4,048 research outputs found
The impact of polarized extragalactic radio sources on the detection of CMB anisotropies in polarization
Recent polarimetric surveys of extragalactic radio sources (ERS) at
frequencies \nu>1GHz are reviewed. By exploiting all the most relevant data on
the polarized emission of ERS we study the frequency dependence of polarization
properties of ERS between 1.4 and 86GHz. For flat-spectrum sources the median
(mean) fractional polarization increases from 1.5% (2-2.5%) at 1.4GHz to 2.5-3%
(3-3.5%) at \nu>10GHz. Steep-spectrum sources are typically more polarized,
especially at high frequencies where Faraday depolarization is less relevant.
As a general result, we do not find that the fractional polarization of ERS
depends on the total flux density at high radio frequencies, i.e >20GHz.
Moreover, in this frequency range, current data suggest a moderate increase of
the fractional polarization of ERS with frequency. A formalism to estimate ERS
number counts in polarization and the contribution of unresolved polarized ERS
to angular power spectra at Cosmic Microwave Background (CMB) frequencies is
also developed and discussed. As a first application, we present original
predictions for the Planck satellite mission. Our current results show that
only a dozen polarized ERS will be detected by the Planck Low Frequency
Instrument (LFI), and a few tens by the High Frequency Instrument (HFI). As for
CMB power spectra, ERS should not be a strong contaminant to the CMB E-mode
polarization at frequencies \nu>70GHz. On the contrary, they can become a
relevant constraint for the detection of the cosmological B--mode polarization
if the tensor-to-scalar ratio is <0.01.Comment: 14 pages, 7 figures. Accepted for publication in Advances in
Astronomy, Special Issue: "Astrophysical Foregrounds in Microwave Surveys",
editor-in-chief C. Burigana, (www.hindawi.com
Phase growth in bistable systems with impurities
A system of coupled chaotic bistable maps on a lattice with randomly
distributed impurities is investigated as a model for studying the phenomenon
of phase growth in nonuniform media. The statistical properties of the system
are characterized by means of the average size of spatial domains of equivalent
spin variables that define the phases. It is found that the rate at which phase
domains grow becomes smaller when impurities are present and that the average
size of the resulting domains in the inhomogeneous state of the system
decreases when the density of impurities is increased. The phase diagram
showing regions where homogeneous, heterogeneous, and chessboard patterns occur
on the space of parameters of the system is obtained. A critical boundary that
separates the regime of slow growth of domains from the regime of fast growth
in the heterogeneous region of the phase diagram is calculated. The transition
between these two growth regimes is explained in terms of the stability
properties of the local phase configurations. Our results show that the
inclusion of spatial inhomogeneities can be used as a control mechanism for the
size and growth velocity of phase domains forming in spatiotemporal systems.Comment: 7 pages, 12 figure
Cross-correlation of the CMB and radio galaxies in real, harmonic and wavelet spaces: detection of the integrated Sachs-Wolfe effect and dark energy constraints
We report the first detection of the ISW effect in wavelet space, at scales
in the sky around 7 degrees with a significance of around 3.3 sigma, by
cross-correlating the WMAP first-year data and the NRAO VLA Sky Survey (NVSS).
In addition, we present a detailed comparison among the capabilities of three
different techniques for two different objectives: to detect the ISW and to put
constraints in the nature of the dark energy. The three studied techniques are:
the cross-angular power spectrum (CAPS, harmonic space), the correlation
function (CCF, real space) and the covariance of the Spherical Mexican Hat
Wavelet (SMHW) coefficients (CSMHW, wavelet space). We prove that the CSMHW is
expected to provide a higher detection of the ISW effect for a certain scale.
