8,904 research outputs found
The empirical accuracy of uncertain inference models
Uncertainty is a pervasive feature of the domains in which expert systems are designed to function. Research design to test uncertain inference methods for accuracy and robustness, in accordance with standard engineering practice is reviewed. Several studies were conducted to assess how well various methods perform on problems constructed so that correct answers are known, and to find out what underlying features of a problem cause strong or weak performance. For each method studied, situations were identified in which performance deteriorates dramatically. Over a broad range of problems, some well known methods do only about as well as a simple linear regression model, and often much worse than a simple independence probability model. The results indicate that some commercially available expert system shells should be used with caution, because the uncertain inference models that they implement can yield rather inaccurate results
Rapid X-ray Variability of Seyfert 1 Galaxies
The rapid and seemingly random fluctuations in X-ray luminosity of Seyfert
galaxies provided early support for the standard model in which Seyferts are
powered by a supermassive black hole fed from an accretion disc. However, since
EXOSAT there has been little opportunity to advance our understanding of the
most rapid X-ray variability. Observations with XMM-Newton have changed this.
We discuss some recent results obtained from XMM-Newton observations of Seyfert
1 galaxies. Particular attention will be given to the remarkable similarity
found between the timing properties of Seyferts and black hole X-ray binaries,
including the power spectrum and the cross spectrum (time delays and
coherence), and their implications for the physical processes at work in
Seyferts.Comment: To appear in From X-ray Binaries to Quasars: Black Hole Accretion on
All Mass Scales, ed. T. J. Maccarone, R. P. Fender, and L. C. Ho (Dordrecht:
Kluwer
EPR entanglement strategies in two-well BEC
Criteria suitable for measuring entanglement between two different potential
wells in a Bose- Einstein condensation (BEC) are evaluated. We show how to
generate the required entanglement, utilizing either an adiabatic two-mode or
dynamic four-mode interaction strategy, with techniques that take advantage of
s-wave scattering interactions to provide the nonlinear coupling. The dynamic
entanglement method results in an entanglement signature with spatially
separated detectors, as in the Einstein-Podolsky-Rosen (EPR) paradox.Comment: 4 pages, 4 figure
Effects of Buoyancy on Laminar, Transitional, and Turbulent Gas Jet Diffusion Flames
Gas jet diffusion flames have been a subject of research for many years. However, a better understanding of the physical and chemical phenomena occurring in these flames is still needed, and, while the effects of gravity on the burning process have been observed, the basic mechanisms responsible for these changes have yet to be determined. The fundamental mechanisms that control the combustion process are in general coupled and quite complicated. These include mixing, radiation, kinetics, soot formation and disposition, inertia, diffusion, and viscous effects. In order to understand the mechanisms controlling a fire, laboratory-scale laminar and turbulent gas-jet diffusion flames have been extensively studied, which have provided important information in relation to the physico-chemical processes occurring in flames. However, turbulent flames are not fully understood and their understanding requires more fundamental studies of laminar diffusion flames in which the interplay of transport phenomena and chemical kinetics is more tractable. But even this basic, relatively simple flame is not completely characterized in relation to soot formation, radiation, diffusion, and kinetics. Therefore, gaining an understanding of laminar flames is essential to the understanding of turbulent flames, and particularly fires, in which the same basic phenomena occur. In order to improve and verify the theoretical models essential to the interpretation of data, the complexity and degree of coupling of the controlling mechanisms must be reduced. If gravity is isolated, the complication of buoyancy-induced convection would be removed from the problem. In addition, buoyant convection in normal gravity masks the effects of other controlling parameters on the flame. Therefore, the combination of normal-gravity and microgravity data would provide the information, both theoretical and experimental, to improve our understanding of diffusion flames in general, and the effects of gravity on the burning process in particular
Modeling the skin pattern of fishes
Complicated patterns showing various spatial scales have been obtained in the past by coupling Turing systems in such a way that the scales of the independent systems resonate. This produces superimposed patterns with different length scales. Here we propose a model consisting of two identical reaction-diffusion systems coupled together in such a way that one of them produces a simple Turing pattern of spots or stripes, and the other traveling wave fronts that eventually become stationary. The basic idea is to assume that one of the systems becomes fixed after some time and serves as a source of morphogens for the other system. This mechanism produces patterns very similar to the pigmentation patterns observed in different species of stingrays and other fishes. The biological mechanisms that support the realization of this model are discussed
Long-Term X-ray Spectral Variability in Seyfert 1 Galaxies
Direct time-resolved spectral fitting has been performed on continuous RXTE
monitoring of seven Seyfert 1 galaxies in order to study their broadband
spectral variability and Fe K alpha variability characteristics on time scales
of days to years. Variability in the Fe K alpha line is not detected in some
objects but is present in others, e.g., in NGC 3516, NGC 4151 and NGC 5548
there are systematic decreases in line flux by factors of ~2-5 over 3-4 years.
The Fe K alpha line varies less strongly than the broadband continuum, but,
like the continuum, exhibits stronger variability towards longer time scales.
Relatively less model-dependent broadband fractional variability amplitude
(Fvar) spectra also show weaker line variability compared to the continuum
variability. Comparable systematic long-term decreases in the line and
continuum are present in NGC 5548. Overall, however, there is no evidence for
correlated variability between the line and continuum, severely challenging
models in which the line tracks continuum variations modified only by a
light-travel time delay. Local effects such as the formation of an ionized skin
at the site of line emission may be relevant. The spectral fitting and Fvar
spectra both support spectral softening as continuum flux increases.Comment: Accepted for publication in ApJ. 29 page
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