3,249 research outputs found
The resolved structure of the extragalactic supernova remnant SNR 4449-1
We present very long baseline interferometry (VLBI) observations of the
milliarcsecond-scale radio structure of the supernova remnant SNR 44491 in
the galaxy NGC 4449. This young and superluminous remnant was observed at 1.6
GHz (\,cm) with the European VLBI Network. The observations
confirm earlier identifications of this object with a supernova remnant (SNR)
while revealing a somewhat different morphology compared with the structure
reported by Bietenholz et al. from VLBI observations at 1.4 GHz. This
difference is discussed here in the context of structural sensitivity of both
observations. The 1.6 GHz image yields accurate estimates of the size (0.0422
arcsec 0.0285 arcsec and 0.8 0.5 pc) and age (55 yr) of
SNR 44491. With a total flux of 6.1 0.6 mJy measured in the VLBI
image, the historical lightcurve of the source can be well represented by a
power-law decay with a power index of 1.19 0.07. The SNR exhibits a
decline rate of the radio emission of 2.2 0.1 yr and a radio
luminosity of 1.74 10 erg s.Comment: 7 pages, 6 figures, MNRAS preprint, arXiv:1309.401
VLBI observations of SN2011dh: imaging of the youngest radio supernova
We report on the VLBI detection of supernova SN2011dh at 22GHz using a subset
of the EVN array. The observations took place 14 days after the discovery of
the supernova, thus resulting in a VLBI image of the youngest radio-loud
supernova ever. We provide revised coordinates for the supernova with
milli-arcsecond precision, linked to the ICRF. The recovered flux density is a
factor 2 below the EVLA flux density reported by other authors at the same
frequency and epoch of our observations. This discrepancy could be due to
extended emission detected with the EVLA or to calibration problems in the VLBI
and/or EVLA observations.Comment: Letter. Accepted in A&
Assessment of a high-resolution central scheme for the solution of the relativistic hydrodynamics equations
We assess the suitability of a recent high-resolution central scheme
developed by Kurganov & Tadmor (2000) for the solution of the relativistic
hydrodynamics equations. The novelty of this approach relies on the absence of
Riemann solvers in the solution procedure. The computations we present are
performed in one and two spatial dimensions in Minkowski spacetime. Standard
numerical experiments such as shock tubes and the relativistic flat-faced step
test are performed. As an astrophysical application the article includes
two-dimensional simulations of the propagation of relativistic jets using both
Cartesian and cylindrical coordinates. The simulations reported clearly show
the capabilities of the numerical scheme to yield satisfactory results, with an
accuracy comparable to that obtained by the so-called high-resolution
shock-capturing schemes based upon Riemann solvers (Godunov-type schemes), even
well inside the ultrarelativistic regime. Such central scheme can be
straightforwardly applied to hyperbolic systems of conservation laws for which
the characteristic structure is not explicitly known, or in cases where the
exact solution of the Riemann problem is prohibitively expensive to compute
numerically. Finally, we present comparisons with results obtained using
various Godunov-type schemes as well as with those obtained using other
high-resolution central schemes which have recently been reported in the
literature.Comment: 14 pages, 12 figures, to appear in A&
Robustness of a high-resolution central scheme for hydrodynamic simulations in full general relativity
A recent paper by Lucas-Serrano et al. indicates that a high-resolution
central (HRC) scheme is robust enough to yield accurate hydrodynamical
simulations of special relativistic flows in the presence of ultrarelativistic
speeds and strong shock waves. In this paper we apply this scheme in full
general relativity (involving {\it dynamical} spacetimes), and assess its
suitability by performing test simulations for oscillations of rapidly rotating
neutron stars and merger of binary neutron stars. It is demonstrated that this
HRC scheme can yield results as accurate as those by the so-called
high-resolution shock-capturing (HRSC) schemes based upon Riemann solvers.
