3,404 research outputs found
An evaluation of the utilization of remote sensing in resource and environmental management of the Chesapeake Bay region
A nine-month study was conducted to assess the effectiveness of the NASA Wallops Chesapeake Bay Ecological Program in remote sensing. The study consisted of a follow-up investigation and information analysis of actual cases in which remote sensing was utilized by management and research personnel in the Chesapeake Bay region. The study concludes that the NASA Wallops Chesapeake Bay Ecological Program is effective, both in terms of costs and performance
Many-Spin Effects and Tunneling Properties of Magnetic Molecules
Spin tunneling in molecular magnets has attracted much attention, however
theoretical considerations of this phenomenon up to now have not taken into
account the many-spin nature of molecular magnets. We present, to our
knowledge, the first successful attempt of a realistic calculation of tunneling
splittings for Mn molecules, thus achieving a quantitatively accurate
many-spin description of a real molecular magnet in the energy interval ranging
from about 100 K down to 10 K. Comparison with the results of the
standard single-spin model shows that many-spin effects affect the tunneling
splittings considerably. The values of ground state splitting given by
single-spin and many-spin models differ from each other by a factor of five.Comment: 3REVTeX pages, 2 figure
A Search for Hard X-Ray Emission from Globular Clusters - Constraints from BATSE
We have monitored a sample of 27 nearby globular clusters in the hard X-ray
band (20-120 keV) for approximately 1400 days using the BATSE instrument on
board the Compton Gamma-Ray Observatory. Globular clusters may contain a large
number of compact objects (e.g., pulsars or X-ray binaries containing neutron
stars) which can produce hard X-ray emission. Our search provides a sensitive
(~50 mCrab) monitor for hard X-ray transient events on time scales of >1 day
and a means for observing persistent hard X-ray emission. We have discovered no
transient events from any of the clusters and no persistent emission. Our
observations include a sensitive search of four nearby clusters containing dim
X-ray sources: 47 Tucanae, NGC 5139, NGC 6397, and NGC 6752. The non-detection
in these clusters implies a lower limit for the recurrence time of transients
of 2 to 6 years for events with luminosities >10^36 erg s^-1 (20-120 keV) and
~20 years if the sources in these clusters are taken collectively. This
suggests that the dim X-ray sources in these clusters are not transients
similar to Aql~X-1. We also place upper limits on the persistent emission in
the range 2-10*10^34 erg s^-1 (2 sigma, 20-120 keV) for these four clusters.
For 47 Tuc the upper limit is more sensitive than previous measurements by a
factor of 3. We find a model dependent upper limit of 19 isolated millisecond
pulsars (MSPs) producing gamma-rays in 47 Tuc, compared to the 11 observed
radio MSPs in this cluster.Comment: 20 pages; accepted, ApJ; uu encoded tar file; 7 figure
Isogrid design handbook
Handbook has been published which presents information needed for design of isogrid triangular integral-stiffened structures. It develops equations, methods, and graphs to handle wide variety of loadings, materials, and geometry. Handbook is divided into seven sections. Handbook may be used by marine and civil engineers and by students and designers without access to computers
Discovery of Pulsed X-ray Emission from the SMC Transient RX J0117.6-7330
We report on the detection of pulsed, broad-band, X-ray emission from the
transient source RX J0117.6-7330. The pulse period of 22 seconds is detected by
the ROSAT/PSPC instrument in a 1992 Sep 30 - Oct 2 observation and by the
CGRO/BATSE instrument during the same epoch. Hard X-ray pulsations are
detectable by BATSE for approximately 100 days surrounding the ROSAT
observation (1992 Aug 28 - Dec 8). The total directly measured X-ray luminosity
during the ROSAT observation is 1.0E38 (d/60 kpc)^2 ergs s-1. The pulse
frequency increases rapidly during the outburst, with a peak spin-up rate of
1.2E-10 Hz s-1 and a total frequency change 1.8%. The pulsed percentage is
11.3% from 0.1-2.5 keV, increasing to at least 78% in the 20-70 keV band. These
results establish RX J0117.6-7330 as a transient Be binary system.Comment: 17 pages, Latex, aasms, accepted for publication in ApJ Letter
Dynamical control of electron spin coherence in a quantum dot
We investigate the performance of dynamical decoupling methods at suppressing
electron spin decoherence from a low-temperature nuclear spin reservoir in a
quantum dot. The controlled dynamics is studied through exact numerical
simulation, with emphasis on realistic pulse delays and long-time limit. Our
results show that optimal performance for this system is attained by a periodic
protocol exploiting concatenated design, with control rates substantially
slower than expected from the upper spectral cutoff of the bath. For a known
initial electron spin state, coherence can saturate at long times, signaling
the creation of a stable ``spin-locked'' decoherence-free subspace. Analytical
insight on saturation is obtained for a simple echo protocol, in good agreement
with numerical results.Comment: 4 pages, 4 figures with 3 of them in colo
Teachers\u27 Beliefs and Practices of Vocabulary Instruction With Social Studies Textbooks in Grades 4-8
This investigation focused on the vocabulary beliefs and instructional practices of social studies teachers in intermediate and middle school grades as well as their use of teachers\u27 manuals. Using a self-reporting survey to measure these beliefs and practices, we found some discrepancy between what teachers believe about vocabulary learning and their actual instructional practices for supporting vocabulary in teaching social studies. While their reported beliefs appear to mirror what is currently accepted as effective vocabulary instruction, their reported practices reflect more traditional notions like those found in many social studies textbook manuals. While all teachers surveyed held many beliefs and practices in common, three beliefs and three practices were differentially affected by grade level, economic status, or number of years of teacher experience
Long-term source monitoring with BATSE
The uncollimated Burst and Transient Source Experiment (BATSE) large area detectors (LADs) are well suited to nearly continuous monitoring of the stronger hard x-ray sources, and time series analysis for pulsars. An overview of the analysis techniques presently being applied to the data are discussed, including representative observations of the Crab Nebula, Crab pulsar, and summaries of the sources detected to data. Results of a search for variability in the Crab Pulsar pulse profile are presented
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