16,542 research outputs found
On the continuous dependence with respect to sampling of the linear quadratic regulator problem for distributed parameter systems
The convergence of solutions to the discrete or sampled time linear quadratic regulator problem and associated Riccati equation for infinite dimensional systems to the solutions to the corresponding continuous time problem and equation, as the length of the sampling interval (the sampling rate) tends toward zero (infinity) is established. Both the finite and infinite time horizon problems are studied. In the finite time horizon case, strong continuity of the operators which define the control system and performance index together with a stability and consistency condition on the sampling scheme are required. For the infinite time horizon problem, in addition, the sampled systems must be stabilizable and detectable, uniformly with respect to the sampling rate. Classes of systems for which this condition can be verified are discussed. Results of numerical studies involving the control of a heat/diffusion equation, a hereditary of delay system, and a flexible beam are presented and discussed
The XMM-Newton spectral-fit database
The XMM-Newton spectral-fit database is an ongoing ESA funded project aimed
to construct a catalogue of spectral-fitting results for all the sources within
the XMM-Newton serendipitous source catalogue for which spectral data products
have been pipeline-extracted (~ 120,000 X-ray source detections). The
fundamental goal of this project is to provide the astronomical community with
a tool to construct large and representative samples of X-ray sources by
allowing source selection according to spectral properties.Comment: Conference proceedings of IAU Symposium 304: Multiwavelength AGN
surveys and studie
Numerical studies of identification in nonlinear distributed parameter systems
An abstract approximation framework and convergence theory for the identification of first and second order nonlinear distributed parameter systems developed previously by the authors and reported on in detail elsewhere are summarized and discussed. The theory is based upon results for systems whose dynamics can be described by monotone operators in Hilbert space and an abstract approximation theorem for the resulting nonlinear evolution system. The application of the theory together with numerical evidence demonstrating the feasibility of the general approach are discussed in the context of the identification of a first order quasi-linear parabolic model for one dimensional heat conduction/mass transport and the identification of a nonlinear dissipation mechanism (i.e., damping) in a second order one dimensional wave equation. Computational and implementational considerations, in particular, with regard to supercomputing, are addressed
Quasi-Particle Degrees of Freedom versus the Perfect Fluid as Descriptors of the Quark-Gluon Plasma
The hot nuclear matter created at the Relativistic Heavy Ion Collider (RHIC)
has been characterized by near-perfect fluid behavior. We demonstrate that this
stands in contradiction to the identification of QCD quasi-particles with the
thermodynamic degrees of freedom in the early (fluid) stage of heavy ion
collisions. The empirical observation of constituent quark ``'' scaling of
elliptic flow is juxtaposed with the lack of such scaling behavior in
hydrodynamic fluid calculations followed by Cooper-Frye freeze-out to hadrons.
A ``quasi-particle transport'' time stage after viscous effects break down the
hydrodynamic fluid stage, but prior to hadronization, is proposed to reconcile
these apparent contradictions. However, without a detailed understanding of the
transitions between these stages, the ``'' scaling is not a necessary
consequence of this prescription. Also, if the duration of this stage is too
short, it may not support well defined quasi-particles. By comparing and
contrasting the coalescence of quarks into hadrons with the similar process of
producing light nuclei from nucleons, it is shown that the observation of
``'' scaling in the final state does not necessarily imply that the
constituent degrees of freedom were the relevant ones in the initial state.Comment: 9 pages, 7 figures, Updated text and figure
ORFEUS II and IUE Spectroscopy of EX Hydrae
Using ORFEUS-SPAS II FUV spectra, IUE UV spectra, and archival EUVE deep
survey photometry, we present a detailed picture of the behavior of the
magnetic cataclysmic variable EX Hydrae. Like HUT spectra of this source, the
FUV and UV spectra reveal broad emission lines of He II, C II-IV, N III and V,
O VI, Si III-IV, and Al III superposed on a continuum which is blue in the UV
and nearly flat in the FUV. Like ORFEUS spectra of AM Her, the O VI doublet is
resolved into broad and narrow emission components. Consistent with its
behavior in the optical, the FUV and UV continuum flux densities, the FUV and
UV broad emission line fluxes, and the radial velocity of the O VI broad
emission component all vary on the spin phase of the white dwarf, with the
maximum of the FUV and UV continuum and broad emission line flux light curves
coincident with maximum blueshift of the broad O VI emission component. On the
binary phase, the broad dip in the EUV light curve is accompanied by strong
eclipses of the UV emission lines and by variations in both the flux and radial
velocity of the O VI narrow emission component. The available data are
consistent with the accretion funnel being the source of the FUV and UV
