1,300 research outputs found
Twin-Photon Confocal Microscopy
A recently introduced two-channel confocal microscope with correlated
detection promises up to 50% improvement in transverse spatial resolution
[Simon, Sergienko, Optics Express {\bf 18}, 9765 (2010)] via the use of photon
correlations. Here we achieve similar results in a different manner,
introducing a triple-confocal correlated microscope which exploits the
correlations present in optical parametric amplifiers. It is based on tight
focusing of pump radiation onto a thin sample positioned in front of a
nonlinear crystal, followed by coincidence detection of signal and idler
photons, each focused onto a pinhole. This approach offers further resolution
enhancement in confocal microscopy
Complex Langevin dynamics for dynamical QCD at nonzero chemical potential: a comparison with multi-parameter reweighting
We study lattice QCD at non-vanishing chemical potential using the complex
Langevin equation. We compare the results with multi-parameter reweighting both
from and phase quenched ensembles. We find a good agreement for lattice
spacings below 0.15 fm. On coarser lattices the complex Langevin
approach breaks down. Four flavors of staggered fermions are used on
and 8 lattices. For one ensemble we also use two flavors to investigate the
effects of rooting.Comment: 10 pages, 11 figures, PRD version, minor change
Quasiperiodic oscillations in a strong gravitational field around neutron stars testing braneworld models
The strong gravitational field of neutron stars in the brany universe could
be described by spherically symmetric solutions with a metric in the exterior
to the brany stars being of the Reissner-Nordstrom type containing a brany
tidal charge representing the tidal effect of the bulk spacetime onto the star
structure. We investigate the role of the tidal charge in orbital models of
high-frequency quasiperiodic oscillations (QPOs) observed in neutron star
binary systems. We focus on the relativistic precession model. We give the
radial profiles of frequencies of the Keplerian (vertical) and radial epicyclic
oscillations. We show how the standard relativistic precession model modified
by the tidal charge fits the observational data, giving estimates of the
allowed values of the tidal charge and the brane tension based on the processes
going in the vicinity of neutron stars. We compare the strong field regime
restrictions with those given in the weak-field limit of solar system
experiments.Comment: 26 pages, 6 figure
Ideal kink instability of a magnetic loop equilibrium
The force-free coronal loop model by Titov & D\'emoulin (1999} is found to be
unstable with respect to the ideal kink mode, which suggests this instability
as a mechanism for the initiation of flares. The long-wavelength () mode
grows for average twists \Phi\ga3.5\pi (at a loop aspect ratio of
5). The threshold of instability increases with increasing major loop radius,
primarily because the aspect ratio then also increases. Numerically obtained
equilibria at subcritical twist are very close to the approximate analytical
equilibrium; they do not show indications of sigmoidal shape. The growth of
kink perturbations is eventually slowed down by the surrounding potential
field, which varies only slowly with radius in the model. With this field a
global eruption is not obtained in the ideal MHD limit. Kink perturbations with
a rising loop apex lead to the formation of a vertical current sheet below the
apex, which does not occur in the cylindrical approximation.Comment: Astron. Astrophys. Lett., accepte
Testing magnetofrictional extrapolation with the Titov-D\'emoulin model of solar active regions
We examine the nonlinear magnetofrictional extrapolation scheme using the
solar active region model by Titov and D\'emoulin as test field. This model
consists of an arched, line-tied current channel held in force-free equilibrium
by the potential field of a bipolar flux distribution in the bottom boundary. A
modified version, having a parabolic current density profile, is employed here.
We find that the equilibrium is reconstructed with very high accuracy in a
representative range of parameter space, using only the vector field in the
bottom boundary as input. Structural features formed in the interface between
the flux rope and the surrounding arcade-"hyperbolic flux tube" and "bald patch
separatrix surface"-are reliably reproduced, as are the flux rope twist and the
energy and helicity of the configuration. This demonstrates that force-free
fields containing these basic structural elements of solar active regions can
be obtained by extrapolation. The influence of the chosen initial condition on
the accuracy of reconstruction is also addressed, confirming that the initial
field that best matches the external potential field of the model quite
naturally leads to the best reconstruction. Extrapolating the magnetogram of a
Titov-D\'emoulin equilibrium in the unstable range of parameter space yields a
sequence of two opposing evolutionary phases which clearly indicate the
unstable nature of the configuration: a partial buildup of the flux rope with
rising free energy is followed by destruction of the rope, losing most of the
free energy.Comment: 14 pages, 10 figure
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