122,627 research outputs found
Satellite personal communications system
Voice channel communication between low power mobile stations dispersed over a large area is provided by a system which includes a geostationary satellite utilizing a large UHF antenna that can receive a transmission from a caller and retransmit it over any one beam of a matrix of narrow beams, so the chosen beam covers an area in which a designated called party is located. A single up-link control channel occupying a narrow frequency band, can be utilized to receive dial up signals from a caller, and another single down link control channel can be utilized to ring up the called party located anywhere within the continental United States. The satellite antenna includes a matrix of feed horns that not only direct the beams in a controlled matrix onto the area of the continental United States, but also permit detection of the region from which the caller's signal is transmitted and the region from which the called party's answer is received, to enable the interconnection of signals received from these two regions. The system is particularly useful for rural areas
Design and construction of the IMACS-IFU, a 2000-element integral field unit
The IMACS-IFU is an Integral Field Unit built for the IMACS spectrograph at
the Magellan-I-Telescope at Las Campanas Observatory. It consists of two
rectangular fields of 5 by 7 arcseconds, separated by roughly one arcminute.
With a total number of 2000 spatial elements it is the second largest
fiber-lenslet based IFU worldwide, working in a wavelength range between 400
and 900 nm. Due to the equally sized fields classical background subtraction,
beam switching and shuffling are possible observation techniques. One
particular design challenge was the single, half a metre long curved slit in
combination with a non telecentric output. Besides the construction some
preliminary results are described.Comment: 10 pages, 7 figures, 1 table. Proceedings for SPIE poster 5492-175 of
SPIE Symposium "Astronomical Telescopes and Instrumentation", June 2004,
Glasgo
An implicit finite-difference solution to the viscous shock layer, including the effects of radiation and strong blowing
An implicit finite-difference scheme is developed for the fully coupled solution of the viscous, radiating stagnation-streamline equations, including strong blowing. Solutions are presented for both air injection and injection of carbon-phenolic ablation products into air at conditions near the peak radiative heating point in an earth entry trajectory from interplanetary return missions. A detailed radiative-transport code that accounts for the important radiative exchange processes for gaseous mixtures in local thermodynamic and chemical equilibrium is utilized in the study. With minimum number of assumptions for the initially unknown parameters and profile distributions, convergent solutions to the full stagnation-line equations are rapidly obtained by a method of successive approximations. Damping of selected profiles is required to aid convergence of the solutions for massive blowing. It is shown that certain finite-difference approximations to the governing differential equations stabilize and improve the solutions. Detailed comparisons are made with the numerical results of previous investigations. Results of the present study indicate lower radiative heat fluxes at the wall for carbonphenolic ablation than previously predicted
Radio and X-ray study of two multi-shell Supernova Remnants: Kes79 and G352.7-0.1
We investigate two multi-shell galactic supernova remnants (SNRs), Kes79 and
G352.7-0.1, to understand the causes of such morphology. The research was
carried out based on new and reprocessed archival VLA observations and
XMM-Newton archival data. The surrounding was investigated based on data
extracted from the HI Canadian Galactic Plane Survey, the 13^CO Galactic Ring
Survey and the HI Southern Galactic Plane Survey. The present study revealed
that the overall morphology of both SNRs is the result of the mass-loss history
of their respective progenitor stars. Kes79 would be the product of the
gravitational collapse of a massive O9 star evolving near a molecular cloud and
within the precursor's wind-driven bubble, while G352.7-0.1 would be the result
of interactions of the SNR with an asymmetric wind from the progenitor together
with projection effects. No radio point source or pulsar wind nebula was found
associated with the X-ray pulsar CXOU J185238.6+004020 in Kes79. The X-ray
study of G352.7-0.1, on its hand, revealed that most of the thermal X-ray
radiation completely fills in the interior of the remnant and originates in
heated ejecta. Characteristic parameters, like radio flux, radio spectral
index, age, distance, shock velocity, initial energy and luminosity, were
estimated for both SNRs.Comment: 14 pages, 13 figures. Accepted to be published in Astronomy and
Astrophysic
Using Big Bang Nucleosynthesis to Extend CMB Probes of Neutrino Physics
We present calculations showing that upcoming Cosmic Microwave Background
(CMB) experiments will have the power to improve on current constraints on
neutrino masses and provide new limits on neutrino degeneracy parameters. The
latter could surpass those derived from Big Bang Nucleosynthesis (BBN) and the
observationally-inferred primordial helium abundance. These conclusions derive
from our Monte Carlo Markov Chain (MCMC) simulations which incorporate a full
BBN nuclear reaction network. This provides a self-consistent treatment of the
helium abundance, the baryon number, the three individual neutrino degeneracy
parameters and other cosmological parameters. Our analysis focuses on the
effects of gravitational lensing on CMB constraints on neutrino rest mass and
degeneracy parameter. We find for the PLANCK experiment that total (summed)
neutrino mass eV could be ruled out at or better.
Likewise neutrino degeneracy parameters and could be detected or ruled out at
confidence, or better. For POLARBEAR we find that the corresponding detectable
values are , , and , while for EPIC we obtain ,
, and . Our forcast for
EPIC demonstrates that CMB observations have the potential to set constraints
on neutrino degeneracy parameters which are better than BBN-derived limits and
an order of magnitude better than current WMAP-derived limits.Comment: 27 pages, 11 figures, matches published version in JCA
The most complete and detailed X-ray view of the SNR Puppis A
With the purpose of producing the first detailed full view of Puppis A in
X-rays, we carried out new XMM-Newton observations covering the missing regions
in the southern half of the supernova remnant (SNR) and combined them with
existing XMM-Newton and Chandra data. The new images were produced in the
0.3-0.7, 0.7-1.0 and 1.0-8.0 energy bands. We investigated the SNR morphology
in detail, carried out a multi-wavelength analysis and estimated the flux
density and luminosity of the whole SNR. The complex structure observed across
the remnant confirms that Puppis A evolves in an inhomogeneous, probably knotty
interstellar medium. The southwestern corner includes filaments that perfectly
correlate with radio features suggested to be associated with shock/cloud
interaction. In the northern half of Puppis A the comparison with Spitzer
infrared images shows an excellent correspondence between X-rays and 24 and 70
microns emission features, while to the south there are some matched and other
unmatched features. X-ray flux densities of 12.6 X 10^-9, 6.2 X 10^-9, and 2.8
X 10^-9 erg cm^-2 s^-1 were derived for the 0.3-0.7, 0.7-1.0 and 1.0-8.0 keV
bands, respectively. At the assumed distance of 2.2 kpc, the total X-ray
luminosity between 0.3 and 8.0 keV is 1.2 X 10^37 erg s^-1. We also collected
and updated the broad-band data of Puppis A between radio and GeV gamma-ray
range, producing its spectral energy distribution. To provide constraints to
the high-energy emission models, we re-analyzed radio data, estimating the
energy content in accelerated particles to be Umin=4.8 X 10^49 erg and the
magnetic field strength B=26 muG.Comment: Article accepted to be published in the Astronomy and Astrophysics
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