619 research outputs found
Constant flux relation for diffusion-limited cluster-cluster aggregation
In a nonequilibrium system, a constant flux relation (CFR) expresses the fact that a constant flux of a conserved quantity exactly determines the scaling of the particular correlation function linked to the flux of that conserved quantity. This is true regardless of whether mean-field theory is applicable or not. We focus on cluster-cluster aggregation and discuss the consequences of mass conservation for the steady state of aggregation models with a monomer source in the diffusion-limited regime. We derive the CFR for the flux-carrying correlation function for binary aggregation with a general scale-invariant kernel and show that this exponent is unique. It is independent of both the dimension and of the details of the spatial transport mechanism, a property which is very atypical in the diffusion-limited regime. We then discuss in detail the "locality criterion" which must be satisfied in order for the CFR scaling to be realizable. Locality may be checked explicitly for the mean-field Smoluchowski equation. We show that if it is satisfied at the mean-field level, it remains true over some finite range as one perturbatively decreases the dimension of the system below the critical dimension, d(c)=2, entering the fluctuation-dominated regime. We turn to numerical simulations to verify locality for a range of systems in one dimension which are, presumably, beyond the perturbative regime. Finally, we illustrate how the CFR scaling may break down as a result of a violation of locality or as a result of finite size effects and discuss the extent to which the results apply to higher order aggregation processes
A New Analysis Method for Reconstructing the Arrival Direction of TeV Gamma-rays Using a Single Imaging Atmospheric Cherenkov Telescope
We present a method of atmospheric Cherenkov imaging which reconstructs the
unique arrival direction of TeV gamma rays using a single telescope. The method
is derived empirically and utilizes several features of gamma-ray induced air
showers which determine, to a precision of 0.12 degrees, the arrival direction
of photons, on an event-by-event basis. Data from the Whipple Observatory's 10
m gamma-ray telescope is utilized to test selection methods based on source
location. The results compare these selection methods with traditional
techniques and three different camera fields of view. The method will be
discussed in the context of a search for a gamma-ray signal from a point source
located anywhere within the field of view and from regions of extended
emission.Comment: 24 pages, 16 figures, accepted for publication in Astroparticle
Physics May 11, 200
Wave turbulence in the two-layer ocean model
This paper looks at the two-layer ocean model from a wave turbulence
perspective. A symmetric form of the two-layer kinetic equation for Rossby
waves is derived using canonical variables, allowing the turbulent cascade of
energy between the barotropic and baroclinic modes to be studied. It turns out
that energy is transferred via local triad interactions from the large-scale
baroclinic modes to the baroclinic and barotropic modes at the Rossby
deformation scale. From there it is then transferred to the large-scale
barotropic modes via a nonlocal inverse transfer. Using scale separation a sys-
tem of coupled equations were obtained for the small-scale baroclinic component
and the large-scale barotropic component. Since the total energy of the
small-scale component is not conserved, but the total barotropic plus
baroclinic energy is conserved, the baroclinic energy loss at small scales will
be compensated by the growth of the barotropic energy at large scales. It is
found that this transfer is mostly anisotropic and mostly to the zonal
component
Swift and Fermi observations of X-ray flares: the case of Late Internal Shock
Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a
unique broadband view of their afterglow emission, spanning more than ten
decades in energy. We present the sample of X-ray flares observed by both Swift
and Fermi during the first three years of Fermi operations. While bright in the
X-ray band, X-ray flares are often undetected at lower (optical), and higher
(MeV to GeV) energies. We show that this disfavors synchrotron self-Compton
processes as origin of the observed X-ray emission. We compare the broadband
properties of X-ray flares with the standard late internal shock model, and
find that, in this scenario, X-ray flares can be produced by a late-time
relativistic (Gamma>50) outflow at radii R~10^13-10^14 cm. This conclusion
holds only if the variability timescale is significantly shorter than the
observed flare duration, and implies that X-ray flares can directly probe the
activity of the GRB central engine.Comment: 13 pages, 4 figures, accepted for publication in Ap
Growing condensate in two-dimensional turbulence
We report a numerical study, supplemented by phenomenological explanations,
of ``energy condensation'' in forced 2D turbulence in a biperiodic box.
