65,730 research outputs found
Automatic analysis of Swift-XRT data
The Swift spacecraft detects and autonomously observes ~100 Gamma Ray Bursts
(GRBs) per year, ~96% of which are detected by the X-ray telescope (XRT). GRBs
are accompanied by optical transients and the field of ground-based follow-up
of GRBs has expanded significantly over the last few years, with rapid response
instruments capable of responding to Swift triggers on timescales of minutes.
To make the most efficient use of limited telescope time, follow-up astronomers
need accurate positions of GRBs as soon as possible after the trigger.
Additionally, information such as the X-ray light curve, is of interest when
considering observing strategy. The Swift team at Leicester University have
developed techniques to improve the accuracy of the GRB positions available
from the XRT, and to produce science-grade X-ray light curves of GRBs. These
techniques are fully automated, and are executed as soon as data are available.Comment: 4 pages, 2 figures, to appear in the proceedings of ADASS XVII (ASP
Conference Series
A model colloidal fluid with competing interactions: bulk and interfacial properties
Using a simple mean-field density functional theory theory (DFT), we
investigate the structure and phase behaviour of a model colloidal fluid
composed of particles interacting via a pair potential which has a hard core of
diameter , is attractive Yukawa at intermediate separations and
repulsive Yukawa at large separations. We analyse the form of the asymptotic
decay of the bulk fluid correlation functions, comparing results from our DFT
with those from the self consistent Ornstein-Zernike approximation (SCOZA). In
both theories we find rich crossover behaviour, whereby the ultimate decay of
correlation functions changes from monotonic to long-wavelength damped
oscillatory decay on crossing certain lines in the phase diagram, or sometimes
from oscillatory to oscillatory with a longer wavelength. For some choices of
potential parameters we find, within the DFT, a -line at which the
fluid becomes unstable with respect to periodic density fluctuations. SCOZA
fails to yield solutions for state points near such a -line. The
propensity to clustering of particles, which is reflected by the presence of a
long wavelength , slowly decaying oscillatory pair correlation
function, and a structure factor that exhibits a very sharp maximum at small
but non zero wavenumbers, is enhanced in states near the -line. We
present density profiles for the planar liquid-gas interface and for fluids
adsorbed at a planar hard wall. The presence of a nearby -transition
gives rise to pronounced long-wavelength oscillations in the one-body densities
at both types of interface.Comment: 14 pages, 11 figure
X-ray and UV observations of V751 Cyg in an optical high state
Aims: The VY Scl system (anti-dwarf nova) V751 Cyg is examined following a
claim of a super-soft spectrum in the optical low state. Methods: A
serendipitous XMM-Newton X-ray observation and, 21 months later, Swift X-ray
and UV observations, have provided the best such data on this source so far.
These optical high-state datasets are used to study the flux and spectral
variability of V751 Cyg. Results: Both the XMM-Newton and Swift data show
evidence for modulation of the X-rays for the first time at the known 3.467 hr
orbital period of V751 Cyg. In two Swift observations, taken ten days apart,
the mean X-ray flux remained unchanged, while the UV source brightened by half
a magnitude. The X-ray spectrum was not super-soft during the optical high
state, but rather due to multi-temperature optically thin emission, with
significant (10^{21-22} cm^-2) absorption, which was higher in the observation
by Swift than that of XMM-Newton. The X-ray flux is harder at orbital minimum,
suggesting that the modulation is related to absorption, perhaps linked to the
azimuthally asymmetric wind absorption seen previously in H-alpha.Comment: 6 pages, 9 figures, accepted for publication in A&
Johnson-Kendall-Roberts theory applied to living cells
Johnson-Kendall-Roberts (JKR) theory is an accurate model for strong adhesion
energies of soft slightly deformable material. Little is known about the
validity of this theory on complex systems such as living cells. We have
addressed this problem using a depletion controlled cell adhesion and measured
the force necessary to separate the cells with a micropipette technique. We
show that the cytoskeleton can provide the cells with a 3D structure that is
sufficiently elastic and has a sufficiently low deformability for JKR theory to
be valid. When the cytoskeleton is disrupted, JKR theory is no longer
applicable
Interactions between unidirectional quantized vortex rings
We have used the vortex filament method to numerically investigate the
interactions between pairs of quantized vortex rings that are initially
