We employed our recent model of the cosmic-ray (CR) halo to compute the Galactic spectra of stable and unstable secondary nuclei. In this model, confinement of the Galactic CRs is entirely determined ...by the self-generated Alfvénic turbulence whose spectrum is controlled by nonlinear Landau damping. We analyzed the physical parameters affecting propagation characteristics of CRs and estimated the best set of free parameters providing accurate description of available observational data. We also show that agreement with observations at lower energies may be further improved by taking into account the effect of ion-neutral damping that operates near the Galactic disk.
Abstract
A flux of cosmic rays (CRs) propagating through a diffuse ionized gas can excite MHD waves, thus generating magnetic disturbances. We propose a generic model of CR penetration into molecular ...clouds through their diffuse envelopes, and identify the leading physical processes controlling their transport on the way from a highly ionized interstellar medium to the dense interior of the cloud. The model allows us to describe a transition between a free streaming of CRs and their diffusive propagation, determined by the scattering on the self-generated disturbances. A self-consistent set of equations, governing the diffusive transport regime in an envelope and the MHD turbulence generated by the modulated CR flux, is characterized by two dimensionless numbers. We demonstrate a remarkable mutual complementarity of different mechanisms leading to the onset of the diffusive regime, which results in a universal energy spectrum of the modulated CRs. In conclusion, we briefly discuss implications of our results for several fundamental astrophysical problems, such as the spatial distribution of CRs in the Galaxy as well as the ionization, heating, and chemistry in dense molecular clouds.
We analyze the processes governing cosmic-ray (CR) penetration into molecular clouds and the resulting generation of gamma-ray emission. The density of CRs inside a cloud is depleted at lower ...energies due to the self-excited MHD turbulence. The depletion depends on the effective gas column density ("size") of the cloud. We consider two different environments where the depletion effect is expected to be observed. For the central molecular zone, the expected range of CR energy depletion is E 10 GeV, leading to the depletion of gamma-ray flux below Eγ 2 GeV. This effect can be important for the interpretation of the GeV gamma-ray excess in the Galactic Center, which has been revealed from the standard model of CR propagation (assuming the CR spectrum inside a cloud to be equal to the interstellar spectrum). Furthermore, recent observations of some local molecular clouds suggest the depletion of the gamma-ray emission, indicating possible self-modulation of the penetrating low-energy CRs.
Abstract
We analyze properties of nonthermal radio emission from the Central Molecular Zone (CMZ) and from 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 ∼10
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 ∼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.
Abstract
We present a nonlinear model of a self-consistent Galactic halo, where the processes of cosmic-ray (CR) propagation and excitation/damping of MHD waves are included. The MHD turbulence that ...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 ∼1
μ
G, we estimate
z
cr
∼ 1 kpc for GeV protons. The derived proton spectrum is in a good agreement with observational data.
A self-consistent model of a one-dimensional cosmic-ray (CR) halo around the Galactic disk is formulated with the restriction of a minimum number of free parameters. It is demonstrated that the ...turbulent cascade of MHD waves does not necessarily play an essential role in the halo formation. Instead, an increase of the Alfvén velocity with distance to the disk leads to an efficient generic mechanism of the turbulent redshift, enhancing CR scattering by the self-generated MHD waves. As a result, the calculated size of the CR halo at lower energies is determined by the halo sheath, an energy-dependent region around the disk beyond which the CR escape becomes purely advective. At sufficiently high energies, the halo size is set by the characteristic thickness of the ionized gas distribution. The calculated Galactic spectrum of protons shows a remarkable agreement with observations, reproducing the position of the spectral break at TeV and the spectral shape up to ∼10 TeV.
