Crossover temperature of Bose-Einstein condensation in an atomic Fermi gas

We show that in an atomic Fermi gas near a Feshbach resonance the crossover between a Bose-Einstein condensate of diatomic molecules and a Bose-Einstein condensate of Cooper pairs occurs at positive detuning, i.e., when the molecular energy level lies in the two-atom continuum. We determine the crossover temperature as a function of the applied magnetic field and find excellent agreement with the experiment of Regal et al. [Phys. Rev. Lett. 92, 040403 (2004)] that has recently observed this crossover temperature.Comment: 4 pages, 2 figure

Simulation and modelling of transient electric fields in hvdc insulation systems based on polarization current measurements

Simulating and modelling electric field dynamics in the insulation of medium-and high-voltage DC electrical systems is needed to support insulation design optimization and to evaluate the impact of voltage transients on ageing mechanisms and insulation reliability. In order to perform accurate simulations, appropriate physical models must be adopted for the insulating material properties, particularly conductivity, which drives the electric field in a steady-state condition and contributes to determining the field behavior during voltage and load transients. In order to model insulation conductivity, polarization, and conduction, mechanisms must be inferred through charging and discharging current measurements, generally performed at different values of electric field and temperatures in flat specimens of the material under study. In general, both mechanisms are present, but one of them may be predominant with respect to the other depending on type of material. In this paper, we showed that models based on predominant polarization mechanisms were suitable to describe impregnated paper, but not polymers used for HV and MV DC insulation. In the latter case, indeed, trapping–detrapping and conduction phenomena were predominant compared to polarization, thus conductivity models had to be considered, in addition to or as a replacement of the polarization model, in order to carry out proper electric field simulations

Tunneling mediated by conical waves in a 1D lattice

The nonlinear propagation of 3D wave-packets in a 1D Bragg-induced band-gap system, shows that tranverse effects (free space diffraction) affect the interplay of periodicity and nonlinearity, leading to the spontaneous formation of fast and slow conical localized waves. Such excitation corresponds to enhanced nonlinear transmission (tunneling) in the gap, with peculiar features which differ on the two edges of the band-gap, as dictated by the full dispersion relationship of the localized waves.Comment: 5 pages, 6 figure

GPUs Based Material Point Method for Compressible Flows

Particle-In-Cell (PIC) methods such as the Material Point Method (MPM) can be cast in formulations suitable to the requirements of data locality and fine-grained parallelism of modern hardware accelerators such as Graphics Processing Units (GPUs). While continuum mechanics simulations have already shown the capabilities of MPM on a wide range of phenomena, the use of the method in compressible gas dynamics is less frequent. This contribution aims to show the potential of a GPU-based MPM parallel implementation for compressible fluid dynamics, as well as to assess the reliability of this approach in reproducing supersonic gas flows against solid obstacles. The results in the paper represent a stepping stone towards a highly parallel, Multi-GPU, MPM-base solver for M ach > 1 Fluid-Structure Interaction problems

An Integral Formulation for Rectangular Wires in a 3D Magneto-Quasi-Static Field

This paper proposes an integral formulation for calculating the magnetic and ohmic losses in rectangular wires immersed in an external 3D magneto-quasi-static field. The formulation is based on some simplifying assumptions that allow to employ a collocation method with constant elements for the discretization, reducing the computational burden. Even if the assumptions introduce an approximation, for the application at hand the numerical tests have shown that the accuracy is still acceptable. Moreover, the computational time is drastically reduced with respect to other approaches based on the finite element method

A software engine for multi-criteria decision support in flight management - Use of dynamic cost functions - Architecture and first results

Tactical trajectory optimisation should consider the total expected cost of the flight (fuel and delay). The cost of fuel can be estimated from the expected fuel usage. The cost of delay can be approximated by simple non-linear functions but, we propose a methodology to explicitly consider its different components: passenger related (regulation 261, duty of care, missed connections and soft costs), crew and maintenance, and reactionary costs (delay and curfew). This explicit modelling captures the non-continuous aspects of the cost function, which can significantly impact the optimisation profile, e.g. ensure that missed connections are reduced. The cost of delay, dependent on the arrival time at the gate, can be subject to uncertainties which are inherent (e.g. if a passenger will or not miss a connection) and external (e.g. taxi-in or holding times). Therefore, the optimisation framework should estimate the arrival time to the gate (not the runway) while considering these associated uncertainties. The described architecture models the processes affecting the cost (e.g. considering probabilities of missed connections or explicit propagation of delay) and operational aspects at arrival which impact the realisation of the planned optimised trajectory (holding time, sequencing and merging distance (tromboning), and taxi-in time). The consideration of the operational uncertainties enables the estimation of the probability of achieving the flight on-time performance. All these operational uncertainties are integrated into the cost function producing a total expected cost as a function of arrival to FL100 during the descent at the arrival airport. The trajectory is then optimised in its vertical and speed profile finding the cost index which is expected to minimise the total costs with a simulated annealing framework. The first results presented describe how the cost functions are generated, uncertainties considered and trajectories optimised for a flight in the LEDM-EDDF route

