Fermi bubbles as a source of cosmic rays above 1015 eV

Abstract

Fermi bubbles are giant gamma-ray structures extended north and south of the Galactic center with characteristic sizes of order of 10 kpc recently discovered by Fermi Large Area Telescope. Good correlation between radio and gamma-ray emission in the region covered by Fermi bubbles implies the presence of high-energy electrons in this region. Since it is relatively difficult for relativistic electrons of this energy to travel all the way from the Galactic sources toward Fermi bubbles one can assume that they accelerated in-situ. The corresponding acceleration mechanism should also affect the distribution of the relativistic protons in the Galaxy. Since protons have much larger lifetimes the effect may even be observed near the Earth. In our model we suggest that Fermi bubbles are created by acceleration of electrons on series of shocks born due to periodic star accretions by supermassive black hole Sgr A*. We propose that hadronic CR within the "knee" of the observed CR spectrum are produced by Galactic supernova remnants distributed in the Galactic disk. Reacceleration of these particles in the Fermi Bubble produces CRs beyond the knee. This model provides a natural explanation of the observed CR flux, spectral indexes, and matching of spectra at the knee.

Fermi Bubble Edges: Spectrum and Diffusion Function

“…In spite of their dramatic appearance in the γ-ray sky, the nature of the FBs is still debated. Different models were proposed (S10, F14), interpreting the FB edge as an outgoing shock (Fujita et al 2013), a termination shock of a wind (Lacki 2014;Mou et al 2014), or a discontinuity (Crocker 2012;Guo & Mathews 2012;Sarkar et al 2015); the γ-ray emission mechanism as either hadronic (Crocker & Aharonian 2011;Fujita et al 2013) or leptonic (Yang et al 2013); the underlying engine as a starburst (Carretti et al 2013;Lacki 2014;Sarkar et al 2015), an SMBH jet (Cheng et al 2011;Guo & Mathews 2012;Zubovas & Nayakshin 2012;Mou et al 2014), or steady star-formation (Crocker 2012); and the CR acceleration mechanism as first-order Fermi accelera-tion, second-order Fermi acceleration (Mertsch & Sarkar 2011;Chernyshov et al 2014), or injection at the GC (Guo & Mathews 2012;Thoudam 2013).…”

Cosmic Ray (Stochastic) Acceleration from a Background Plasma

“…[29,30,31,33]), whose problems were presented in [34] On the other hand, the stochastic acceleration of protons in the Fermi bubbles may explain the origin of CRs with energies above the "knee" (> 10 15 eV) as shown by Cheng et al in [35]. We presented, however, these results in details at the last San-Vito conference which were published in [36].…”

Fermi bubbles: high-latitude X-ray supersonic shell

“…In spite of their dramatic appearance in the γ-ray sky, the nature of the FBs is still debated. Different models were proposed (S10, F14), interpreting the FB edge as an outgoing shock (Fujita et al 2013), a termination shock of a wind (Lacki 2014;Mou et al 2014), or a discontinuity (Crocker 2012;Guo & Mathews 2012;Sarkar et al 2015); the γ-ray emission mechanism as either hadronic (Crocker & Aharonian 2011;Fujita et al 2013) or leptonic (Yang et al 2013); the underlying engine as a starburst (Carretti et al 2013;Lacki 2014;Sarkar et al 2015), a jet from the the central massive black hole (Cheng et al 2011;Guo & Mathews 2012;Zubovas & Nayakshin 2012;Mou et al 2014), or steady star-formation (Crocker 2012); and the cosmic-ray (CR) acceleration mechanism as first order Fermi acceleration, second order Fermi acceleration (Mertsch & Sarkar 2011;Chernyshov et al 2014), or injection at the GC (Guo & Mathews 2012;Thoudam 2013).…”