Institute of Space science

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Institute of Space science

Spain
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Garcia-Piquer A.,Institute of Space science | Fornells A.,Ramon Llull University | Bacardit J.,Newcastle University | Orriols-Puig A.,Ramon Llull University | Golobardes E.,Ramon Llull University
IEEE Transactions on Evolutionary Computation | Year: 2014

Multiobjective evolutionary clustering algorithms are based on the optimization of several objective functions that guide the search following a cycle based on evolutionary algorithms. Their capabilities allow them to find better solutions than with conventional clustering algorithms if the suitable individual representation is selected. This paper provides a detailed analysis of the three most relevant and useful representations-prototype-based, label-based, and graph-based-through a wide set of synthetic data sets. Moreover, they are also compared to relevant conventional clustering algorithms. Experiments show that multiobjective evolutionary clustering is competitive with regard to other clustering algorithms. Furthermore, the best scenario for each representation is also presented. © 2013 IEEE.


PubMed | German Electron Synchrotron, Institute Of Fisica Daltes Energies, ETH Zurich, Bhabha Atomic Research Center and 28 more.
Type: Journal Article | Journal: Science (New York, N.Y.) | Year: 2014

Supermassive black holes with masses of millions to billions of solar masses are commonly found in the centers of galaxies. Astronomers seek to image jet formation using radio interferometry but still suffer from insufficient angular resolution. An alternative method to resolve small structures is to measure the time variability of their emission. Here we report on gamma-ray observations of the radio galaxy IC 310 obtained with the MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescopes, revealing variability with doubling time scales faster than 4.8 min. Causality constrains the size of the emission region to be smaller than 20% of the gravitational radius of its central black hole. We suggest that the emission is associated with pulsar-like particle acceleration by the electric field across a magnetospheric gap at the base of the radio jet.


Boniface K.,Environment Canada | Aparicio J.M.,Environment Canada | Cardellach E.,Institute of Space science
Atmospheric Measurement Techniques | Year: 2011

Vertical profiles of the atmosphere can be obtained globally with the radio-occultation technique. However, the lowest layers of the atmosphere are less accurately extracted. A good description of these layers is important for the good performance of Numerical Weather Prediction (NWP) systems, and an improvement of the observational data available for the low troposphere would thus be of great interest for data assimilation. We outline here how supplemental meteorological information close to the surface can be extracted whenever reflected signals are available. We separate the reflected signal through a radioholographic filter, and we interpret it with a ray tracing procedure, analyzing the trajectories of the electromagnetic waves over a 3-D field of refractive index. A perturbation approach is then used to perform an inversion, identifying the relevant contribution of the lowest layers of the atmosphere to the properties of the reflected signal, and extracting some supplemental information to the solution of the inversion of the direct propagation signals. It is found that there is a significant amount of useful information in the reflected signal, which is sufficient to extract a stand-alone profile of the low atmosphere, with a precision of approximately 0.1 %. The methodology is applied to one reflection case. © 2011 Author(s).


Witek H.,University of Lisbon | Witek H.,University of Cambridge | Cardoso V.,University of Lisbon | Cardoso V.,University of Mississippi | And 5 more authors.
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

Light bosonic degrees of freedom have become a serious candidate for dark matter, which seems to pervade our entire Universe. The evolution of these fields around curved spacetimes is poorly understood but is expected to display interesting effects. In particular, the interaction of light bosonic fields with supermassive black holes, key players in most galaxies, could provide colorful examples of superradiance and nonlinear bosenovalike collapse. In turn, the observation of spinning black holes is expected to impose stringent bounds on the mass of putative massive bosonic fields in our Universe. Our purpose here is to present a comprehensive study of the evolution of linearized massive scalar and vector fields in the vicinities of rotating black holes. The evolution of generic initial data has a very rich structure, depending on the mass of the field and of the black hole. Quasinormal ringdown or exponential decay followed by a power-law tail at very late times is a generic feature of massless fields at intermediate times. Massive fields generically show a transition to power-law tails early on. For a certain boson field mass range, the field can become trapped in a potential barrier outside the horizon and transition to a bound state. Because there are a number of such quasibound states, the generic outcome is an amplitude modulated sinusoidal, or beating, signal, whose envelope is well described by the two lowest overtones. We believe that the appearance of such beatings has gone unnoticed in the past, and in fact mistaken for exponential growth. The amplitude modulation of the signal depends strongly on the relative excitation of the overtones, which in turn is strongly tied to the bound state geography. A fine-tuning of the initial data allows one to see the evolution of the nearly pure bound state mode that turns unstable for sufficiently large black hole (BH) rotation. For the first time we explore massive vector fields in a generic black hole background that are difficult, if not impossible, to separate in the Kerr background. Our results show that spinning BHs are generically strongly unstable against massive vector fields. © 2013 American Physical Society.