This prediction has been corroborated by the analysis of the data. The SMHW
analysis shows that the cross-correlation signal is caused neither by
systematic effects nor foreground contamination. However, by taking into
account the information encoded in all the multipoles/scales/angles, the CAPS
provides slightly better constraints than the SMHW in the cosmological
parameters that define the nature of the dark energy. The limits provided by
the CCF are wider than for the other two methods. Two different cases have been
studied: 1) a flat Lambda-CDM universe and 2) a flat universe with an equation
of state parameter different from -1. In the first case, the CAPS provides (for
a bias value of b = 1.6) 0.59 < Lambda density < 0.84 (at 1 sigma CL).
Moreover, the CAPS rejects the range Lambda density < 0.1 at 3.5 sigma, which
is the highest detection of the dark energy reported up to date. In the second
case, the CAPS gives 0.50 < dark energy density < 0.82 and -1.16 < w < 0.43 (at
1 sigma CL).Comment: 12 pages, 7 figures, accepted for publication in MNRAS. Analysis
redone. Changes in the estimation of the cosmological parametres. Additional
comparison between wavelets and more standard technique
Phase ordering induced by defects in chaotic bistable media
The phase ordering dynamics of coupled chaotic bistable maps on lattices with
defects is investigated. The statistical properties of the system are
characterized by means of the average normalized size of spatial domains of
equivalent spin variables that define the phases. It is found that spatial
defects can induce the formation of domains in bistable spatiotemporal systems.
The minimum distance between defects acts as parameter for a transition from a
homogeneous state to a heterogeneous regime where two phases coexist The
critical exponent of this transition also exhibits a transition when the
coupling is increased, indicating the presence of a new class of domain where
both phases coexist forming a chessboard pattern.Comment: 3 pages, 3 figures, Accepted in European Physics Journa
Entanglement of 2xK quantum systems
We derive an analytical expression for the lower bound of the concurrence of
mixed quantum states of composite 2xK systems. In contrast to other, implicitly
defined entanglement measures, the numerical evaluation of our bound is
straightforward. We explicitly evaluate its tightness for general mixed states
of 2x3 systems, and identify a large class of states where our expression gives
the exact value of the concurrence.Comment: 7 pages, 1 figure, to be published in Europhysics Lette
Expected optimal feedback with Time-Varying Parameters
In this paper we derive the closed loop form of the Expected Optimal Feedback rule, sometimes called passive learning stochastic control, with time varying parameters. As such this paper extends the work of Kendrick (1981,2002, Chapter 6) where parameters are assumed to vary randomly around a known constant mean. Furthermore, we show that the cautionary myopic rule in Beck and Wieland (2002) model, a test bed for comparing various stochastic optimizations approaches, can be cast into this framework and can be treated as a special case of this solution.Optimal experimentation, stochastic optimization, time-varying parameters, expected optimal feedback
Expected optimal feedback with Time-Varying Parameters
In this paper we derive, by using dynamic programming, the closed loop form of the Expected Optimal Feedback rule with time varying parameter. As such this paper extends the work of Kendrick (1981, 2002, Chapter 6) for the time varying parameter case. Furthermore, we show that the Beck and Wieland (2002) model can be cast into this framework and can be treated as a special case of this solution.
Mesoscopic Model for Diffusion-Influenced Reaction Dynamics
A hybrid mesoscopic multi-particle collision model is used to study
diffusion-influenced reaction kinetics. The mesoscopic particle dynamics
conserves mass, momentum and energy so that hydrodynamic effects are fully
taken into account. Reactive and non-reactive interactions with catalytic
solute particles are described by full molecular dynamics. Results are
presented for large-scale, three-dimensional simulations to study the influence
of diffusion on the rate constants of the A+CB+C reaction. In the limit of
a dilute solution of catalytic C particles, the simulation results are compared
with diffusion equation approaches for both the irreversible and reversible
reaction cases. Simulation results for systems where the volume fraction of
catalytic spheres is high are also presented, and collective interactions among
reactions on catalytic spheres that introduce volume fraction dependence in the
rate constants are studied.Comment: 9 pages, 5 figure
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