393 research outputs found
Multi-wavelength Emission from the Fermi Bubble III. Stochastic (Fermi) Re-Acceleration of Relativistic Electrons Emitted by SNRs
We analyse the model of stochastic re-acceleration of electrons, which are
emitted by supernova remnants (SNRs) in the Galactic Disk and propagate then
into the Galactic halo, in order to explain the origin on nonthermal (radio and
gamma-ray) emission from the 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 about several kpc above the disk. The
energy of SNR electrons that reach these regions does not exceed several GeV
because of synchrotron and inverse Compton energy losses. At appropriate
parameters of re-acceleration these electrons can be re-accelerated up to the
energy 10E12 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 the observed by WMAP and Planck. If adiabatic losses due to plasma
outflow from the Galactic central regions are taken into account, then the
re-acceleration model nicely reproduces the Planck datapoints.Comment: 33 pages, 8 figures, accepted by Ap
The origin of the 6.4 keV line emission and H ionization in the diffuse molecular gas of the Galactic center region
We investigate the origin of the diffuse 6.4 keV line emission recently
detected by Suzaku and the source of H_2ionization in the diffuse molecular gas
of the Galactic Center (GC) region. We show that Fe atoms and H_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 > 10^38} erg
s^{-1}. The H_2 molecules of the diffuse gas can not 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.Comment: 5 pages, 4 figues, accepted for publication in the Astrophysical
Journal Letter
The Origin of Gamma-Rays from Globular Clusters
Fermi has detected gamma-ray emission from eight globular clusters. We
suggest that the gamma-ray emission from globular clusters may result from the
inverse Compton scattering between relativistic electrons/positrons in the
pulsar wind of MSPs in the globular clusters and background soft photons
including cosmic microwave/relic photons, background star lights in the
clusters, the galactic infrared photons and the galactic star lights. We show
that the gamma-ray spectrum from 47 Tuc can be explained equally well by upward
scattering of either the relic photons, the galactic infrared photons or the
galactic star lights whereas the gamma-ray spectra from other seven globular
clusters are best fitted by the upward scattering of either the galactic
infrared photons or the galactic star lights. We also find that the observed
gamma-ray luminosity is correlated better with the combined factor of the
encounter rate and the background soft photon energy density. Therefore the
inverse Compton scattering may also contribute to the observed gamma-ray
emission from globular clusters detected by Fermi in addition to the standard
curvature radiation process. Furthermore, we find that the emission region of
high energy photons from globular cluster produced by inverse Compton
scattering is substantially larger than the core of globular cluster with a
radius >10pc. The diffuse radio and X-rays emitted from globular clusters can
also be produced by synchrotron radiation and inverse Compton scattering
respectively. We suggest that future observations including radio, X-rays, and
gamma-rays with energy higher than 10 GeV and better angular resolution can
provide better constraints for the models.Comment: Accepted by ApJ, Comments may send to Prof. K.S. Cheng:
[email protected]
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: III. Interference Caused by a Double Pulse
In order to study consequences of the differences between the
ionic-to-neutral and neutral-to-ionic transitions in the one-dimensional
extended Peierls-Hubbard model with alternating potentials for the TTF-CA
complex, we introduce a double pulse of oscillating electric field in the
time-dependent Schr\"odinger equation and vary the interval between the two
pulses as well as their strengths. When the dimerized ionic phase is
photoexcited, the interference effect is clearly observed owing to the
coherence of charge density and lattice displacements. Namely, the two pulses
constructively interfere with each other if the interval is a multiple of the
period of the optical lattice vibration, while they destructively interfere if
the interval is a half-odd integer times the period, in the processes toward
the neutral phase. The interference is strong especially when the pulse is
strong and short because the coherence is also strong. Meanwhile, when the
neutral phase is photoexcited, the interference effect is almost invisible or
weakly observed when the pulse is weak. The photoinduced lattice oscillations
are incoherent due to random phases. The strength of the interference caused by
a double pulse is a key quantity to distinguish the two transitions and to
evaluate the coherence of charge density and lattice displacements.Comment: 16 pages, 8 figure
Gamma-Ray Emission from Molecular Clouds Generated by Penetrating Cosmic Rays
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 GeV, leading to the depletion of
gamma-ray flux below 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.Comment: 10 pages, 5 figures, accepted for publication in Ap
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