continuum and the O VI broad emission component, and the white dwarf being the
source of the O VI narrow emission component.Comment: 21 pages, 10 Postscript figures; LaTeX format, uses aaspp4.sty;
table2.tex included separately because it must be printed sideways - see
instructions in the file; accepted on 1999 Feb 20 for publication in The
Astrophysical Journa
Nonet Symmetry and Two-Body Decays of Charmed Mesons
The decay of charmed mesons into pseudoscalar (P) and vector (V) mesons is
studied in the context of nonet symmetry. We have found that it is badly broken
in the PP channels and in the P sector of the PV channels as expected from the
non-ideal mixing of the \eta and the \eta'. In the VV channels, it is also
found that nonet symmetry does not describe the data well. We have found that
this discrepancy cannot be attributed entirely to SU(3) breaking at the usual
level of 20--30%. At least one, or both, of nonet and SU(3) symmetry must be
very badly broken. The possibility of resolving the problem in the future is
also discussed.Comment: 9 pages, UTAPHY-HEP-
Laser driven launch vehicles for continuous access to space
The availability of megawatt laser systems in the next century will make laser launch systems from ground to orbit feasible and useful. Systems studies indicate launch capabilities of 1 ton payload per gigawatt laser power. Recent research in ground to orbit laser propulsion has emphasized laser supported detonation wave thrusters driven by repetitively pulsed infrared lasers. In this propulsion concept each laser repetition cycle consists of two pulses. A lower energy first pulse is used to vaporize a small amount of solid propellant and then after a brief expansion period, a second and higher energy laser pulse is used to drive a detonation wave through the expanded vapor. The results are reported of numerical studies comparing the detonation wave properties of various candidate propellants, and the simulation of thruster performance under realistic conditions. Experimental measurements designed to test the theoretical predictions are also presented. Measurements are discussed of radiance and opacity in absorption waves, and mass loss and momentum transfer. These data are interpreted in terms of specific impulse and energy conversion efficiency
Searching for highly obscured AGN in the XMM-Newton serendipitous source catalog
The majority of active galactic nuclei (AGN) are obscured by large amounts of
absorbing material that makes them invisible at many wavelengths. X-rays, given
their penetrating power, provide the most secure way for finding these AGN. The
XMM-Newton serendipitous source catalog is the largest catalog of X-ray sources
ever produced; it contains about half a million detections. These sources are
mostly AGN. We have derived X-ray spectral fits for very many 3XMM-DR4 sources
( 114 000 observations, corresponding to 77 000 unique
sources), which contain more than 50 source photons per detector. Here, we use
a subsample of 1000 AGN in the footprint of the SDSS area (covering
120 deg) with available spectroscopic redshifts. We searched for highly
obscured AGN by applying an automated selection technique based on X-ray
spectral analysis that is capable of efficiently selecting AGN. The selection
is based on the presence of either a) flat rest-frame spectra; b) flat observed
spectra; c) an absorption turnover, indicative of a high rest-frame column
density; or d) an Fe K line with an equivalent width > 500 eV. We found
81 highly obscured candidate sources. Subsequent detailed manual spectral fits
revealed that 28 of them are heavily absorbed by column densities higher than
10 cm. Of these 28 AGN, 15 are candidate Compton-thick AGN on the
basis of either a high column density, consistent within the 90% confidence
level with N 10 cm, or a large equivalent width
(>500 eV) of the Fe K line. Another six are associated with
near-Compton-thick AGN with column densities of 510
cm. A combination of selection criteria a) and c) for low-quality
spectra, and a) and d) for medium- to high-quality spectra, pinpoint highly
absorbed AGN with an efficiency of 80%.Comment: 18 pages, 10 figures, accepted for publication in A&
Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds
Two three-dimensional magnetohydrodynamical simulations of strongly
magnetized conical jets, one with a poloidal and one with a helical magnetic
field, have been performed. In the poloidal simulation a significant sheath
(wind) of magnetized moving material developed and partially stabilized the jet
to helical twisting. The fundamental pinch mode was not similarly affected and
emission knots developed in the poloidal simulation. Thus, astrophysical jets
surrounded by outflowing winds could develop knotty structures along a straight
jet triggered by pinching. Where helical twisting dominated the dynamics,
magnetic field orientation along the line-of-sight could be organized by the
toroidal flow field accompanying helical twisting. On astrophysical jets such
structure could lead to a reversal of the direction of Faraday rotation in
adjacent zones along a jet. Theoretical analysis showed that the different
dynamical behavior of the two simulations could be entirely understood as a
result of dependence on the velocity shear between jet and wind which must
exceed a surface Alfven speed before the jet becomes unstable to helical and
higher order modes of jet distortion.Comment: 25 pages, 15 figures, in press Astrophysical Journal (September
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