Sperhake U.,University of Cambridge | Sperhake U.,Institute of Space science | Sperhake U.,University of Lisbon | Sperhake U.,California Institute of Technology | And 6 more authors.
Physical Review Letters | Year: 2013

We explore the impact of black hole spins on the dynamics of high-energy black hole collisions. We report results from numerical simulations with γ factors up to 2.49 and dimensionless spin parameter χ=+0.85, +0.6, 0, -0.6, -0.85. We find that the scattering threshold becomes independent of spin at large center-of-mass energies, confirming previous conjectures that structure does not matter in ultrarelativistic collisions. It has further been argued that in this limit all of the kinetic energy of the system may be radiated by fine tuning the impact parameter to threshold. On the contrary, we find that only about 60% of the kinetic energy is radiated for γ=2.49. By monitoring apparent horizons before and after scattering events we show that the "missing energy" is absorbed by the individual black holes in the encounter, and moreover the individual black-hole spins change significantly. We support this conclusion with perturbative calculations. An extrapolation of our results to the limit γ→∞ suggests that about half of the center-of-mass energy of the system can be emitted in gravitational radiation, while the rest must be converted into rest-mass and spin energy. © 2013 American Physical Society.


Baleanu D.,Cankaya University | Baleanu D.,Institute of Space science | Asad J.H.,University of Tabuk | Petras I.,Technical University of Košice
Romanian Reports in Physics | Year: 2012

In this paper we study the fractional Lagrangian of the two-electric pendulum. We obtained the fractional Euler-Lagrangian equation of the system and then we studied the obtained Euler-Lagrangian equation analytically, and numerically. The numerical method used here is based on Grünwald-Letnikov definition of left and right fractional derivatives.


Sperhake U.,University of Cambridge | Sperhake U.,Institute of Space science | Sperhake U.,California Institute of Technology | Sperhake U.,University of Lisbon
International Journal of Modern Physics D | Year: 2013

We present an overview of recent developments in numerical relativity studies of higher dimensional spacetimes with a focus on time evolutions of black hole (BH) systems. After a brief review of the numerical techniques employed for these studies, we summarize results grouped into the following three areas: (i) numerical studies of fundamental properties of BHs, (ii) applications of BH collisions to the modeling of Trans-Planckian scattering and (iii) numerical studies of asymptotically anti-de Sitter spacetimes in the context of the gauge-gravity duality. © 2013 World Scientific Publishing Company.


Faruque M.R.I.,Institute of Space Science | Faruque M.R.I.,National University of Malaysia | Islam M.T.,Institute of Space Science | Misran N.,Institute of Space Science | Misran N.,National University of Malaysia
Informacije MIDEM | Year: 2012

The reducing electromagnetic (EM) absorption reduction with a new type of triangular split ring resonators (TSSRs) based on triangular metamaterials (TMMs) attachment is investigated. The finite-difference time-domain method with lossy-Drude model is adopted in this investigation. The technique of EM absorption reduction is discussed and the effects of position, distance, and size of new design of metamaterials are investigated. Metamaterials have achieved a 63.40% reduction of the initial SAR value for the case of 10 gm SAR.


Yang X.-J.,China University of Mining and Technology | Baleanu D.,Cankaya University | Baleanu D.,Institute of Space science | Machado J.A.T.,Polytechnic Institute of Porto
Fractional Dynamics | Year: 2015

In this chapter an efficient numerical algorithm for solving ODEs using the extended differential transform method via the generalized local fractional Taylor theorem is presented. Four examples are studied in order to illustrate the proposed technique. © 2015 Xiao-Jun Yang et al.

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