Condensation is a finite size effect which occurs after the standard inverse
cascade reaches the size of the system. It leads to emergence of a coherent
vortex dipole. We show that the time growth of the dipole is self-similar, and
it contains most of the injected energy, thus resulting in an energy spectrum
which is markedly steeper than the standard one. Once the coherent
component is subtracted, however, the remaining fluctuations have a spectrum
close to . The fluctuations decay slowly as the coherent part grows.Comment: 4 pages, 4 figures. This version includes some additional
phenomenological explanations of the results, additional references and
improved figure
Breakdown of Kolmogorov scaling in models of cluster aggregation with deposition
The steady state of the model of cluster aggregation with deposition is
characterized by a constant flux of mass directed from small masses towards
large masses. It can therefore be studied using phenomenological theories of
turbulence, such as Kolmogorov's 1941 theory. On the other hand, the large
scale behavior of the aggregation model in dimensions lower than or equal to
two is governed by a perturbative fixed point of the renormalization group
flow, which enables an analytic study of the scaling properties of correlation
functions in the steady state. In this paper, we show that the correlation
functions have multifractal scaling, which violates linear Kolmogorov scaling.
The analytical results are verified by Monte Carlo simulations.Comment: 5 pages 4 figure
Stationary mass distribution and non-locality in models of coalescence and shattering
We study the asymptotic properties of the steady state mass distribution for a class of collision kernels in an aggregation-shattering model in the limit of small shattering probabilities. It is shown that the exponents characterizing the large and small mass asymptotic behavior of the mass distribution depend on whether the collision kernel is local (the aggregation mass flux is essentially generated by collisions between particles of similar masses) or nonlocal (collision between particles of widely different masses give the main contribution to the mass flux). We show that the nonlocal regime is further divided into two subregimes corresponding to weak and strong nonlocality. We also observe that at the boundaries between the local and nonlocal regimes, the mass distribution acquires logarithmic corrections to scaling and calculate these corrections. Exact solutions for special kernels and numerical simulations are used to validate some nonrigorous steps used in the analysis. Our results show that for local kernels, the scaling solutions carry a constant flux of mass due to aggregation, whereas for the nonlocal case there is a correction to the constant flux exponent. Our results suggest that for general scale-invariant kernels, the universality classes of mass distributions are labeled by two parameters: the homogeneity degree of the kernel and one further number measuring the degree of the nonlocality of the kernel
Stationary Kolmogorov Solutions of the Smoluchowski Aggregation Equation with a Source Term
In this paper we show how the method of Zakharov transformations may be used
to analyze the stationary solutions of the Smoluchowski aggregation equation
for arbitrary homogeneous kernel. The resulting massdistributions are of
Kolmogorov type in the sense that they carry a constant flux of mass from small
masses to large. We derive a ``locality criterion'', expressed in terms of the
asymptotic properties of the kernel, that must be satisfied in order for the
Kolmogorov spectrum to be an admissiblesolution. Whether a given kernel leads
to a gelation transition or not can be determined by computing the mass
capacity of the Kolmogorov spectrum. As an example, we compute the exact
stationary state for the family of
kernels, which includes both gelling and
non-gelling cases, reproducing the known solution in the case .
Surprisingly, the Kolmogorov constant is the same for all kernels in this
family.Comment: This article is an expanded version of a talk given at IHP workshop
"Dynamics, Growth and Singularities of Continuous Media", Paris July 2003.
Updated 01/04/04. Revised version with additional discussion, references
added, several typographical errors corrected. Revised version accepted for
publication by Phys. Rev.
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