2,687 research outputs found
Spectral Theory of Time Dispersive and Dissipative Systems
We study linear time dispersive and dissipative systems. Very often such
systems are not conservative and the standard spectral theory can not be
applied. We develop a mathematically consistent framework allowing (i) to
constructively determine if a given time dispersive system can be extended to a
conservative one; (ii) to construct that very conservative system -- which we
show is essentially unique. We illustrate the method by applying it to the
spectral analysis of time dispersive dielectrics and the damped oscillator with
retarded friction. In particular, we obtain a conservative extension of the
Maxwell equations which is equivalent to the original Maxwell equations for a
dispersive and lossy dielectric medium.Comment: LaTeX, 57 Pages, incorporated revisions corresponding with published
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Quasi-periodic X-ray brightness fluctuations in an accreting millisecond pulsar
The relativistic plasma flows onto neutron stars that are accreting material
from stellar companions can be used to probe strong-field gravity as well as
the physical conditions in the supranuclear-density interiors of neutron stars.
Plasma inhomogeneities orbiting a few kilometres above the stars are observable
as X-ray brightness fluctuations on the millisecond dynamical timescale of the
flows. Two frequencies in the kilohertz range dominate these fluctuations: the
twin kilohertz quasi-periodic oscillations (kHz QPOs). Competing models for the
origins of these oscillations (based on orbital motions) all predict that they
should be related to the stellar spin frequency, but tests have been difficult
because the spins were not unambiguously known. Here we report the detection of
kHz QPOs from a pulsar whose spin frequency is known. Our measurements
establish a clear link between kHz QPOs and stellar spin, but one not predicted
by any current model. A new approach to understanding kHz QPOs is now required.
We suggest that a resonance between the spin and general relativistic orbital
and epicyclic frequencies could provide the observed relation between QPOs and
spin.Comment: Published in the 2003 July 3 issue of Natur
Liquid-gas phase transition in hot asymmetric nuclear matter with density-dependent relativistic mean-field models
The liquid-gas phase transition in hot asymmetric nuclear matter is studied
within density-dependent relativistic mean-field models where the density
dependence is introduced according to the Brown-Rho scaling and constrained by
available data at low densities and empirical properties of nuclear matter. The
critical temperature of the liquid-gas phase transition is obtained to be 15.7
MeV in symmetric nuclear matter falling on the lower edge of the small
experimental error bars. In hot asymmetric matter, the boundary of the
phase-coexistence region is found to be sensitive to the density dependence of
the symmetry energy. The critical pressure and the area of phase-coexistence
region increases clearly with the softening of the symmetry energy. The
critical temperature of hot asymmetric matter separating the gas phase from the
LG coexistence phase is found to be higher for the softer symmetry energy.Comment: To be published in Phys. Lett.