COSMOGRAIL: the COSmological MOnitoring of GRAvItational Lenses VII. Time delays and the Hubble constant from WFI J2033-4723

Gravitationally lensed quasars can be used to map the mass distribution in lensing galaxies and to estimate the Hubble constant H0 by measuring the time delays between the quasar images. Here we report the measurement of two independent time delays in the quadruply imaged quasar WFI J2033-4723 (z = 1.66). Our data consist of R-band images obtained with the Swiss 1.2 m EULER telescope located at La Silla and with the 1.3 m SMARTS telescope located at Cerro Tololo. The light curves have 218 independent epochs spanning 3 full years of monitoring between March 2004 and May 2007, with a mean temporal sampling of one observation every 4th day. We measure the time delays using three different techniques, and we obtain Dt(B-A) = 35.5 +- 1.4 days (3.8%) and Dt(B-C) = 62.6 +4.1/-2.3 days (+6.5%/-3.7%), where A is a composite of the close, merging image pair. After correcting for the time delays, we find R-band flux ratios of F_A/F_B = 2.88 +- 0.04, F_A/F_C = 3.38 +- 0.06, and F_A1/F_A2 = 1.37 +- 0.05 with no evidence for microlensing variability over a time scale of three years. However, these flux ratios do not agree with those measured in the quasar emission lines, suggesting that longer term microlensing is present. Our estimate of H0 agrees with the concordance value: non-parametric modeling of the lensing galaxy predicts H0 = 67 +13/-10 km s-1 Mpc-1, while the Single Isothermal Sphere model yields H0 = 63 +7/-3 km s-1 Mpc-1 (68% confidence level). More complex lens models using a composite de Vaucouleurs plus NFW galaxy mass profile show twisting of the mass isocontours in the lensing galaxy, as do the non-parametric models. As all models also require a significant external shear, this suggests that the lens is a member of the group of galaxies seen in field of view of WFI J2033-4723.Comment: 14 pages, 12 figures, published in A&

The KX method for producing K-band flux-limited samples of quasars

The longstanding question of the extent to which the quasar population is affected by dust extinction, within host galaxies or galaxies along the line of sight, remains open. More generally, the spectral energy distributions of quasars vary significantly and flux-limited samples defined at different wavelengths include different quasars. Surveys employing flux measurements at widely separated wavelengths are necessary to characterise fully the spectral properties of the quasar population. The availability of panoramic near-infrared detectors on large telescopes provides the opportunity to undertake surveys capable of establishing the importance of extinction by dust on the observed population of quasars. We introduce an efficient method for selecting K-band, flux-limited samples of quasars, termed ``KX'' by analogy with the UVX method. This method exploits the difference between the power-law nature of quasar spectra and the convex spectra of stars: quasars are relatively brighter than stars at both short wavelengths (the UVX method) and long wavelengths (the KX method). We consider the feasibility of undertaking a large-area KX survey for damped Ly-alpha galaxies and gravitational lenses using the planned UKIRT wide-field near-infrared camera.Comment: 6 pages, 3 figures, to appear in MNRA

GRB 050408: An Atypical Gamma-Ray Burst as a Probe of an Atypical Galactic Environment

The bright GRB 050408 was localized by HETE-II near local midnight, enabling an impressive ground-based followup effort as well as space-based followup from Swift. The Swift data from the X-Ray Telescope (XRT) and our own optical photometry and spectrum of the afterglow provide the cornerstone for our analysis. Under the traditional assumption that the visible waveband was above the peak synchrotron frequency and below the cooling frequency, the optical photometry from 0.03 to 5.03 days show an afterglow decay corresponding to an electron energy index of p_lc = 2.05 +/- 0.04, without a jet break as suggested by others. A break is seen in the X-ray data at early times (at ~12600 sec after the GRB). The spectral slope of the optical spectrum is consistent with p_lc assuming a host-galaxy extinction of A_V = 1.18 mag. The optical-NIR broadband spectrum is also consistent with p = 2.05, but prefers A_V = 0.57 mag. The X-ray afterglow shows a break at 1.26 x 10^4 sec, which may be the result of a refreshed shock. This burst stands out in that the optical and X-ray data suggest a large H I column density of N_HI ~ 10^22 cm^-2; it is very likely a damped Lyman alpha system and so the faintness of the host galaxy (M_V > -18 mag) is noteworthy. Moreover, we detect extraordinarily strong Ti II absorption lines with a column density through the GRB host that exceeds the largest values observed for the Milky Way by an order of magnitude. Furthermore, the Ti II equivalent width is in the top 1% of Mg II absorption-selected QSOs. This suggests that the large-scale environment of GRB 050408 has significantly lower Ti depletion than the Milky Way and a large velocity width (delta v > 200 km/s).Comment: ApJ submitte