ABSTRACT We analyze the model of stochastic re-acceleration of electrons that are emitted by supernova remnants (SNRs) in the Galactic Disk and then propagate into the Galactic Halo in order to ...explain the origin of nonthermal (radio and gamma-ray) emission from Fermi bubbles (FB). We assume that the energy for re-acceleration in the Halo is supplied by shocks generated by processes of star accretion onto the central black hole. Numerical simulations show that regions with strong turbulence (places for electron re-acceleration) are located high up in the Galactic Halo several kpc above the disk. The energy of the SNR electrons that reach these regions does not exceed several GeV due to synchrotron and inverse Compton energy losses. At appropriate parameters of re-acceleration these electrons can be re-accelerated up to an energy of 1012 eV, which explains in this model the origin of the observed radio and gamma-ray emission from the FB. However, although the model gamma-ray spectrum is consistent with the Fermi results, the model radio spectrum is steeper than that observed by WMAP and Planck. If adiabatic losses due to plasma outflows from the Galactic central regions are taken into account, then the re-acceleration model nicely reproduces the Planck data points.
We analyze processes of electron acceleration in the Fermi bubbles in order to define parameters and restrictions of the models, which are suggested for the origin of these giant radio and gamma-ray ...structures. In the case of the leptonic origin of the nonthermal radiation from the bubbles, these electrons should be produced somehow in situ because of the relatively short lifetime of high-energy electrons, which lose their energy by synchrotron and inverse-Compton processes. It has been suggested that electrons in bubbles may be accelerated by shocks produced by tidal disruption of stars accreting onto the central black hole or a process of re-acceleration of electrons ejected by supernova remnants. These processes will be investigated in subsequent papers. In this paper, we focus on in situ stochastic (Fermi) acceleration by a hydromagnetic/supersonic turbulence, in which electrons can be directly accelerated from the background plasma. We showed that the acceleration from the background plasma is able to explain the observed fluxes of radio and gamma-ray emission from the bubbles, but the range of permitted parameters of the model is strongly restricted.
Capture and tidal disruption of stars by the supermassive black hole in the Galactic center (GC) should occur regularly. The energy released and dissipated by these processes will affect both the ...ambient environment of the GC and the Galactic halo. The single star of a super-Eddington eruption generates a subsonic outflow with an energy release of more than 1052 erg, which still is not high enough to push shock heated gas into the halo. Only routine tidal disruption of stars near the GC can provide enough cumulative energy to form and maintain large-scale structures like the Fermi Bubbles. The average rate of disruption events is expected to be 10−4 ∼ 10−5 yr−1, providing the average power of energy release from the GC into the halo of erg s−1, which is needed to support the Fermi Bubbles. The GC black hole is surrounded by molecular clouds in the disk, but their overall mass and filling factor are too low to significantly stall the shocks from tidal disruption events. The de facto continuous energy injection on timescales of megayears will lead to the propagation of strong shocks in a density stratified Galactic halo and thus create elongated bubble-like features that are symmetric to the Galactic midplane.
ABSTRACT From the rate of hydrogen ionization and the gamma-ray flux, we derived the spectrum of relativistic and subrelativistic cosmic rays (CRs) nearby and inside the molecular cloud Sgr B2 near ...the Galactic Center. We studied two cases of CR propagation in molecular clouds: free propagation and scattering of particles by magnetic fluctuations excited by the neutral gas turbulence. We showed that in the latter case CR propagation inside the cloud can be described as diffusion with a coefficient of cm2 s−1. For the case of hydrogen ionization by subrelativistic protons, we showed that their spectrum outside the cloud is quite hard with a spectral index of . The energy density of subrelativistic protons ( eV cm−3) is one order of magnitude higher than that of relativistic CRs. These protons generate the 6.4 keV emission from Sgr B2, which was about 30% of the flux observed by Suzaku in 2013. Future observations for the period after 2013 may discover the background flux generated by subrelativistic CRs in Sgr B2. Alternatively, hydrogen ionization of the molecular gas in Sgr B2 may be caused by high energy electrons. We showed that the spectrum of electron bremsstrahlung is harder than the observed continuum from Sgr B2, and in principle, this X-ray component provided by electrons could be seen from the INTEGRAL data as a stationary high energy excess above the observed spectrum .
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