High-energy neutrino emission from the Milky Way

Technical Summary: High-energy neutrino emission from the Milky Way

Problem and Context
For decades, the origin of high-energy astrophysical neutrinos has been a central question in multi-messenger astronomy. While IceCube established a diffuse, isotropic flux of high-energy neutrinos a decade ago, the specific contribution from the Milky Way remained unresolved. Although the Galaxy hosts cosmic-ray accelerators capable of energies beyond terrestrial reach, and theoretical models predict that cosmic rays (CRs) interacting with interstellar gas should produce a diffuse flux of high-energy neutrinos, previous searches had only provided hints of emission from the Galactic plane (GP). The challenge lies in distinguishing this faint Galactic signal from the overwhelming background of atmospheric muons and neutrinos, particularly given the low flux of extraterrestrial neutrinos at TeV energies and above. Furthermore, neutrinos produced in the Galaxy are expected to arrive as a mixture of flavors ($\nu_e, \nu_\mu, \nu_\tau$) due to oscillations, necessitating an analysis sensitive to all three.

Methodology
This study presents a unified, all-flavor analysis of 12 years of data from the IceCube Neutrino Observatory. The analysis combines three complementary event selection samples to maximize sensitivity:

1. Shower-dominated sample: Extending a previous 10-year analysis, this sample targets neutrino-induced particle showers with interaction vertices inside or near the instrumented volume. It utilizes updated detector calibration, simulation, and reconstruction.
2. Starting-track sample: Targets muon tracks with vertices inside the detector, offering better angular resolution than showers but with lower statistics.
3. Northern-track sample: Consists of through-going muons from the northern sky ($\delta > -5^\circ$), which are dominated by atmospheric neutrinos but provide high statistics and superior angular resolution.

Key Technical Improvements:

• Ice Modeling: The analysis incorporates recent improvements in the modeling of South Pole ice, including ice birefringence, layer undulations, and the optical properties of refrozen ice within drill holes. These updates correct for azimuth-dependent reconstruction biases and improve the median angular resolution of shower events by a factor of 1.5–2.
• Unified Likelihood: Unlike previous searches that treated samples separately, this work employs a single likelihood framework that simultaneously incorporates shower, starting-track, and through-going-track samples. The likelihood is constructed from a mixture of signal and background probability density functions (PDFs), utilizing both directional and energy information.
• Signal Templates: The analysis tests four emission templates based on different assumptions of CR propagation and gas distribution: Fermi-LAT $\pi^0$, KRA$\gamma^5$, KRA$\gamma^{50}$, and CRINGE.
• Background Estimation: Backgrounds are modeled using a data-driven approach that exploits the symmetry of the detector and the uniformity of the sky in right ascension, avoiding reliance on Monte Carlo (MC) simulations for the background PDF.

Key Results

• Discovery Significance: The analysis establishes high-energy neutrino emission from the Galactic plane with a post-trial significance of 5.7 $\sigma$. This exceeds the 5 $\sigma$ threshold required for a discovery, identifying the Milky Way as the first astrophysical source of high-energy neutrinos to do so.
• Spatial Distribution: The signal is concentrated toward the inner region of the Galaxy. In the central longitude bin ($|\hat{\ell}| < 20^\circ$), the analysis observes 217 shower events with visible energy above 5 TeV, compared to an expected background of $154.4 \pm 4.1$. This results in a residual excess of $62.6 \pm 15.3$ events.
• Flux Normalization: The measured flux depends on the assumed template. The Fermi-LAT $\pi^0$ template requires a normalization factor of $4.99^{+0.90}_{-0.86}$, indicating the nominal model underestimates the observed flux. Conversely, the KRA$\gamma$ templates prefer normalizations below unity ($0.53$ and $0.37$), while the CRINGE template is consistent with its nominal prediction ($0.91$).
• Spectral Characteristics: The fitted flux from the inner Galaxy reaches a level comparable to the all-sky averaged astrophysical neutrino flux. However, because the inner Galaxy spans only $\sim 3\%$ of the sky, the total Galactic component integrated over the full sky corresponds to approximately 10% of the total astrophysical flux at $E_\nu \sim 20$ TeV.
• Sample Contributions: The significance is primarily driven by the shower-dominated sample (local significance of 5.29 $\sigma$), with the track samples contributing a further $\sim 17\%$ to the discovery potential. The track samples show less significant excesses individually but are consistent with the shower sample's best-fit normalizations.

Significance and Claims
The paper claims that these results herald a new era of Galactic multi-messenger astronomy. By establishing the Milky Way as a confirmed source of high-energy neutrinos, the study provides a new tool to:

1. Probe Cosmic-Ray Propagation: The neutrino flux traces collisions of cosmic rays with the ambient interstellar medium, preserving information about CR origins and propagation even in regions inaccessible to electromagnetic observations.
2. Test Neutrino Properties: The detection of neutrinos over kiloparsec baselines offers new opportunities to probe neutrino properties, such as flavor composition and potential decay or secret interactions, complementing terrestrial and extragalactic measurements.

The authors note that current data do not yet uniquely separate diffuse interstellar emission from unresolved point-source populations or other subdominant contributions (e.g., dark matter). Future work involving improved angular resolution, higher statistics, and joint comparisons with high-energy $\gamma$-ray observations will be required to disentangle these components.