The Fermi Bubble as a source of cosmic rays in the energy range >10 15eV

K. S. Cheng, D. O. Chernyshov, V. A. Dogiel, C. M. Ko, W. H. Ip, Y. Wang

Research output: Contribution to journalArticlepeer-review

24 Scopus citations

Abstract

The Fermi Large Area Telescope has recently discovered two giant gamma-ray bubbles that extend north and south of the Galactic center with diameters and heights of the order of H 10kpc. We suggest that the periodic star capture processes by the Galactic supermassive black hole Sgr A*, with a capture rate of τ-1cap 3 × 10-5yr -1 and an energy release of W 3 × 1052erg per capture, can result in hot plasma injecting into the Galactic halo at a wind velocity of u 108cms-1. The periodic injection of hot plasma can produce a series of shocks. Energetic protons in the bubble are re-accelerated when they interact with these shocks. We show that for energy larger than E > 1015eV, the acceleration process can be better described by the stochastic second-order Fermi acceleration. We propose that hadronic cosmic rays (CRs) within the "knee" of the observed CR spectrum are produced by Galactic supernova remnants distributed in the Galactic disk. Re-acceleration of these particles in the Fermi Bubble produces CRs beyond the knee. With a mean CR diffusion coefficient in this energy range in the bubble DB 3 × 1030cm2s-1, we can reproduce the spectral index of the spectrum beyond the knee and within it. The conversion efficiency from shock energy of the bubble into CR energy is about 10%. This model provides a natural explanation of the observed CR flux, spectral indices, and matching of spectra at the knee.

Original languageEnglish
Article number116
JournalAstrophysical Journal
Volume746
Issue number2
DOIs
StatePublished - 20 Feb 2012

Keywords

  • acceleration of particles
  • galaxies: jets
  • Galaxy: halo
  • shock waves

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