6,804 research outputs found
Solar drum positioner mechanisms
The need for additional power on spinning satellites required development of deployable solar arrays activated, as on a 3-axis vehicle, after separation from a booster or shuttle orbiter. Mechanisms were developed for telescopically extending a secondary 36.3 kg (80 lb.), 2.13 m (84 in.) diameter spinning solar drum for a distance of 2.0 m (80 in.) or more along the spin axis. After extension, the system has the capability of dynamically controlling the drum tilt angle about the spin axis to provide precision in-orbit balancing of the spacecraft. This approach was selected for the SBS, ANIK C, ANIK D, WESTAR B and PALAPA B satellites. It was successfully demonstrated during the in orbit deployment of the aft solar panels of the SBS F-3 and F-1 satellites, subsequent to the November 1980 and September 1981 launches
Self-Consistent Cosmological Simulations of DGP Braneworld Gravity
We perform cosmological N-body simulations of the Dvali-Gabadadze-Porrati
braneworld model, by solving the full non-linear equations of motion for the
scalar degree of freedom in this model, the brane bending mode. While coupling
universally to matter, the brane-bending mode has self-interactions that become
important as soon as the density field becomes non-linear. These
self-interactions lead to a suppression of the field in high-density
environments, and restore gravity to General Relativity. The code uses a
multi-grid relaxation scheme to solve the non-linear field equation in the
quasi-static approximation. We perform simulations of a flat self-accelerating
DGP model without cosmological constant. The results of the DGP simulations are
compared with standard gravity simulations assuming the same expansion history,
and with DGP simulations using the linearized equation for the brane bending
mode. This allows us to isolate the effects of the non-linear self-couplings of
the field which are noticeable already on quasi-linear scales. We present
results on the matter power spectrum and the halo mass function, and discuss
the behavior of the brane bending mode within cosmological structure formation.
We find that, independently of CMB constraints, the self-accelerating DGP model
is strongly constrained by current weak lensing and cluster abundance
measurements.Comment: 21 pages; 10 figures. Revised version matching published versio
Cosmological Simulations of Normal-Branch Braneworld Gravity
We introduce a cosmological model based on the normal branch of DGP
braneworld gravity with a smooth dark energy component on the brane. The
expansion history in this model is identical to LambdaCDM, thus evading all
geometric constraints on the DGP cross-over scale r_c. This model can serve as
a first approximation to more general braneworld models whose cosmological
solutions have not been obtained yet. We study the formation of large scale
structure in this model in the linear and non-linear regime using N-body
simulations for different values of r_c. The simulations use the code presented
in (F.S., arXiv:0905.0858) and solve the full non-linear equation for the
brane-bending mode in conjunction with the usual gravitational dynamics. The
brane-bending mode is attractive rather than repulsive in the DGP normal
branch, hence the sign of the modified gravity effects is reversed compared to
those presented in arXiv:0905.0858. We compare the simulation results with
those of ordinary LambdaCDM simulations run using the same code and initial
conditions. We find that the matter power spectrum in this model shows a
characteristic enhancement peaking at k ~ 0.7 h/Mpc. We also find that the
abundance of massive halos is significantly enhanced. Other results presented
here include the density profiles of dark matter halos, and signatures of the
brane-bending mode self-interactions (Vainshtein mechanism) in the simulations.
Independently of the expansion history, these results can be used to place
constraints on the DGP model and future generalizations through their effects
on the growth of cosmological structure.Comment: 17 pages, 10 figures; v2: minor changes; v3: references added; v4:
added appendix on comparison with previous results; matches published
version; v5: corrected Eqs. (2.4-2.5) and Fig. 1 following Ref. [28]; all
following results unchange
Load programmer, static switches, and annunciator for inverters and converters
Load programmer, switches, and annunciator for static inverters and converters operating in paralle
BATSE Gamma-Ray Burst Line Search: V. Probability of Detecting a Line in a Burst
The physical importance of the apparent discrepancy between the detections by
pre-BATSE missions of absorption lines in gamma-ray burst spectra and the
absence of a BATSE line detection necessitates a statistical analysis of this
discrepancy. This analysis requires a calculation of the probability that a
line, if present, will be detected in a given burst. However, the connection
between the detectability of a line in a spectrum and in a burst requires a
model for the occurrence of a line within a burst. We have developed the
necessary weighting for the line detection probability for each spectrum
spanning the burst. The resulting calculations require a description of each
spectrum in the BATSE database. With these tools we identify the bursts in
which lines are most likely to be detected. Also, by assuming a small frequency
with which lines occur, we calculate the approximate number of BATSE bursts in
which lines of various types could be detected. Lines similar to the Ginga
detections can be detected in relatively few BATSE bursts; for example, in only
~20 bursts are lines similar to the GB 880205 pair of lines detectable. Ginga
reported lines at ~20 and ~40 keV whereas the low energy cutoff of the BATSE
spectra is typically above 20 keV; hence BATSE's sensitivity to lines is less
than that of Ginga below 40 keV, and greater above. Therefore the probability
that the GB 880205 lines would be detected in a Ginga burst rather than a BATSE
burst is ~0.2. Finally, we adopted a more appropriate test of the significance
of a line feature.Comment: 20 pages, AASTeX 4.0, 5 figures, Ap.J. in pres
An Efficient Parallel Solver for SDD Linear Systems
We present the first parallel algorithm for solving systems of linear
equations in symmetric, diagonally dominant (SDD) matrices that runs in
polylogarithmic time and nearly-linear work. The heart of our algorithm is a
construction of a sparse approximate inverse chain for the input matrix: a
sequence of sparse matrices whose product approximates its inverse. Whereas
other fast algorithms for solving systems of equations in SDD matrices exploit
low-stretch spanning trees, our algorithm only requires spectral graph
sparsifiers
The BATSE Gamma-Ray Burst Spectral Catalog. I. High Time Resolution Spectroscopy of Bright Bursts using High Energy Resolution Data
This is the first in a series of gamma-ray burst spectroscopy catalogs from
the Burst And Transient Source Experiment (BATSE) on the Compton Gamma Ray
Observatory, each covering a different aspect of burst phenomenology. In this
paper, we present time-sequences of spectral fit parameters for 156 bursts
selected for either their high peak flux or fluence. All bursts have at least
eight spectra in excess of 45 sigma above background and span burst durations
from 1.66 to 278 s. Individual spectral accumulations are typically 128 ms long
at the peak of the brightest events, but can be as short as 16 ms, depending on
the type of data selected. We have used mostly high energy resolution data from
the Large Area Detectors, covering an energy range of typically 28 - 1800 keV.
The spectral model chosen is from a small empirically-determined set of
functions, such as the well-known `GRB' function, that best fits the
time-averaged burst spectra. Thus, there are generally three spectral shape
parameters available for each of the 5500 total spectra: a low-energy power-law
index, a characteristic break energy and possibly a high-energy power-law
index. We present the distributions of the observed sets of these parameters
and comment on their implications. The complete set of data that accompanies
this paper is necessarily large, and thus is archived electronically at:
http://www.journals.uchicago.edu/ApJ/journal/.Comment: Accepted for publication: ApJS, 125. 38 pages, 9 figures;
supplementary electronic archive to be published by ApJ; available from lead
author upon reques
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