We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H sub(2) ionization in the diffuse molecular gas of the Galactic center (GC) region. We ...show that Fe atoms and H sub(2) molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A* lasted at least several hundred years and released a mean 2-100 keV luminosity > ~10 super(38) erg s sub(-1). The H sub(2) molecules of the diffuse gas cannot be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.
We investigate the origin of the diffuse 6.4 keV line emission recently detected by Suzaku and the source of H{sub 2} ionization in the diffuse molecular gas of the Galactic center (GC) region. We ...show that Fe atoms and H{sub 2} molecules in the diffuse interstellar medium of the GC are not ionized by the same particles. The Fe atoms are most likely ionized by X-ray photons emitted by Sgr A* during a previous period of flaring activity of the supermassive black hole. The measured longitudinal intensity distribution of the diffuse 6.4 keV line emission is best explained if the past activity of Sgr A* lasted at least several hundred years and released a mean 2-100 keV luminosity {approx}> 10{sup 38} erg s{sup -1}. The H{sub 2} molecules of the diffuse gas cannot be ionized by photons from Sgr A*, because soft photons are strongly absorbed in the interstellar gas around the central black hole. The molecular hydrogen in the GC region is most likely ionized by low-energy cosmic rays, probably protons rather than electrons, whose contribution into the diffuse 6.4 keV line emission is negligible.
We present a nonlinear model of self-consistent Galactic halo, where the processes of cosmic ray (CR) propagation and excitation/damping of MHD waves are included. The MHD-turbulence, which prevents ...CR escape from the Galaxy, is entirely generated by the resonant streaming instability. The key mechanism controlling the halo size is the nonlinear Landau (NL) damping, which suppresses the amplitude of MHD fluctuations and, thus, makes the halo larger. The equilibrium turbulence spectrum is determined by a balance of CR excitation and NL damping, which sets the regions of diffusive and advective propagation of CRs. The boundary \(z_{cr}(E)\) between the two regions is the halo size, which slowly increases with the energy. For the vertical magnetic field of \(\sim 1~\mu G\), we estimate \(z_{cr} \sim 1\) kpc for GeV protons. The derived proton spectrum is in a good agreement with observational data.
We analyze properties of non-thermal radio emission from the Central Molecular Zone (CMZ) and individual molecular clouds, and argue that the observed features can be interpreted in the framework of ...our recent theory of self-modulation of cosmic rays (CRs) penetrating dense molecular regions. For clouds with gas column densities of \(\sim10^{23}\) cm\(^{-2}\), the theory predicts depletion of sub-GeV CR electrons, occurring due to self-modulation of CR protons and leading to harder synchrotron spectra in the sub-GHz range. The predicted imprints of electron depletion in the synchrotron spectra agree well with the spectral hardening seen in available radio observations of the CMZ. A similar, but even stronger effect on the synchrotron emission is predicted for individual (denser) CMZ clouds, such as the Sgr B2. However, the emission at frequencies above \(\sim\) GHz, where observational data are available, is completely dominated by the thermal component, and therefore new observations at lower frequencies are needed to verify the predictions.
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 tau super(-1) sub(cap) ~ 3 x 10 super(-5) yr super(-1) and an energy release of W ~ 3 x 10 super(52) erg per capture, can result in hot plasma injecting into the Galactic halo at a wind velocity of u ~ 10 super(8) cm s super(-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 super(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 sub(B) ~ 3 x 10 super(30) cm super(2) s super(-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.
Acceleration of charged particles by neutral gas turbulence in giant molecular clouds is considered. The gamma-ray emission from these clouds is estimated. It is shown that molecular clouds can be ...the counterparts of some of unidentified sources.
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