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Researchers analyzed ten years of gamma-ray data from the Fermi Bubbles, massive structures extending 50 degrees above and below the Milky Way's center, to determine their origin and power source. Using pixel-by-pixel spectral analysis, they found that both hadronic (proton-driven) and leptonic (electron-driven) processes could equally explain the observed gamma-ray emissions, with cosmic ray spectra showing hardening toward the southern tip and requiring specific energy distributions. The spatial distribution of cosmic ray electrons contradicts scenarios where electrons are accelerated near the galactic center and then transported outward.
Why it matters
Understanding the Fermi Bubbles' origin helps explain energetic processes at the Milky Way's core and the mechanisms that accelerate cosmic rays to extreme energies. These findings constrain theoretical models of galactic evolution and particle acceleration in our own galaxy, which has broader implications for understanding similar phenomena in other galaxies.
Understand the Science
arXiv:2511.07066v2 Announce Type: replace
Abstract: The Fermi Bubbles are gamma-ray structures extending from the center of the Milky Way to +/-50 degree Galactic latitude that were discovered in data obtained by the Fermi/LAT instrument. Their origin and power source remain uncertain. To help address this uncertainty, here we use a template-free reconstruction of ten years of all-sky Fermi/LAT data provided by Platz et al. (2023) to carry out a pixel-by-pixel spectral analysis of the Bubbles. We recover the position-dependent spectral shape and normalization that would be required for parent proton or electron cosmic ray populations to produce the Bubbles’ observed gamma-ray spectra. We find that models in which the gamma-ray emission is driven by either hadronic or leptonic processes can explain the data equally well. The cosmic ray population driving the emission must have either broken power-law or exponentially cut-off spectra, with break or cutoff energies that are almost constant with latitude but spectral indices below the break that harden towards the Bubbles’ southern tip. For the leptonic channel, reproducing the observed position-dependent gamma-ray spectrum also requires a cosmic ray electron energy density that grows with distance from the Galactic plane and increases towards the edges of the Bubbles. This finding disfavors scenarios for the origin of the Bubbles where a population of cosmic ray electrons is accelerated near the Milky Way center and subsequently advected out to the extremities of the Bubbles.
Source: Analysis and implications of the spatio-spectral morphology of the Fermi Bubbles