7,005 research outputs found
Exposure interlock for oscilloscope cameras
An exposure interlock has been developed for oscilloscope cameras which cuts off ambient light from the oscilloscope screen before the shutter of the camera is tripped. A flap is provided which may be selectively positioned to an open position which enables viewing of the oscilloscope screen and a closed position which cuts off the oscilloscope screen from view and simultaneously cuts off ambient light from the oscilloscope screen. A mechanical interlock is provided between the flap to be activated to its closed position before the camera shutter is tripped, thereby preventing overexposure of the film
Evolution of the Dark Matter Distribution at the Galactic Center
Annihilation radiation from neutralino dark matter at the Galactic center
(GC) would be greatly enhanced if the dark matter were strongly clustered
around the supermassive black hole (SBH). The existence of a dark-matter
"spike" is made plausible by the observed, steeply-rising stellar density near
the GC SBH. Here the time-dependent equations describing gravitational
interaction of the dark matter particles with the stars are solved. Scattering
of dark matter particles by stars would substantially lower the dark matter
density near the GC SBH over 10^10 yr, due both to kinetic heating, and to
capture of dark matter particles by the SBH. This result suggests that
enhancements in the dark matter density around a SBH would be modest whether or
not the host galaxy had experienced the scouring effects of a binary SBH.Comment: 5 pages, 3 figures. Submitted to Physical Review Letter
Dynamics of a disordered, driven zero range process in one dimension
We study a driven zero range process which models a closed system of
attractive particles that hop with site-dependent rates and whose steady state
shows a condensation transition with increasing density. We characterise the
dynamical properties of the mass fluctuations in the steady state in one
dimension both analytically and numerically and show that the transport
properties are anomalous in certain regions of the density-disorder plane. We
also determine the form of the scaling function which describes the growth of
the condensate as a function of time, starting from a uniform density
distribution.Comment: Revtex4, 5 pages including 2 figures; Revised version; To appear in
Phys. Rev. Let
Boundary conditions and defect lines in the Abelian sandpile model
We add a defect line of dissipation, or crack, to the Abelian sandpile model.
We find that the defect line renormalizes to separate the two-dimensional plane
into two half planes with open boundary conditions. We also show that varying
the amount of dissipation at a boundary of the Abelian sandpile model does not
affect the universality class of the boundary condition. We demonstrate that a
universal coefficient associated with height probabilities near the defect can
be used to classify boundary conditions.Comment: 8 pages, 1 figure; suggestions from referees incorporated; to be
published in Phys. Rev.
Distance-redshift from an optical metric that includes absorption
We show that it is possible to equate the intensity reduction of a light wave
caused by weak absorption with a geometrical reduction in intensity caused by a
"transverse" conformal transformation of the spacetime metric in which the wave
travels. We are consequently able to modify Gordon's optical metric to account
for electromagnetic properties of ponderable material whose properties include
both refraction and absorption. Unlike refraction alone however, including
absorption requires a modification of the optical metric that depends on the
eikonal of the wave itself. We derive the distance-redshift relation from the
modified optical metric for Friedman-Lema\^itre-Robertson-Walker spacetimes
whose cosmic fluid has associated refraction and absorption coefficients. We
then fit the current supernovae data and provide an alternate explanation
(other than dark energy) of the apparent acceleration of the universe.Comment: 2 figure
Condensation Transitions in a One-Dimensional Zero-Range Process with a Single Defect Site
Condensation occurs in nonequilibrium steady states when a finite fraction of
particles in the system occupies a single lattice site. We study condensation
transitions in a one-dimensional zero-range process with a single defect site.
The system is analysed in the grand canonical and canonical ensembles and the
two are contrasted. Two distinct condensation mechanisms are found in the grand
canonical ensemble. Discrepancies between the infinite and large but finite
systems' particle current versus particle density diagrams are investigated and
an explanation for how the finite current goes above a maximum value predicted
for infinite systems is found in the canonical ensemble.Comment: 18 pages, 4 figures, revtex
First-principles calculations for the adsorption of water molecules on the Cu(100) surface
First-principles density-functional theory and supercell models are employed
to calculate the adsorption of water molecules on the Cu(100) surface. In
agreement with the experimental observations, the calculations show that a H2O
molecule prefers to bond at a one-fold on-top (T1) surface site with a tilted
geometry. At low temperatures, rotational diffusion of the molecular axis of
the water molecules around the surface normal is predicted to occur at much
higher rates than lateral diffusion of the molecules. In addition, the
calculated binding energy of an adsorbed water molecule on the surfaces is
significantly smaller than the water sublimation energy, indicating a tendency
for the formation of water clusters on the Cu(100) surface.Comment: 5 pages, 3 figures, submitted to Phys. Rev.
Feedback Heating by Cosmic Rays in Clusters of Galaxies
Recent observations show that the cooling flows in the central regions of
galaxy clusters are highly suppressed. Observed AGN-induced cavities/bubbles
are a leading candidate for suppressing cooling, usually via some form of
mechanical heating. At the same time, observed X-ray cavities and synchrotron
emission point toward a significant non-thermal particle population. Previous
studies have focused on the dynamical effects of cosmic-ray pressure support,
but none have built successful models in which cosmic-ray heating is
significant. Here we investigate a new model of AGN heating, in which the
intracluster medium is efficiently heated by cosmic-rays, which are injected
into the ICM through diffusion or the shredding of the bubbles by
Rayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermal
conduction as well. Using numerical simulations, we show that the cooling
catastrophe is efficiently suppressed. The cluster quickly relaxes to a
quasi-equilibrium state with a highly reduced accretion rate and temperature
and density profiles which match observations. Unlike the conduction-only case,
no fine-tuning of the Spitzer conduction suppression factor f is needed. The
cosmic ray pressure, P_c/P_g <~ 0.1 and dP_c/dr <~ 0.1 \rho g, is well within
observational bounds. Cosmic ray heating is a very attractive alternative to
mechanical heating, and may become particularly compelling if GLAST detects the
gamma-ray signature of cosmic-rays in clusters.Comment: Revised version accepted for publication in MNRAS. Significantly
expanded discussion and new simulations exploring parameter space/model
robustness; conclusions unchange
Phase Transition in Two Species Zero-Range Process
We study a zero-range process with two species of interacting particles. We
show that the steady state assumes a simple factorised form, provided the
dynamics satisfy certain conditions, which we derive. The steady state exhibits
a new mechanism of condensation transition wherein one species induces the
condensation of the other. We study this mechanism for a specific choice of
dynamics.Comment: 8 pages, 3 figure
Locating the minimum : Approach to equilibrium in a disordered, symmetric zero range process
We consider the dynamics of the disordered, one-dimensional, symmetric zero
range process in which a particle from an occupied site hops to its nearest
neighbour with a quenched rate . These rates are chosen randomly from the
probability distribution , where is the lower cutoff.
For , this model is known to exhibit a phase transition in the steady
state from a low density phase with a finite number of particles at each site
to a high density aggregate phase in which the site with the lowest hopping
rate supports an infinite number of particles. In the latter case, it is
interesting to ask how the system locates the site with globally minimum rate.
We use an argument based on local equilibrium, supported by Monte Carlo
simulations, to describe the approach to the steady state. We find that at
large enough time, the mass transport in the regions with a smooth density
profile is described by a diffusion equation with site-dependent rates, while
the isolated points where the mass distribution is singular act as the
boundaries of these regions. Our argument implies that the relaxation time
scales with the system size as with for and
suggests a different behaviour for .Comment: Revtex, 7 pages including 3 figures. Submitted to Pramana -- special
issue on mesoscopic and disordered system
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