Suspended-sediment induced stratification inferred from concentration and velocity profile measurements in the lower Yellow River, China
Despite a multitude of models predicting sediment transport dynamics in openâchannel flow, selfâorganized vertical density stratification that dampens flow turbulence due to the interaction between fluid and sediment, has not been robustly validated with field observations from natural rivers. Turbulenceâsuppressing density stratification can develop in channels with low channelâbed slope and high sediment concentration. As the Yellow River, China, maintains one of the highest sediment loads in the world for a low sloping system, this location is ideal for documenting particle and fluid interactions that give rise to density stratification. Herein, we present analyses from a study conducted over a range of discharge conditions (e.g., low flow, rising limb, and flood peak) from a lower reach of the Yellow River, whereby water samples were collected at targeted depths to measure sediment concentration and, simultaneously, velocity measurements were collected throughout the flow depth. Importantly, sediment concentration varied by an order of magnitude between base and flood flows. By comparing measured concentration and velocity profiles to predictive models, we show that the magnitude of density stratification increases with sediment concentration. Furthermore, a steadyâstate calculation of sediment suspension is used to determine that sediment diffusivity increases with grain size. Finally, we calculate concentration and velocity profiles, showing that steadyâstate sediment suspensions are reliably predicted over a range of stratification conditions larger than had been previously documented in natural river flows. We determine that the magnitude of density stratification can be predicted by a function considering an entrainment parameter, sediment concentration, and bed slope
Pasireotide Long-Acting Release Treatment for Diabetic Cats with Underlying Hypersomatotropism
BACKGROUND: Longâterm medical management of hypersomatotropism (HS) in cats has proved unrewarding. Pasireotide, a novel somatostatin analogue, decreases serum insulinâlike growth factor 1 (IGFâ1) and improves insulin sensitivity in cats with HS when administered as a shortâacting preparation. OBJECTIVES: Assess onceâmonthly administration of longâacting pasireotide (pasireotide LAR) for treatment of cats with HS. ANIMALS: Fourteen cats with HS, diagnosed based on diabetes mellitus, pituitary enlargement, and serum IGFâ1 > 1000 ng/mL. METHODS: Uncontrolled, prospective cohort study. Cats received pasireotide LAR (6â8 mg/kg SC) once monthly for 6 months. Fructosamine and IGFâ1 concentrations, and 12âhour blood glucose curves (BGCs) were assessed at baseline and then monthly. Product of fructosamine concentration and insulin dose was calculated as an indicator of insulin resistance (Insulin Resistance Index). Linear mixedâeffects modeling assessed for significant change in fructosamine, IGFâ1, mean blood glucose (MBG) of BGCs, insulin dose (U/kg) and Insulin Resistance Index. RESULTS: Eight cats completed the trial. Three cats entered diabetic remission. Median IGFâ1 (baseline: 1962 ng/mL [range 1051â2000 ng/mL]; month 6: 1253 ng/mL [524â1987 ng/mL]; P < .001) and median Insulin Resistance Index (baseline: 812 ÎŒmolU/L kg [173â3565 ÎŒmolU/L kg]; month 6: 135 ÎŒmolU/L kg [0â443 ÎŒmolU/L kg]; P = .001) decreased significantly. No significant change was found in mean fructosamine (baseline: 494 ± 127 ÎŒmol/L; month 6: 319 ± 113.3 ÎŒmol/L; P = .07) or MBG (baseline: 347.7 ± 111.0 mg/dL; month 6: 319.5 ± 113.3 mg/dL; P = .11), despite a significant decrease in median insulin dose (baseline: 1.5 [0.4â5.2] U/kg; 6 months: 0.3 [0.0â1.4] U/kg; P < .001). Adverse events included diarrhea (n = 11), hypoglycemia (n = 5), and worsening polyphagia (n = 2). CONCLUSIONS AND CLINICAL IMPORTANCE: Pasireotide LAR is the first drug to show potential as a longâterm management option for cats with HS
Effects of isospin and momentum dependent interactions on liquid-gas phase transition in hot asymmetric nuclear matter
The liquid-gas phase transition in hot neutron-rich nuclear matter is
investigated within a self-consistent thermal model using an isospin and
momentum dependent interaction (MDI) constrained by the isospin diffusion data
in heavy-ion collisions, a momentum-independent interaction (MID), and an
isoscalar momentum-dependent interaction (eMDYI). The boundary of the
phase-coexistence region is shown to be sensitive to the density dependence of
the nuclear symmetry energy with a softer symmetry energy giving a higher
critical pressure and a larger area of phase-coexistence region. Compared with
the momentum-independent MID interaction, the isospin and momentum-dependent
MDI interaction is found to increase the critical pressure and enlarge the area
of phase-coexistence region. For the isoscalar momentum-dependent eMDYI
interaction, a limiting pressure above which the liquid-gas phase transition
cannot take place has been found and it is shown to be sensitive to the
stiffness of the symmetry energy.Comment: 6 pages, 4 figures, revised version, to appear in PL
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