IceCube measures how the Milky Way’s neutrino emission varies across the sky

From detecting the Galaxy to comparing its regions

The Milky Way produces high-energy neutrinos, but establishing that overall signal is different from measuring how it changes across the sky. Earlier IceCube searches used templates that prescribed the relative emission from different Galactic regions. Those searches could adjust the Galaxy’s overall brightness, but not independently measure each region’s brightness and energy spectrum.

The new analysis takes that next step. The researchers divide the Galactic plane—the band along the Milky Way’s disk—into segments and fit their emission simultaneously. Each segment gets two adjustable quantities: its neutrino intensity and a spectral index describing how quickly that intensity falls with increasing energy.

This avoids fixing the regional relationships to a particular Galactic emission model. It is not assumption-free: the analysis assumes uniform emission within each segment and describes its energy spectrum with a single power law.

Ten years of particle showers

IceCube detects light produced when charged particles travel through Antarctic ice. A neutrino interaction can create a particle shower, or cascade, whose light lets researchers estimate the incoming neutrino’s energy and direction.

The study uses 59,592 selected events recorded between May 2011 and May 2021. These are not all identified Galactic neutrinos: the analysis statistically separates Galactic emission from backgrounds, including atmospheric neutrinos and an approximately uniform astrophysical component.

Cascades provide sensitivity across the whole sky, but their typical directional uncertainty is about seven degrees. That favors measurements of broad regions rather than a detailed map of individual sources. The generic regional comparisons use a band extending eight degrees above and below the Galactic plane.

A brighter inner plane, with an uncertain spectral difference

At a reference energy of 5 teraelectronvolts, or TeV, the best-fit neutrino intensity per unit sky area is approximately five times higher in the innermost region than in the outer plane. The inner region lies within 30 degrees of Galactic longitude zero, toward the Galactic center; the outer region lies more than 60 degrees away.

The sky-area qualification matters. This is a comparison of average intensity, not the total emission summed over two differently sized regions. The factor of five also applies at the stated energy, rather than necessarily across the entire spectrum.

The fitted spectral index is 2.76, with a 68% confidence interval extending 0.15 lower and 0.18 higher, for the innermost region. For the outer plane it is 3.13, with substantially larger uncertainties. A smaller index means a harder spectrum: the intensity declines less steeply as energy increases.

The best fits therefore suggest harder emission toward the inner Galaxy, but the indices remain compatible within uncertainties. The study does not establish a spectral difference. Statistical uncertainty still dominates after the researchers include known detector and ice-modeling uncertainties.

Comparing neutrinos with gamma rays

Neutrinos can help distinguish processes that produce high-energy gamma rays. Collisions between cosmic-ray particles and interstellar matter can produce pions: neutral pions decay into gamma rays, while charged-pion decay chains produce neutrinos. Gamma rays can also arise through processes involving electrons that do not supply the same neutrino counterpart.

The researchers compare IceCube measurements with LHAASO gamma-ray observations of the same sky region. They convert the gamma-ray measurements into an expected neutrino signal assuming production through collisions of protons with protons and negligible gamma-ray attenuation.

IceCube’s best-fit intensity falls between LHAASO’s source-masked, diffuse-only estimate and its total emission estimate. Both are compatible with the neutrino measurement within uncertainties. That agreement permits a hadronic interpretation, but does not prove it or rule out electron-driven emission from gamma-ray sources.

What remains unresolved

The measured signal includes both diffuse Galactic emission and any unresolved sources. Some diffuse-emission models predict less inner-Galaxy emission than measured; others agree within uncertainties. Consequently, the results do not uniquely establish an extra population of sources.

The advance is a regional measurement against which such explanations can be tested. More precise neutrino data will be needed to determine whether the spectrum genuinely changes across the Galaxy and how much of its gamma-ray emission comes from particle collisions.