37,299 research outputs found
Communications satellite systems operations with the space station. Volume 1: Executive summary
The benefits of new space-based activities are quantified and the impacts on the satellite design and the space station are assessed
Evidence for accretion rate change during type I X-ray bursts
The standard approach for time-resolved X-ray spectral analysis of
thermonuclear bursts involves subtraction of the pre-burst emission as
background. This approach implicitly assumes that the persistent flux remains
constant throughout the burst. We reanalyzed 332 photospheric radius expansion
bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing
a multiplicative factor to the persistent emission contribution in our
spectral fits. We found that for the majority of spectra the best-fit value of
is significantly greater than 1, suggesting that the persistent emission
typically increases during a burst. Elevated values were not found solely
during the radius expansion interval of the burst, but were also measured in
the cooling tail. The modified model results in a lower average value of the
fit statistic, indicating superior spectral fits, but not yet to the
level of formal statistical consistency for all the spectra.
We interpret the elevated values as an increase of the mass accretion
rate onto the neutron star during the burst, likely arising from the effects of
Poynting-Robertson drag on the disk material. We measured an inverse
correlation of with the persistent flux, consistent with theoretical
models of the disc response. We suggest that this modified approach may provide
more accurate burst spectral parameters, as well as offering a probe of the
accretion disk structure.Comment: 15 pages, 12 figures, 4 table
Evidence for enhanced persistent emission during sub-Eddington thermonuclear bursts
The standard approach for time-resolved X-ray spectral analysis of
thermonuclear bursts involves subtraction of the pre-burst emission as
background. This approach implicitly assumes that the persistent flux remains
constant throughout the burst. We reanalyzed 332 photospheric radius expansion
bursts observed from 40 sources by the Rossi X-ray Timing Explorer, introducing
a multiplicative factor to the persistent emission contribution in our
spectral fits. We found that for the majority of spectra the best-fit value of
is significantly greater than 1, suggesting that the persistent emission
typically increases during a burst. Elevated values were not found solely
during the radius expansion interval of the burst, but were also measured in
the cooling tail. The modified model results in a lower average value of the
fit statistic, indicating superior spectral fits, but not yet to the
level of formal statistical consistency for all the spectra.
We interpret the elevated values as an increase of the mass accretion
rate onto the neutron star during the burst, likely arising from the effects of
Poynting-Robertson drag on the disk material. We measured an inverse
correlation of with the persistent flux, consistent with theoretical
models of the disc response. We suggest that this modified approach may provide
more accurate burst spectral parameters, as well as offering a probe of the
accretion disk structure.Comment: 15 pages, 9 figure
An integrated LANDSAT/ancillary data classification of desert rangeland
Range inventorying methods using LANDSAT MSS data, coupled with ancillary data were examined. The study area encompassed nearly 20,000 acres in Rush Valley, Utah. The vegetation is predominately desert shrub and annual grasses, with some annual forbs. Three LANDSAT scenes were evaluated using a Kauth-Thomas brightness/greenness data transformation (May, June, and August dates). The data was classified using a four-band maximum-likelihood classifier. A print map was taken into the field to determine the relationship between print symbols and vegetation. It was determined that classification confusion could be greatly reduced by incorporating geomorphic units and soil texture (coarse vs fine) into the classification. Spectral data, geomorphic units, and soil texture were combined in a GIS format to produce a final vegetation map identifying 12 vegetation types
Survival of fossils under extreme shocks induced by hypervelocity impacts
Experimental data are shown for survival of fossilized diatoms undergoing shocks in the GPa range. The results were obtained from hypervelocity impact experiments which fired fossilized diatoms frozen in ice into water targets. After the shots, the material recovered from the target water was inspected for diatom fossils. Nine shots were carried out, at speeds from 0.388 to 5.34?km?s?1, corresponding to mean peak pressures of 0.2–19?GPa. In all cases, fragmented fossilized diatoms were recovered, but both the mean and the maximum fragment size decreased with increasing impact speed and hence peak pressure. Examples of intact diatoms were found after the impacts, even in some of the higher speed shots, but their frequency and size decreased significantly at the higher speeds. This is the first demonstration that fossils can survive and be transferred from projectile to target in hypervelocity impacts, implying that it is possible that, as suggested by other authors, terrestrial rocks ejected from the Earth by giant impacts from space, and which then strike the Moon, may successfully transfer terrestrial fossils to the Moon
Atlantic Exchange: Case Studies of Housing and Community Redevelopment in the United States and the United Kingdom
Examines lessons learned from community redevelopment initiatives in Birmingham, England, and Chicago. Explores physical, managerial, and demographic changes and issues of place identity, community cohesion, and the communities' place in city initiatives
Photoproduction of Xi off nucleons
The photoproduction reaction is investigated based on
a relativistic meson-exchange model of hadronic interactions. The production
amplitude is calculated in the tree-level approximation from relevant effective
Lagrangians, whose (coupling constant) parameters are mostly fixed from the
empirical data and/or quark models together with SU(3) symmetry considerations.
Gauge invariance of the resulting amplitude is maintained by introducing the
contact currents by extending the gauge-invariant approach of Haberzettl for
one-meson photoproduction to two-meson photoproduction. The role of the
intermediate low-lying hyperons and of the intermediate higher-mass hyperon
resonances are analyzed in detail. In particular, the basic features of the
production of in and their possible
manifestations in the forthcoming experimental data are discussed.Comment: 18 pages, 17 figures, REVTeX, 1 figure added, to appear in Phys. Rev.
- …