8,235 research outputs found
Comparison of phase-coherent and non-phase- coherent coded communications
Word-error probability versus signal-to-noise ratio evaluation for five communications system
Digital voltage-controlled oscillator
Digital voltage-controlled oscillator generates a variable frequency signal controlled linearly about a center frequency with high stability and is phase controlled by an applied voltage. Integration ahead of the digital circuitry provides linear operation with control voltage having appreciable noise components
Diffuse continuum gamma rays from the Galaxy
A new study of the diffuse Galactic gamma-ray continuum radiation is
presented, using a cosmic-ray propagation model which includes nucleons,
antiprotons, electrons, positrons, and synchrotron radiation. Our treatment of
the inverse Compton (IC) scattering includes the effect of anisotropic
scattering in the Galactic interstellar radiation field (ISRF) and a new
evaluation of the ISRF itself. Models based on locally measured electron and
nucleon spectra and synchrotron constraints are consistent with gamma-ray
measurements in the 30-500 MeV range, but outside this range excesses are
apparent. A harder nucleon spectrum is considered but fitting to gamma rays
causes it to violate limits from positrons and antiprotons. A harder
interstellar electron spectrum allows the gamma-ray spectrum to be fitted above
1 GeV as well, and this can be further improved when combined with a modified
nucleon spectrum which still respects the limits imposed by antiprotons and
positrons. A large electron/IC halo is proposed which reproduces well the
high-latitude variation of gamma-ray emission. The halo contribution of
Galactic emission to the high-latitude gamma-ray intensity is large, with
implications for the study of the diffuse extragalactic component and
signatures of dark matter. The constraints provided by the radio synchrotron
spectral index do not allow all of the <30 MeV gamma-ray emission to be
explained in terms of a steep electron spectrum unless this takes the form of a
sharp upturn below 200 MeV. This leads us to prefer a source population as the
origin of the excess low-energy gamma rays.Comment: Final version accepted for publication in The Astrophysical Journal
(vol. 537, July 10, 2000 issue); Many Updates; 20 pages including 49
ps-figures, uses emulateapj.sty. More details can be found at
http://www.gamma.mpe-garching.mpg.de/~aws/aws.htm
The Transition Between Quantum Coherence and Incoherence
We show that a transformed Caldeira-Leggett Hamltonian has two distinct
families of fixed points, rather than a single unique fixed point as often
conjectured based on its connection to the anisotropic Kondo model. The two
families are distinguished by a sharp qualitative difference in their quantum
coherence properties and we argue that this distinction is best understood as
the result of a transition in the model between degeneracy and non-degeneracy
in the spectral function of the ``spin-flip'' operator.Comment: some typos corrected and a reference adde
The Distance to the Soft Gamma Repeater SGR 1627-41
We report millimeter observations of the line of sight to the recently
discovered Soft Gamma Repeater, SGR 1627-41, which has been tentatively
associated with the supernova remnant SNR G337.0-0.1 Among the eight molecular
clouds along the line of sight to SGR 1627-41, we show that SNR G337.0-0.1 is
probably interacting with one of the most massive giant molecular clouds (GMC)
in the Galaxy, at a distance of 11 kpc from the sun. Based on the high
extinction to the persistent X-ray counterpart of SGR 1627-41, we present
evidence for an association of this new SGR with the SNR G337.0-0.1; they both
appear to be located on the near side of the GMC. This is the second SGR
located near an extraordinarily massive GMC. We suggest that SGR 1627-41 is a
neutron star with a high transverse velocity (~ 1,000 \kms) escaping the young
(~ 5,000 years) supernova remnant G337.0-0.1Comment: 17 pages, including 2 figures. Accepted for publication in the
Astrophysical Journal Letter
Evidence for the Galactic X-ray Bulge II
A mosaic of 5 \ros~PSPC pointed observations in the Galactic plane
() reveals X-ray shadows in the keV band cast by
distant molecular clouds. The observed on-cloud and off-cloud X-ray fluxes
indicate that % and % of the diffuse X-ray background in this
direction in the \tq~keV and 1.5 keV bands, respectively, originates behind the
molecular gas which is located at 3 kpc from the Sun. The implication of
the derived background X-ray flux beyond the absorbing molecular cloud is
consistent with, and lends further support to recent observations of a Galactic
X-ray bulge.Comment: 19 pages, 5 figures, 2 table
Effective chiral-spin Hamiltonian for odd-numbered coupled Heisenberg chains
An system of odd number of coupled Heisenberg spin chains
is studied using a degenerate perturbation theory, where is the number of
coupled chains. An effective chain Hamiltonian is derived explicitly in terms
of two spin half degrees of freedom of a closed chain of sites, valid in
the regime the inter-chain coupling is stronger than the intra-chain coupling.
