21,599 research outputs found
A new formulation of compartmental epidemic modelling for arbitrary distributions of incubation and removal times
The paradigm for compartment models in epidemiology assumes exponentially
distributed incubation and removal times, which is not realistic in actual
populations. Commonly used variations with multiple exponentially distributed
variables are more flexible, yet do not allow for arbitrary distributions. We
present a new formulation, focussing on the SEIR concept that allows to include
general distributions of incubation and removal times. We compare the solution
to two types of agent-based model simulations, a spatially homogeneous one
where infection occurs by proximity, and a model on a scale-free network with
varying clustering properties, where the infection between any two agents
occurs via their link if it exists. We find good agreement in both cases.
Furthermore a family of asymptotic solutions of the equations is found in terms
of a logistic curve, which after a non-universal time shift, fits extremely
well all the microdynamical simulations. The formulation allows for a simple
numerical approach; software in Julia and Python is provided.Comment: 21 pages, 11 figures. v2 matches published version: improved
presentation (including title, abstract and references), results and
conclusions unchange
Hall-MHD small-scale dynamos
Much of the progress in our understanding of dynamo mechanisms has been made
within the theoretical framework of magnetohydrodynamics (MHD). However, for
sufficiently diffuse media, the Hall effect eventually becomes non-negligible.
We present results from three dimensional simulations of the Hall-MHD equations
subjected to random non-helical forcing. We study the role of the Hall effect
in the dynamo efficiency for different values of the Hall parameter, using a
pseudospectral code to achieve exponentially fast convergence. We also study
energy transfer rates among spatial scales to determine the relative importance
of the various nonlinear effects in the dynamo process and in the energy
cascade. The Hall effect produces a reduction of the direct energy cascade at
scales larger than the Hall scale, and therefore leads to smaller energy
dissipation rates. Finally, we present results stemming from simulations at
large magnetic Prandtl numbers, which is the relevant regime in hot and diffuse
media such a the interstellar medium.Comment: 11 pages and 11 figure
Diffusion dynamics on multiplex networks
We study the time scales associated to diffusion processes that take place on
multiplex networks, i.e. on a set of networks linked through interconnected
layers. To this end, we propose the construction of a supra-Laplacian matrix,
which consists of a dimensional lifting of the Laplacian matrix of each layer
of the multiplex network. We use perturbative analysis to reveal analytically
the structure of eigenvectors and eigenvalues of the complete network in terms
of the spectral properties of the individual layers. The spectrum of the
supra-Laplacian allows us to understand the physics of diffusion-like processes
on top of multiplex networks.Comment: 6 Pages including supplemental material. To appear in Physical Review
Letter
Gravitational waveforms with controlled accuracy
A partially first-order form of the characteristic formulation is introduced
to control the accuracy in the computation of gravitational waveforms produced
by highly distorted single black hole spacetimes. Our approach is to reduce the
system of equations to first-order differential form on the angular
derivatives, while retaining the proven radial and time integration schemes of
the standard characteristic formulation. This results in significantly improved
accuracy over the standard mixed-order approach in the extremely nonlinear
post-merger regime of binary black hole collisions.Comment: Revised version, published in Phys. Rev. D, RevTeX, 16 pages, 4
figure
The milliarcsecond-scale jet of PKS 0735+178 during quiescence
We present polarimetric 5 GHz to 43 GHz VLBI observations of the BL Lacertae
object PKS 0735+178, spanning March 1996 to May 2000. Comparison with previous
and later observations suggests that the overall kinematic and structural
properties of the jet are greatly influenced by its activity. Time intervals of
enhanced activity, as reported before 1993 and after 2000 by other studies, are
followed by highly superluminal motion along a rectilinear jet. In contrast the
less active state in which we performed our observations, shows subluminal or
slow superluminal jet features propagating through a twisted jet with two sharp
bends of about 90 deg. within the innermost three-milliarcsecond jet structure.
Proper motion estimates from the data presented here allow us to constrain the
jet viewing angle to values < 9 deg., and the bulk Lorentz factor to be between
2 and 4.Comment: 11 pages, 12 figures. Accepted for publication in A&
Tunable entanglement distillation of spatially correlated down-converted photons
We report on a new technique for entanglement distillation of the bipartite
continuous variable state of spatially correlated photons generated in the
spontaneous parametric down-conversion process (SPDC), where tunable
non-Gaussian operations are implemented and the post-processed entanglement is
certified in real-time using a single-photon sensitive electron multiplying CCD
(EMCCD) camera. The local operations are performed using non-Gaussian filters
modulated into a programmable spatial light modulator and, by using the EMCCD
camera for actively recording the probability distributions of the
twin-photons, one has fine control of the Schmidt number of the distilled
state. We show that even simple non-Gaussian filters can be finely tuned to a
~67% net gain of the initial entanglement generated in the SPDC process.Comment: 12 pages, 6 figure
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