4,792 research outputs found
The root of the problems of the British banks is the same as that of American banks: shaky mortgage-backed securities
Banks and banking - Great Britain ; Great Britain
Time-Resolved Ultraviolet Observations of the Globular Cluster X-ray Source in NGC 6624: The Shortest Known Period Binary System
Using the Faint Object Spectrograph (FOS) aboard the Hubble Space Telescope,
we have obtained the first time-resolved spectra of the King et al.
ultraviolet-bright counterpart to the 11-minute binary X-ray source in the core
of the globular cluster NGC 6624. This object cannot be readily observed in the
visible, even from HST, due to a much brighter star superposed <0.1'' distant.
Our FOS data show a highly statistically significant UV flux modulation with a
period of 11.46+-0.04 min, very similar to the 685 sec period of the known
X-ray modulation, definitively confirming the association between the King et
al. UV counterpart and the intense X-ray source. The UV amplitude is very large
compared with the observed X-ray oscillations: X-ray variations are generally
reported as 2-3% peak-to-peak, whereas our data show an amplitude of about 16%
in the 126-251 nm range. A model for the system by Arons & King predicts
periodic UV fluctuations in this shortest-known period binary system, due to
the cyclically changing aspect of the X-ray heated face of the secondary star
(perhaps a very low mass helium degenerate). However, prior to our
observations, this predicted modulation has not been detected. Employing the
Arons & King formalism, which invokes a number of different physical
assumptions, we infer a system orbital inclination 35deg<i<50 deg. Amongst the
three best-studied UV/optical counterparts to the intense globular cluster
X-ray sources, two are now thought to consist of exotic double-degenerate
ultrashort period binary systems.Comment: 10 pages including 2 figures in Latex (AASTeX 4.0). Accepted for
publication in vol. 482 (1997 June 10 issue) of The Astrophysical Journal
(Letters
Extended Lifetime in Computational Evolution of Isolated Black Holes
Solving the 4-d Einstein equations as evolution in time requires solving
equations of two types: the four elliptic initial data (constraint) equations,
followed by the six second order evolution equations. Analytically the
constraint equations remain solved under the action of the evolution, and one
approach is to simply monitor them ({\it unconstrained} evolution).
The problem of the 3-d computational simulation of even a single isolated
vacuum black hole has proven to be remarkably difficult. Recently, we have
become aware of two publications that describe very long term evolution, at
least for single isolated black holes. An essential feature in each of these
results is {\it constraint subtraction}. Additionally, each of these approaches
is based on what we call "modern," hyperbolic formulations of the Einstein
equations. It is generally assumed, based on computational experience, that the
use of such modern formulations is essential for long-term black hole
stability. We report here on comparable lifetime results based on the much
simpler ("traditional") - formulation.
We have also carried out a series of {\it constrained} 3-d evolutions of
single isolated black holes. We find that constraint solution can produce
substantially stabilized long-term single hole evolutions. However, we have
found that for large domains, neither constraint-subtracted nor constrained
- evolutions carried out in Cartesian coordinates admit
arbitrarily long-lived simulations. The failure appears to arise from features
at the inner excision boundary; the behavior does generally improve with
resolution.Comment: 20 pages, 6 figure
Intermediate Temperature Fluids Life Tests - Experiments
There are a number of different applications that could use heat pipes or loop heat pipes (LHPs) in the intermediate temperature range of 450 to 725 K (170 to 450 C), including space nuclear power system radiators, fuel cells, and high temperature electronics cooling. Historically, water has been used in heat pipes at temperatures up to about 425 K (150 C). Recent life tests, updated below, demonstrate that titanium/water and Monel/water heat pipes can be used at temperatures up to 550 K (277 C), due to water's favorable transport properties. At temperatures above roughly 570 K (300 C), water is no longer a suitable fluid, due to high vapor pressure and low surface tension as the critical point is approached. At higher temperatures, another working fluid/envelope combination is required, either an organic or halide working fluid. An electromotive force method was used to predict the compatibility of halide working fluids with envelope materials. This procedure was used to reject aluminum and aluminum alloys as envelope materials, due to their high decomposition potential. Titanium and three corrosion resistant superalloys were chosen as envelope materials. Life tests were conducted with these envelopes and six different working fluids: AlBr3, GaCl3, SnCl4, TiCl4, TiBr4, and eutectic diphenyl/diphenyl oxide (Therminol VP-1/Dowtherm A). All of the life tests except for the GaCl3 are ongoing; the GaCl3 was incompatible. As the temperature approaches 725 K (450 C), cesium is a potential heat pipe working fluid. Life tests results are also presented for cesium/Monel 400 and cesium/70-30 copper/nickel heat pipes operating near 750 K (477 C). These materials are not suitable for long term operation, due to copper transport from the condenser to the evaporator
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