Fermi Bubble Edges: Spectrum and Diffusion Function

Uri Keshet, Ilya Gurwich

Research output: Contribution to journalArticlepeer-review

15 Scopus citations


Current measurements of the γ-ray Fermi bubbles (FB) are based on model-dependent tracers, carry substantial systematic uncertainties, and contain some discrepancies between each other. We show that gradient filters pick out the FB edges, which are found to smoothly connect to the bipolar X-ray structure emanating from the Galactic center, thus supporting the interpretation of the FBs as a Galactic-scale phenomenon. The sharp edges facilitate a direct, model-free measurement of the peripheral FB spectrum. The result is strikingly similar to the full FB-integrated spectrum, softened by a power law of index η ≃ (0.2-0.3). This is naturally explained, in both hadronic and leptonic models, if cosmic rays are injected at the edge, and diffuse away preferentially at higher energies E. The inferred, averaged diffusion function in the (more plausible) leptonic model,D(E) ≃ 1029.5(E /10 GeV)0.48 ±0.02 cm2 s-1, is consistent with estimates for Kraichnan-like turbulence. Our results, in particular the minute spatial variations in η, indicate that the FB edge is a strong, Mach ≳;5, forward shock.

Original languageEnglish
Article number7
JournalAstrophysical Journal
Issue number1
StatePublished - 1 May 2017


  • Galaxy: center
  • cosmic rays
  • diffusion
  • gamma rays: ISM
  • shock waves

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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