The spin gap has been calculated numerically using the effective Hamiltonian
for for a finite chain up to ten sites. It is suggested that the
ground state of the effective Hamiltonian is correlated, by examining
variational states for the effective chiral-spin chain Hamiltonian.Comment: 9 Pages, Latex, report ICTP-94-28
Multi-wavelength constraints on cosmic-ray leptons in the Galaxy
Cosmic rays (CRs) interact with the gas, the radiation field and the magnetic
field in the Milky Way, producing diffuse emission from radio to gamma rays.
Observations of this diffuse emission and comparison with detailed predictions
are powerful tools to unveil the CR properties and to study CR propagation. We
present various GALPROP CR propagation scenarios based on current CR
measurements. The predicted synchrotron emission is compared to radio surveys,
and synchrotron temperature maps from WMAP and Planck, while the predicted
interstellar gamma-ray emission is compared to Fermi-LAT observations. We show
how multi-wavelength observations of the Galactic diffuse emission can be used
to help constrain the CR lepton spectrum and propagation. Finally we discuss
how radio and microwave data could be used in understanding the diffuse
Galactic gamma-ray emission observed with Fermi-LAT, especially at low
energies.Comment: 8 pages, 5 figures; in Proceedings of the 34th International Cosmic
Ray Conference (ICRC 2015), The Hague (The Netherlands); Oral contributio
Propagation of cosmic-ray nucleons in the Galaxy
We describe a method for the numerical computation of the propagation of
primary and secondary nucleons, primary electrons, and secondary positrons and
electrons. Fragmentation and energy losses are computed using realistic
distributions for the interstellar gas and radiation fields, and diffusive
reacceleration is also incorporated. The models are adjusted to agree with the
observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and
convection do not account well for the observed energy dependence of B/C, while
models with reacceleration reproduce this easily. The height of the halo
propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc
for diffusion/convection models and 4-12 kpc for reacceleration models. For
convection models we set an upper limit on the velocity gradient of dV/dz < 7
km/s/kpc. The radial distribution of cosmic-ray sources required is broader
than current estimates of the SNR distribution for all halo sizes. Full details
of the numerical method used to solve the cosmic-ray propagation equation are
given.Comment: 15 pages including 23 ps-figures and 3 tables, latex2e, uses
emulateapj.sty (ver. of 11 May 1998, enclosed), apjfonts.sty, timesfonts.sty.
To be published in ApJ 1998, v.509 (December 10 issue). More details can be
found at http://www.gamma.mpe-garching.mpg.de/~aws/aws.html Some references
are correcte
Density Matrix Renormalization Group Study of the Spin 1/2 Heisenberg Ladder with Antiferromagnetic Legs and Ferromagnetic Rungs
The ground state and low lying excitation of the spin 1/2 Heisenberg ladder
with antiferromagnetic leg () and ferromagnetic rung () interaction is studied by means of the density matrix renormalization
group method. It is found that the state remains in the Haldane phase even for
small suggesting the continuous transition to the gapless
phase at . The critical behavior for small is studied by
the finite size scaling analysis. The result is consistent with the recent
field theoretical prediction.Comment: 11 pages, revtex, figures upon reques
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