traveling in the same direction but with their axes offset by a variable impact
parameter. The interaction of two circular rings of comparable radii produce
outcomes that can be categorized into four regimes, dependent only on the
impact parameter; the two rings can either miss each other on the inside or
outside, or they can reconnect leading to final states consisting of either one
or two deformed rings. The fraction of of energy went into ring deformations
and the transverse component of velocity of the rings are analyzed for each
regime. We find that rings of very similar radius only reconnect for a very
narrow range of the impact parameter, much smaller than would be expected from
geometrical cross-section alone. In contrast, when the radii of the rings are
very different, the range of impact parameters producing a reconnection is
close to the geometrical value. A second type of interaction considered is the
collision of circular rings with a highly deformed ring. This type of
interaction appears to be a productive mechanism for creating small vortex
rings. The simulations are discussed in the context of experiments on colliding
vortex rings and quantum turbulence in superfluid helium in the zero
temperature limit
On the dynamics of WKB wave functions whose phase are weak KAM solutions of H-J equation
In the framework of toroidal Pseudodifferential operators on the flat torus
we begin by proving the closure under
composition for the class of Weyl operators with
simbols . Subsequently, we
consider when where and we exhibit the toroidal version of the
equation for the Wigner transform of the solution of the Schr\"odinger
equation. Moreover, we prove the convergence (in a weak sense) of the Wigner
transform of the solution of the Schr\"odinger equation to the solution of the
Liouville equation on written in the measure sense.
These results are applied to the study of some WKB type wave functions in the
Sobolev space with phase functions in the class
of Lipschitz continuous weak KAM solutions (of positive and negative type) of
the Hamilton-Jacobi equation for with , and to the study of the
backward and forward time propagation of the related Wigner measures supported
on the graph of
Nonequilibrium statistical mechanics of shear flow: invariant quantities and current relations
In modeling nonequilibrium systems one usually starts with a definition of
the microscopic dynamics, e.g., in terms of transition rates, and then derives
the resulting macroscopic behavior. We address the inverse question for a class
of steady state systems, namely complex fluids under continuous shear flow: how
does an externally imposed shear current affect the microscopic dynamics of the
fluid? The answer can be formulated in the form of invariant quantities, exact
relations for the transition rates in the nonequilibrium steady state, as
discussed in a recent letter [A. Baule and R. M. L. Evans, Phys. Rev. Lett.
101, 240601 (2008)]. Here, we present a more pedagogical account of the
invariant quantities and the theory underlying them, known as the
nonequilibrium counterpart to detailed balance (NCDB). Furthermore, we
investigate the relationship between the transition rates and the shear current
in the steady state. We show that a fluctuation relation of the
Gallavotti-Cohen type holds for systems satisfying NCDB.Comment: 24 pages, 11 figure
Statistical mechanics far from equilibrium: prediction and test for a sheared system
We report the complete statistical treatment of a system of particles
interacting via Newtonian forces in continuous boundary-driven flow, far from
equilibrium. By numerically time-stepping the force-balance equations of a
model fluid we measure occupancies and transition rates in simulation. The
high-shear-rate simulation data verify the invariant quantities predicted by
our statistical theory, thus demonstrating that a class of non-equilibrium
steady states of matter, namely sheared complex fluids, is amenable to
statistical treatment from first principles.Comment: 4 pages plus a 3-page pdf supplemen
Peeping at chaos: Nondestructive monitoring of chaotic systems by measuring long-time escape rates
One or more small holes provide non-destructive windows to observe
corresponding closed systems, for example by measuring long time escape rates
of particles as a function of hole sizes and positions. To leading order the
escape rate of chaotic systems is proportional to the hole size and independent
of position. Here we give exact formulas for the subsequent terms, as sums of
correlation functions; these depend on hole size and position, hence yield
information on the closed system dynamics. Conversely, the theory can be
readily applied to experimental design, for example to control escape rates.Comment: Originally 4 pages and 2 eps figures incorporated into the text; v2
has more numerical results and discussion: now 6 pages, 4 figure
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