THE FERMI BUBBLE AS A SOURCE OF COSMIC RAYS IN THE ENERGY RANGE >10¹⁵eV

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 ∼ 10 kpc. We suggest that the periodic star capture processes by the Galactic supermassive black hole Sgr A * , with a capture rate of τ −1 cap ∼ 3 × 10 −5 yr −1 and an energy release of W ∼ 3 × 10 52 erg per capture, can result in hot plasma injecting into the Galactic halo at a wind velocity of u ∼ 10 8 cm s −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 > 10 15 eV, 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 D B ∼ 3 × 10 30 cm 2 s −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.

A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center’s Giant Outflows

“…Given the very hard spectrum, a total power in these particles that approaches 10 39 erg s −1 , and that the particles are delivered into the halo but not too far above the disk, these shocks are interesting candidates (see Parizot 2014) for the main accelerators of Galactic cosmic rays in this energy range (see Cheng et al 2012;Lacki 2014;Taylor et al 2014).…”

The Fermi bubbles as starburst wind termination shocks

“…The sheer size of the Bubbles makes them excellent places for the acceleration of extremely high energy particles (c.f., Cheng et al 2012). Electrons are highly radiative particles, so their maximum energy is set by IC or synchrotron losses.…”

Fermi bubbles as sources of cosmic rays above 1 PeV

“…Smooth attachment of the spectra above and below this energy and sharpness of the transition indicates that we are dealing with a single spectrum rather than with the sum of two distinct components. The review of different models suggested to explain this phenomenon can be found in the original paper [17].…”