PDFs). This was done to make the heavy computations feasible. However, this causes a loss of subtle correlations between direction and energy. This makes the likelihood analysis technically "suboptimal." It could be more powerful with non-factorized modeling.

The dawn of multi-messenger astronomy

The Milky Way is officially a high-energy neutrino factory. The 5.7 $\sigma$ signal is a landmark achievement. It moves neutrino physics from "searching for signals" to "mapping structures."

We are only at the start. To understand the Galaxy's engine, we need more data. Future research requires higher statistics and better angular resolution. This will help distinguish smooth interstellar glows from sharp point sources. As the authors suggest, combining neutrino data with gamma-ray observations is key. Upcoming observatories like KM3NeT will also help. These tools will allow us to use neutrinos to probe physics over kiloparsec distances. This may eventually reveal the fundamental properties of the neutrino itself.

Figures from the paper