Furthermore, the adopted HRC scheme has increased computational efficiency as
it avoids the costly solution of Riemann problems and has practical advantages
in the modeling of neutron star spacetimes. Namely, it allows simulations with
stiff equations of state by successfully dealing with very low-density
unphysical atmospheres. These facts not only suggest that such a HRC scheme may
be a desirable tool for hydrodynamical simulations in general relativity, but
also open the possibility to perform accurate magnetohydrodynamical simulations
in curved dynamic spacetimes.Comment: 4 pages, to be published in Phys. Rev. D (brief report
New Relativistic Effects in the Dynamics of Nonlinear Hydrodynamical Waves
In Newtonian and relativistic hydrodynamics the Riemann problem consists of
calculating the evolution of a fluid which is initially characterized by two
states having different values of uniform rest-mass density, pressure and
velocity. When the fluid is allowed to relax, one of three possible
wave-patterns is produced, corresponding to the propagation in opposite
directions of two nonlinear hydrodynamical waves. New effects emerge in a
special relativistic Riemann problem when velocities tangential to the initial
discontinuity surface are present. We show that a smooth transition from one
wave-pattern to another can be produced by varying the initial tangential
velocities while otherwise maintaining the initial states unmodified. These
special relativistic effects are produced by the coupling through the
relativistic Lorentz factors and do not have a Newtonian counterpart.Comment: 4 pages, 5 figure
Excited electron-bubble states in superfluid helium-4: a time-dependent density functional approach
We present a systematic study on the excited electron-bubble states in
superfluid helium-4 using a time-dependent density functional approach. For the
evolution of the 1P bubble state, two different functionals accompanied with
two different time-development schemes are used, namely an accurate
finite-range functional for helium with an adiabatic approximation for electron
versus an efficient zero-range functional for helium with a real-time evolution
for electron. We make a detailed comparison between the quantitative results
obtained from the two methods, which allows us to employ with confidence the
optimal method for suitable problems. Based on this knowledge, we use the
finite-range functional to calculate the time-resolved absorption spectrum of
the 1P bubble, which in principle can be experimentally determined, and we use
the zero-range functional to real-time evolve the 2P bubble for several
hundreds of picoseconds, which is theoretically interesting due to the break
down of adiabaticity for this state. Our results discard the physical
realization of relaxed, metastable 2P electron-bubblesComment: 16 pages, 12 figure
Robust topology optimization of continuum structures using Monte Carlo method and Kriging models
El objetivo de este trabajo es presentar una nueva metodologÃa eficiente y precisa llamada Monte Carlo y Kriging (MCK) para la optimización de topologÃa robusta. El objetivo es minimizar el valor esperado de la compliance considerando la existencia de incertidumbre con cargas concentradas. La incertidumbre en la carga puede presentarse en la magnitud, en la dirección y/o en la posición. La evaluación de la función objetivo se realiza utilizando el método de simulación de Monte Carlo en combinación con un modelo Kriging. Para estimar el valor esperado de la compliance, se transforma el problema probabilÃstico en otro determinÃstico sujeto a múltiples estados de carga mediante el Método de Monte Carlo pero empleando un reducido número de evaluaciones del modelo de simulación. Para ello es necesario construir un modelo Kriging del modelo de simulación a partir de una pequeña muestra obtenida con un hipercubo latino del espacio de diseño y predecir la compliance en cada uno de los puntos utilizados por la simulación de Monte Carlo. Dos ejemplos demuestran la precisión y eficiencia del algoritmo. Para verificar el algoritmo propuesto, los problemas también se resuelven mediante el método de Monte Carlo estándar.The aim of this paper is to introduce an efficient and accurate new approach called Monte Carlo and Kriging (MCK) to robust topology optimization. The objective is to minimize the expected value of compliance under concentrated loading uncertainty. The loading uncertainty may occur in magnitude, direction and/or position. The Monte Carlo simulation method and Kriging model are used to evaluate the objective function. To evaluate the expected value of compliance the probabilistic problem is transformed into a multiple loading deterministic one using of Monte Carlo method but with a reduced evaluations number of simulation model. A small sample obtained with a Latin Hypercube is used to build a Kriging model of the simulation model. This is utilized to estimate the compliance in those points used by Monte Carlo simulation method. Two problems are solved to demonstrate the efficiency and accuracy of the approach. The examples are solved again using a standard Monte Carlo simulation to check the proposed approach.Peer Reviewe
Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves
We report on a simultaneous modelling of the expansion and radio light curves
of SN1993J. We have developed a simulation code capable of generating synthetic
expansion and radio light curves of supernovae by taking into consideration the
evolution of the expanding shock, magnetic fields, and relativistic electrons,
as well as the finite sensitivity of the interferometric arrays used in the
observations. Our software successfully fits all the available radio data of SN
1993J with an standard emission model for supernovae extended with some
physical considerations, as an evolution in the opacity of the ejecta material,
a radial drop of the magnetic fields inside the radiating region, and a
changing radial density profile of the circumstellar medium beyond day 3100
after explosion.Comment: 12 pages, 12 figures, accepted for publication in A&
Experimental demonstration of a graph state quantum error-correction code
Scalable quantum computing and communication requires the protection of
quantum information from the detrimental effects of decoherence and noise.
Previous work tackling this problem has relied on the original circuit model
for quantum computing. However, recently a family of entangled resources known
as graph states has emerged as a versatile alternative for protecting quantum
information. Depending on the graph's structure, errors can be detected and
corrected in an efficient way using measurement-based techniques. In this
article we report an experimental demonstration of error correction using a
graph state code. We have used an all-optical setup to encode quantum
information into photons representing a four-qubit graph state. We are able to
reliably detect errors and correct against qubit loss. The graph we have
realized is setup independent, thus it could be employed in other physical
settings. Our results show that graph state codes are a promising approach for
achieving scalable quantum information processing
Simultaneous measurement of two non-commuting quantum variables: Solution of a dynamical model
The possibility of performing simultaneous measurements in quantum mechanics
is investigated in the context of the Curie-Weiss model for a projective
measurement. Concretely, we consider a spin- system simultaneously
interacting with two magnets, which act as measuring apparatuses of two
different spin components. We work out the dynamics of this process and
determine the final state of the measuring apparatuses, from which we can find
the probabilities of the four possible outcomes of the measurements. The
measurement is found to be non-ideal, as (i) the joint statistics do not
coincide with the one obtained by separately measuring each spin component, and
(ii) the density matrix of the spin does not collapse in either of the measured
observables. However, we give an operational interpretation of the process as a
generalised quantum measurement, and show that it is fully informative: The
expected value of the measured spin components can be found with arbitrary
precision for sufficiently many runs of the experiment.Comment: 24 pages, 9 figures; close to published versio
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