34,833 research outputs found
Integrated voice/data protocols for satellite channels
Several integrated voice/data protocols for satellite channels are studied. The system consists of two types of traffic: voice calls which are blocked-calls-cleared and the data packets which may be stored when no channel is available. The voice calls are operated under a demand assignment protocol. Three different data protocols for data packets are introduced. Under Random Access Data (RAD), the Aloha random access scheme is used. Due to the nature of random access, the channel utilization is low. Under Demand Assignment Data (DAD), a demand assignment protocol is used to improve channel utilization. Since a satellite channel has long propagation delay, DAD may perform worse than RAD. The two protocols are combined to obtain a new protocol called Hybrid Data (HD). The proposed protocols are fully distributed and no central controller is required. Numerical results show that HD enjoys a lower delay than DAD and provides a much higher channel capacity than RAD. The effects of fixed and movable boundaries are compared in partitioning the total frequency band to voice and data users
Possibly New Charmed Baryon States from Decay
We examine the invariant mass spectrum of in decay measured by BABAR and find that through the 2-step processes of
, where denotes a
charmed baryon state, some of the peaks can be identified with the established
, and . Moreover, in
order to account for the measured spectrum, it is necessary to introduce a new
charmed baryon resonance with MeV.Comment: 8 pages, 1 figure, title changed and discussions updated, version
accepted for publication in Phys. Rev.
Soliton Resonances for MKP-II
Using the second flow - the Derivative Reaction-Diffusion system, and the
third one of the dissipative SL(2,R) Kaup-Newell hierarchy, we show that the
product of two functions, satisfying those systems is a solution of the
modified Kadomtsev-Petviashvili equation in 2+1 dimension with negative
dispersion (MKP-II). We construct Hirota's bilinear representation for both
flows and combine them together as the bilinear system for MKP-II. Using this
bilinear form we find one and two soliton solutions for the MKP-II. For special
values of parameters our solution shows resonance behaviour with creation of
four virtual solitons. Our approach allows one to interpret the resonance
soliton as a composite object of two dissipative solitons in 1+1 dimensions.Comment: 11 pages, 2 figures, Talk on International Conference "Nonlinear
Physics. Theory and Experiment. III", 24 June-3 July, 2004, Gallipoli(Lecce),
Ital
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 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]
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 Casimir effect for parallel plates at finite temperature in the presence of one fractal extra compactified dimension
We discuss the Casimir effect for massless scalar fields subject to the
Dirichlet boundary conditions on the parallel plates at finite temperature in
the presence of one fractal extra compactified dimension. We obtain the Casimir
energy density with the help of the regularization of multiple zeta function
with one arbitrary exponent and further the renormalized Casimir energy density
involving the thermal corrections. It is found that when the temperature is
sufficiently high, the sign of the Casimir energy remains negative no matter
how great the scale dimension is within its allowed region. We derive
and calculate the Casimir force between the parallel plates affected by the
fractal additional compactified dimension and surrounding temperature. The
stronger thermal influence leads the force to be stronger. The nature of the
Casimir force keeps attractive.Comment: 14 pages, 2 figure
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