Neutrinos from the Fermi Bubbles: IceCube Confirms Cosmic-Ray Acceleration to PeV Energies ✛ ✎ ✦
Context
The center of our Galaxy occasionally produces powerful bipolar jets of plasma, as evidenced by the giant Fermi and eROSITA bubbles. These structures, observed in X-rays and gamma rays, expand into the interstellar medium, creating shock waves where particles are accelerated to extreme energies via the Fermi acceleration mechanism. However, direct evidence for the acceleration of protons—the main cosmic rays—was missing. Neutrinos, born from collisions of accelerated nuclei with gas, could clarify the picture, but previous searches, including radio astronomy methods, were unsuccessful. The key turned out to be new data from the neutrino telescope IceCube and the active galactic nucleus.
Methods
Researchers compared the HESE event map from the 12-year IceCube dataset with the gamma-ray distribution from the Fermi-LAT telescope and X-ray spectral images from the eROSITA observatory, led by Rashid Sunyaev. To isolate the signal, they used spatial correlation methods and brightness profiles built by stacking along the bubbles' edges. The background was estimated by randomly shuffling the right ascensions of events. The analysis focused on neutrino energies above 100 TeV, where the atmospheric background contribution is minimal.
Results
The correlation of the neutrino map with the gamma-ray map outside the Galactic plane (|b|>30°) reached 3.7σ. In the northern Fermi bubble, an excess of 7 events above 100 TeV (with ~1 expected) was found, corresponding to a global significance of >4σ, and >5σ for track-like events. Analysis of shells localized via X-ray data revealed an excess of >7σ in the northern shell. Modeling of Fermi acceleration in shock waves showed that the energy of cosmic ions in each bubble is about 3×$10^{54}$ erg (with a factor 3 uncertainty), which is ~10% of the total outburst energy. The ratio of electron to ion energy η ~ 7×$10^{-4}$ is consistent with radio and gamma-ray observations.
Implications
The detection of neutrinos from the Fermi bubbles is the first direct evidence of proton acceleration to PeV energies in our Galaxy beyond supernova remnants. This confirms that such large-scale outflows from active galactic centers can be factories of high-energy cosmic rays. The derived acceleration efficiency (around 10%) constrains shock-wave models. Moreover, the predominantly leptonic nature of the bubbles' gamma-ray emission receives independent confirmation, which is important for interpreting observations of other Galactic structures.
Future development
With the accumulation of data from IceCube-Gen2 and other next-generation neutrino telescopes (e.g., KM3NeT), sensitivity will increase, enabling detailed study of the neutrino spectrum from the bubbles. This will help refine the acceleration mechanism and the composition of cosmic rays. It also remains to be determined whether the bubbles are linked to periods of activity of the supermassive black hole Sgr A* or to bursts of star formation.
Impact
The results open a new window into the physics of shock waves and particle acceleration mechanisms. They stimulate the development of multi-messenger astronomy, combining neutrino, radio, and X-ray observations to probe hidden processes in the Milky Way's center.
Next steps
Independent confirmation of the signal by other instruments is needed, for example, by the global network of neutrino telescopes. It is also important to study the contribution of lower-latitude parts of the bubbles and possible time variability.
Key open problems
The discovery is directly connected to the problem of the origin of high-energy Galactic cosmic rays: the Fermi bubbles may be one of the main sources of PeV particles. In addition, it sheds light on the nature of Galactic center activity and the feedback mechanisms between the supermassive black hole and the galaxy.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
Using 12 years of HESE data from the IceCube detector, the authors compared the map of neutrino events with gamma-ray and X-ray maps of the Fermi bubbles and eROSITA bubbles. They found a significant excess of high-energy neutrinos (>100 TeV) from the northeastern shells of the Fermi bubbles, which is interpreted as evidence for the acceleration of cosmic ions to PeV energies at shock waves. They estimated the energy of ions in the bubbles (~3×10^54 erg) and the acceleration efficiency (~10%), and also obtained an indication of neutrinos from the eROSITA bubbles.
Where its strength is
This work provides the first direct observational evidence for the acceleration of protons to PeV energies in large-scale galactic outflows beyond supernova remnants. The use of multi-year IceCube data combined with multi-frequency maps allowed the extraction of a weak neutrino signal and its association with specific astrophysical structures.
Where it could go next
- It might be interesting to compare the neutrino signal from the Fermi bubbles with predictions of models where black holes act as neutrino factories, as in the work on neutrinos from black hole coronae. This would test how universal the acceleration and neutrino production mechanisms are in the vicinity of active galactic nuclei.based on: Ghost Neutrinos: Born in the Coronas of Black Holes arXiv:2601.01533v2
- A related avenue is using neutrino telescopes to search for dark matter signals. Similar methods of background analysis and weak source extraction could be useful for further studying neutrinos from the eROSITA bubbles, where the signal is still preliminary.based on: Symphony of Shadows: Neutrino Telescopes Catch the Whisper of Dark Matter arXiv:2607.05335v1
- It is worth examining how neutrino tomography of the Earth performed by IceCube uses similar techniques for handling large datasets and background suppression. These approaches might help improve sensitivity to extended structures such as bubble shells.based on: Neutrino Tomography of Earth: A New Look at the Planet's Interior arXiv:2607.02644v2
Worth trying
- Test whether applying machine learning methods for signal/background separation, similar to those used in neutrino tomography of the Earth, would improve the significance of the detection from the Fermi bubble shells.
- Compare the neutrino spectrum from the Fermi bubbles with predictions of acceleration models in black hole coronae to assess the contribution of different source populations.
- Use future IceCube-Gen2 data to verify the presence of a neutrino signal from the eROSITA bubbles and refine its morphology.
Why this may matter
Confirming proton acceleration in galactic bubbles expands our understanding of the origin of high-energy cosmic rays and the role of active galactic nuclei in this process. It also paves the way for using neutrino observations as an independent tool to study the dynamics of large-scale outflows from galactic centers.
Nearby works in our archive
- Black Holes: Neutrino Factoriessimilarity 0.66
- Ghost Neutrinos: Born in the Coronas of Black Holessimilarity 0.64
- Symphony of Shadows: Neutrino Telescopes Catch the Whisper of Dark Mattersimilarity 0.64
- Ghost Particles Will Reveal the Star's Fur Coatsimilarity 0.64
- Black Holes as Ghost Particle Acceleratorssimilarity 0.64
- The Earth’s Neutrino Confession: IceCube Reads the Planet in the Language of Weak Interactionssimilarity 0.64
- Neutrino Tomography of Earth: A New Look at the Planet's Interiorsimilarity 0.61
The closest work in the archive sits at 0.66 by meaning — the usual level for our corpus.
Although the Fermi bubbles were discovered in 2010, their exact origin remains debated: some scientists link them to black hole jets, others to a powerful burst of star formation. The neutrino signal for the first time gives a 'taste' of the accelerated particle physics, helping to solve this mystery.
Key numbers
- neutrino energy: >100 TeV
- global significance (northern bubble): >4σ
- significance including tracks: >5σ
- local excess in the shell: >7σ
- ion energy in one bubble: ~1054.5} erg