Nonlinear electrodynamics and the gravitational redshift of highly magnetised neutron stars

The idea that the nonlinear electromagnetic interaction, i. e., light propagation in vacuum, can be geometrized was developed by Novello et al. (2000) and Novello & Salim (2001). Since then a number of physical consequences for the dynamics of a variety of systems have been explored. In a recent paper Mosquera Cuesta & Salim (2003) presented the first astrophysical study where such nonlinear electrodynamics (NLEDs) effects were accounted for in the case of a highly magnetized neutron star or pulsar. In that paper the NLEDs was invoked {\it a l\`a} Euler-Heisenberg, which is an infinite series expansion of which only the first term was used for the analisys. The immediate consequence of that study was an overall modification of the space-time geometry around the pulsar, which is ``perceived'', in principle, only by light propagating out of the star. This translates into an significant change in the surface redshift, as inferred from absorption (emission) lines observed from a super magnetized pulsar. The result proves to be even more dramatic for the so-called magnetars, pulsars endowed with magnetic ($B$) fields higher then the Schafroth quantum electrodynamics critical $B$-field. Here we demonstrate that the same effect still appears if one calls for the NLEDs in the form of the one rigorously derived by Born & Infeld (1934) based on the special relativistic limit for the velocity of approaching of an elementary particle to a pointlike electron [From the mathematical point of view, the Born & Infeld (1934) NLEDs is described by an exact Lagrangean, whose dynamics has been successfully studied in a wide set of physical systems.].Comment: Accepted for publication in Month. Not. Roy. Ast. Soc. latex file, mn-1.4.sty, 5 pages, 2 figure

Neural networks and spectra feature selection for retrival of hot gases temperature profiles

Proceeding of: International Conference on Computational Intelligence for Modelling, Control and Automation, 2005 and International Conference on Intelligent Agents, Web Technologies and Internet Commerce, Vienna, Austria 28-30 Nov. 2005Neural networks appear to be a promising tool to solve the so-called inverse problems focused to obtain a retrieval of certain physical properties related to the radiative transference of energy. In this paper the capability of neural networks to retrieve the temperature profile in a combustion environment is proposed. Temperature profile retrieval will be obtained from the measurement of the spectral distribution of energy radiated by the hot gases (combustion products) at wavelengths corresponding to the infrared region. High spectral resolution is usually needed to gain a certain accuracy in the retrieval process. However, this great amount of information makes mandatory a reduction of the dimensionality of the problem. In this sense a careful selection of wavelengths in the spectrum must be performed. With this purpose principal component analysis technique is used to automatically determine those wavelengths in the spectrum that carry relevant information on temperature distribution. A multilayer perceptron will be trained with the different energies associated to the selected wavelengths. The results presented show that multilayer perceptron combined with principal component analysis is a suitable alternative in this field.Publicad

A white dwarf-neutron star relativistic binary model for soft gamma-ray repeaters

A scenario for SGRs is introduced in which gravitational radiation reaction effects drive the dynamics of an ultrashort orbital period X-ray binary embracing a high-mass donor white dwarf (WD) to a rapidly rotating low magnetised massive neutron star (NS) surrounded by a thick, dense and massive accretion torus. Driven by GR reaction, sparsely, the binary separation reduces, the WD overflows its Roche lobe and the mass transfer drives unstable the accretion disk around the NS. As the binary circular orbital period is a multiple integer number ($m$) of the period of the WD fundamental mode (Pons et al. 2002), the WD is since long pulsating at its fundamental mode; and most of its harmonics, due to the tidal interaction with its NS orbital companion. Hence, when the powerful irradiation glows onto the WD; from the fireball ejected as part of the disk matter slumps onto the NS, it is partially absorbed. This huge energy excites other WD radial ($p$-mode) pulsations (Podsiadlowski 1991,1995). After each mass-transfer episode the binary separation (and orbital period) is augmented significantly (Deloye & Bildsten 2003; Al\'ecyan & Morsink 2004) due to the binary's angular momentum redistribution. Thus a new adiabatic inspiral phase driven by GR reaction starts which brings the binary close again, and the process repeats. This model allows to explain most of SGRs observational features: their recurrent activity, energetics of giant superoutbursts and quiescent stages, and particularly the intriguing subpulses discovered by BeppoSAX (Feroci et al. 1999), which are suggested here to be {\it overtones} of the WD radial fundamental mode (see the accompanying paper: Mosquera Cuesta 2004b).Comment: This paper was submitted as a "Letter to the Editor" of MNRAS in July 17/2004. Since that time no answer or referee report was provided to the Author [MNRAS publication policy limits reviewal process no longer than one month (+/- half more) for the reviewal of this kind of submission). I hope this contribution is not receiving a similar "peer-reviewing" as given to the A. Dar and A. De Rujula's "Cannonball model for gamma-ray bursts", or to the R.K. Williams' "Penrose process for energy extraction from rotating black holes". The author welcomes criticisms and suggestions on this pape

Imperfect Imitation Can Enhance Cooperation

The promotion of cooperation on spatial lattices is an important issue in evolutionary game theory. This effect clearly depends on the update rule: it diminishes with stochastic imitative rules whereas it increases with unconditional imitation. To study the transition between both regimes, we propose a new evolutionary rule, which stochastically combines unconditional imitation with another imitative rule. We find that, surprinsingly, in many social dilemmas this rule yields higher cooperative levels than any of the two original ones. This nontrivial effect occurs because the basic rules induce a separation of timescales in the microscopic processes at cluster interfaces. The result is robust in the space of 2x2 symmetric games, on regular lattices and on scale-free networks.Comment: 4 pages, 4 figure