2,102 research outputs found
Influence of Exposure on Woody Vegetation on the Allen Soils of the Boston Mountain Region of Northwest Arkansas
A Semicoarsening Multigrid Algorithm for SIMD Machines
A semicoarsening multigrid algorithm suitable for use on single instruction multiple data (SIMD) architectures has been implemented on the CM-2. The method performs well for strongly anisotropic problems and for problems with coefficients jumping by orders of magnitude across internal interfaces. The parallel efficiency of this method is analyzed, and its actual performance is compared with its performance on some other machines, both parallel and nonparallel
Surface parameters of stannic oxide in powder, ceramic, and gel form by nitrogen adsorption techniques Interim report
Surface parameters of stannic oxide in powder, ceramic, and gel form by nitrogen adsorption techniques - analysis of adsorption isotherm
Distribution of Spectral Characteristics and the Cosmological Evolution of GRBs
We investigate the cosmological evolution of GRBs, using the total gamma ray
fluence as a measure of the burst strength. This involves an understanding of
the distributions of the spectral parameters of GRBs as well as the total
fluence distribution - both of which are subject to detector selection effects.
We present new non-parametric statistical techniques to account for these
effects, and use these methods to estimate the true distribution of the peak of
the nu F_nu spectrum, E_p, from the raw distribution. The distributions are
obtained from four channel data and therefore are rough estimates. Here, we
emphasize the methods and present qualitative results. Given its spectral
parameters, we then calculate the total fluence for each burst, and compute its
cumulative and differential distributions. We use these distributions to
estimate the cosmological rate evolution of GRBs, for three cosmological
models. Our two main conclusions are the following: 1) Given our estimates of
the spectral parameters, we find that there may exist a significant population
of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving
rate density quite different from that of other extragalactic objects; in
particular, it is different from the recently determined star formation rate.Comment: 20 pages, including 10 postscript figures. Submitted to Ap
No Persistent Pulsations in Aquila X-1 as it Fades into Quiescence
We searched for coherent X-ray pulsations from Aql X-1 in a series of RXTE observations taken shortly after a recent outburst. During the course of these observations, Aql X-1 passes through an apparent "propeller" phase as its luminosity fades to its quiescent value. No pulsations were detected, and we place upper limits (ranging from 0.52% to 9.0%) on the fractional rms amplitude of any periodic signal contained in the various data sets searched. This result has implications for the geometry of the system, if the quiescent luminosity is due to continued low-level accretion. Alternatively, our result supports the idea that the quiescent luminosity may be due to thermal emission
Cosmological versus Intrinsic: The Correlation between Intensity and the Peak of the nu F_nu Spectrum of Gamma Ray Bursts
We present results of correlation studies, examining the association between
the peak of the nu F_nu spectrum of gamma ray bursts, E_p, with the burst's
energy fluence and photon peak flux. We discuss methods to account for data
truncation in E_p and fluence or flux when performing the correlation analyses.
However, because bursts near the detector threshold are not usually able to
provide reliable spectral parameters, we focus on results for the brightest
bursts in which we can better understand the selection effects relevant to E_p
and burst strength.
We find that there is a strong correlation between total fluence and E_p. We
discuss these results in terms of both cosmological and intrinsic effects.
In particular, we show that for realistic distributions of the burst
parameters, cosmological expansion alone cannot account for the correlation
between E_p and total fluence; the observed correlation is likely a result of
an intrinsic relation between the burst rest-frame peak energy and the total
radiated energy. We investigate this latter scenario in the context of
synchrotron radiation from external and internal shock models of GRBs. We find
that the internal shock model is consistent with our interpretation of the
correlation, while the external shock model cannot easily explain this
intrinsic relation between peak energy and burst radiated energy.Comment: 23 pages, including 8 postscript figures. Submitted to Ap
Quiescent X-ray variability from the neutron star transient Aql X-1
A number of studies have revealed variability from neutron star low-mass
X-ray binaries during quiescence. Such variability is not well characterised,
or understood, but may be a common property that has been missed due to lack of
multiple observations. One such source where variability has been observed is
Aql X-1. Here, we analyse 14 Chandra and XMM-Newton observations of Aql X-1 in
quiescence, covering a period of approximately 2 years. There is clear
variability between the epochs, with the most striking feature being a
flare-like increase in the flux by a factor of 5. Spectral fitting is
inconclusive as to whether the power-law and/or thermal component is variable.
We suggest that the variability and flare-like behaviour during quiescence is
due to accretion at low rates which might reach the neutron star surface.Comment: 8 pages, 5 figures, accepted for publication in MNRA
A Comparison of Early Childhood Health Indicators
The abstract for this presentation can be downloaded by clicking on the blue download button
Effects of dust accumulation and removal on radiators surfaces on Mars
Tests were carried out to assess the impact of wind blown dust accumulation and abrasion on radiator surfaces on Mars. High emittance arc-textured copper and niobium-1 percent-zirconium samples were subjected to basaltic dust laden wind at Martian pressure (1000 Pa) at speeds varying from 19 to 97 m/s in the Martian Surface Wind Tunnel. The effect of accumulated dust was also observed by pre-dusting some of the samples before the test. Radiator degradation was determined by measuring the change in the emittance after dust was deposited and/or removed. The principle mode of degradation was abrasion. Arc textured Nb-1 percent-Zr proved to be more susceptible to degradation than Cu, and pre-dusting appeared to have lessened the abrasion
Disposal of Household Wastewater in Soils of High Stone Content (1981-1983)
Four experimental filter fields were constructed with built-in monitoring equipment in Nixa soils. These soils contain many chert fragments and a fragipan about 60 cm below the soil surface. The fragipan restricts downward movement of water and is the designlimitingfeature. The four filter fields were: 1. A standard filter field, 76 cm deep. The bottom of the trench was in the fragipan. 2. A modified standard filter field, 30 cm deep. The bottom of the trench was above the fragipan. 3. A modified pressure filter field, 40 cm deep. The bottom of the trench was above the fragipan. In addition, a pressure-distribution system was used to insure uniform distribution of effluent in the trench. Inadvertently, this field was installed in a different soil, and the results cannot be compared directly with the other three. 4. Another modified pressure filter field with the bottom of the trench only 6 cm below the soil surface. Observation of these systems confirms that placing filter fields higher in the soil above the hydraulically limiting horizon results in improved hydraulic performance. The presence of the fragipan amplified the adverse effects attributable to climatic stress. The seepage beds which are higher in the soil profile are able to handle the effluent load and climate load with less danger of surfacing
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