On a static charged fluid around a magnetized mass

We show that any magnetostatic axially symmetric solution of the Einstein-Maxwell equations can be endowed with a specific charged fluid source of the Polanco et al type via a simple procedure requiring the knowledge of exclusively the magnetostatic seed spacetime. Using this procedure we construct yet another exact solution for a massive magnetic dipole surrounded by a static charged fluid which is different from the Polanco et al metric.Comment: 5 pages, no figures, submitted to Physical Review

Top quark production cross-section at the Tevatron Run 2

The top quark pair production cross-section ${\sigma}_{t\bar{t}}$ has been measured in $p\bar{p}$ collisions at center of mass energies of 1.96 TeV using Tevatron Run 2 data. In the begining of Run 2 both CDF and D\O\ $\sigma_{t\bar{t}}$ measurements in the {\it dilepton} channel $t\bar{t}{\to}WbW\bar{b}{\to}\bar{\ell}{\nu}_{\ell}b{\ell}^{'}\ bar{\nu}_{{\ell}^{'}}\bar{b}$and in the {\it lepton plus jets} channel$t\bar{t}{\to}WbW\bar{b}{\to}q\bar{q}^{'}b{\ell}\bar{\nu}_{\ell }\bar{b}+\bar{\ell}{\nu}_{\ell}bq\bar{q}^{'}\bar{b}$ agree with the NLO (Next-to-Leading-Order) theoretical predictions. The presence of a top signal in Tevatron data has been reestablished.Comment: 4 pages, 3 figure

Utility analysis : current trends and future directions.

Utility analysis procedures offer organizational decision-makers useful information regarding the relative values of different interventions. Years of research have resulted in a number of practically viable utility models and extensions. There is a continued need for research to examine the accuracy of utility estimates and to further compare the different models. A more recent research concern is that of low levels of acceptance of utility analysis results by practitioners. Many researchers are turning their attention to ways in which this acceptance may be increased. This article reviews different utility models as well as a number of important extensions. It then discusses current utility analysis issues, such as the aforementioned acceptance problem and the introduction of a multi-attribute utility model. The article concludes with suggestions for future utility analysis researchUtility theory; Mathematical models; Decision making;

A combined Majumdar-Papapetrou-Bonnor field as extreme limit of the double-Reissner-Nordstrom solution

The general extreme limit of the double-Reissner-Nordstrom solution is worked out in explicit analytical form involving prolate spheroidal coordinates. We name it the combined Majumdar-Papapetrou-Bonnor field to underline the fact that it contains as particular cases the two-body specialization of the well-known Majumdar-Papapetrou solution and Bonnor's three-parameter electrostatic field. To the latter we give a precise physical interpretation as describing a pair of non-rotating extremal black holes with unequal masses and unequal opposite charges kept apart by a strut, the absolute values of charges exceeding the respective (positive) values of masses.Comment: 10 pages, 3 figures, title changed, the result extended, matching the published versio

Talbot effect for dispersion in linear optical fibers and a wavelet approach

We shortly recall the mathematical and physical aspects of Talbot's self-imaging effect occurring in near-field diffraction. In the rational paraxial approximation, the Talbot images are formed at distances z=p/q, where p and q are coprimes, and are superpositions of q equally spaced images of the original binary transmission (Ronchi) grating. This interpretation offers the possibility to express the Talbot effect through Gauss sums. Here, we pay attention to the Talbot effect in the case of dispersion in optical fibers presenting our considerations based on the close relationships of the mathematical representations of diffraction and dispersion. Although dispersion deals with continuous functions, such as gaussian and supergaussian pulses, whereas in diffraction one frequently deals with discontinuous functions, the mathematical correspondence enables one to characterize the Talbot effect in the two cases with minor differences. In addition, we apply, for the first time to our knowledge, the wavelet transform to the fractal Talbot effect in both diffraction and fiber dispersion. In the first case, the self similar character of the transverse paraxial field at irrational multiples of the Talbot distance is confirmed, whereas in the second case it is shown that the field is not self similar for supergaussian pulses. Finally, a high-precision measurement of irrational distances employing the fractal index determined with the wavelet transform is pointed outComment: 15 text pages + 7 gif figs, accepted at Int. J. Mod. Phys. B, final version of a contribution at ICSSUR-Besancon (May/05). Color figs available from the first autho

Stars and brown dwarfs in the sigma Orionis cluster. III. OSIRIS/GTC low-resolution spectroscopy of variable sources

Context. Although many studies have been performed so far, there are still dozens of low-mass stars and brown dwarfs in the young sigma Orionis open cluster without detailed spectroscopic characterisation. Aims. We look for unknown strong accretors and disc hosts that were undetected in previous surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran Telescopio Canarias under very poor weather conditions. These objects display variability in the optical, infrared, Halpha, and/or X-rays on time scales of hours to years. We complemented our spectra with optical and near-/mid-infrared photometry. Results. For seven targets, we detected lithium in absorption, identified Halpha, the calcium doublet, and forbidden lines in emission, and/or determined spectral types for the first time. We characterise in detail a faint, T Tauri-like brown dwarf with an 18 h-period variability in the optical and a large Halpha equivalent width of -125+/-15 AA, as well as two M1-type, X-ray-flaring, low-mass stars, one with a warm disc and forbidden emission lines, the other with a previously unknown cold disc with a large inner hole. Conclusions. New unrevealed strong accretors and disc hosts, even below the substellar limit, await discovery among the list of known sigma Orionis stars and brown dwarfs that are variable in the optical and have no detailed spectroscopic characterisation yet.Comment: A&A, in press (accepted for publication in section 14. Catalogs and data of Astronomy and Astrophysics