End-effects of strongly charged polyelectrolytes - a molecular dynamics study

We investigate end-effects in the ion distribution around strongly charged, flexible polyelectrolytes with a quenched charge distribution by molecular dynamics simulations of dilute polyelectrolyte solutions. We take the counterions explicitly into account and calculate the full Coulomb interaction via an Ewald summation method. We find that the free counterions of the solution are distributed in such a way that a fraction of the chain charges is effectively neutralized. This in turn leads to an effective charge distribution which is similar to those found for weakly charged titrating polyelectrolytes that have an annealed charge distribution. The delicate interplay between the electrostatic interactions, the chain conformation and the counterion distribution is studied in detail as a function of different system parameters such as the chain length Nm, the charge fraction f, the charged particle density rho, the ionic strength and the solvent quality. Comparisons are made with predictions from a scaling theory.Comment: 20 pages, 10 figures. J. Chem. Phys, to appear June 200

Crustal Oscillations of Slowly Rotating Relativistic Stars

We study low-amplitude crustal oscillations of slowly rotating relativistic stars consisting of a central fluid core and an outer thin solid crust. We estimate the effect of rotation on the torsional toroidal modes and on the interfacial and shear spheroidal modes. The results compared against the Newtonian ones for wide range of neutron star models and equations of state.Comment: 15 page

Shear Waves and Giant Flare Oscillations from Soft Gamma-Ray Repeaters

Recent observations of giant flares from soft gamma-ray repeaters have exhibited multiple 25-150 Hz oscillations. Frequencies in this range are expected for toroidal shear waves in a neutron star (NS) crust, lending support to Duncan's proposal that such modes may be excited in these events. This motivates a reassessment of how these waves reflect the NS structure and what role the magnetic field plays in setting their frequencies. We calculate the eigenfrequencies and eigenfunctions of toroidal oscillations for a realistic NS crust, including a vertical magnetic field at magnetar strengths ($B\sim10^{14}-10^{15} {\rm G}$). The lowest radial-order mode has a red-shifted frequency of $\approx28 {\rm Hz}[l(l+1)/6]^{1/2}$, with the prefactor depending on the NS's mass and radius, and its crust's depth and composition. This mode is independent of the magnetic field for $B\lesssim4\times10^{15} {\rm G}$, a limit much greater than the inferred dipole magnetic fields for these objects. Though this is a good fit to the observed oscillations, only rather loose constraints can be made for the NSs' properties because all that can be fit is this prefactor (a single parameter). Modes with shorter radial wavelengths are more sensitive to the magnetic field starting at $B\sim2\times10^{14} {\rm G}$ and have higher frequencies (600-2000 Hz). The discovery of these modes, coupled with the oscillations observed thus far, would provide a powerful probe to the NS crustal structure.Comment: Submitted for publication in The Astrophysical Journal Letters, 4 pages, 3 figure

Design of metallic nanoparticles gratings for filtering properties in the visible spectrum

Plasmonic resonances in metallic nanoparticles are exploited to create efficient optical filtering functions. A Finite Element Method is used to model metallic nanoparticles gratings. The accuracy of this method is shown by comparing numerical results with measurements on a two-dimensional grating of gold nanocylinders with elliptic cross section. Then a parametric analysis is performed in order to design efficient filters with polarization dependent properties together with high transparency over the visible range. The behavior of nanoparticle gratings is also modelled using the Maxwell-Garnett homogenization theory and analyzed by comparison with the diffraction by a single nanoparticle. The proposed structures are intended to be included in optical systems which could find innovative applications.Comment: submitted to Applied Optic

Discovery of Fast X-ray Oscillations During the 1998 Giant Flare from SGR 1900+14

We report the discovery of complex high frequency variability during the August 27, 1998 giant flare from SGR 1900+14 using the Rossi X-ray Timing Explorer (RXTE). We detect an 84 Hz oscillation (QPO) during a 1 s interval beginning approximately 1 min after the initial hard spike. The modulation amplitude is energy dependent, reaching a maximum of 26% (rms) for photons above 30 keV, and is not detected below 11 keV, with a 90% confidence upper limit of 14% (rms). Remarkably, additional QPOs are detected in the average power spectrum of data segments centered on the rotational phase at which the 84 Hz signal was detected. Two signals, at 53.5 and 155.1 Hz, are strongly detected, while a third feature at 28 Hz is found with lower significance. These QPOs are not detected at other rotational phases. The phenomenology seen in the SGR 1900+14 flare is similar to that of QPOs recently reported by Israel et al. from the December 27, 2004 flare from SGR 1806-20, suggesting they may have a common origin, perhaps torsional vibrations of the neutron star crust. Indeed, an association of the four frequencies (in increasing order) found in SGR 1900+14 with l = 2, 4, 7, and 13 toroidal modes appears plausible. We discuss our findings in the context of this model and show that if the stars have similar masses then the magnetic field in SGR 1806-20 must be about twice as large as in SGR 1900+14, broadly consistent with magnetic field estimates from pulse timing.Comment: 13 Pages, 5 figures, AASTeX, accepted for publication in the Astrophysical Journal Letter

A second catalog of gamma ray bursts: 1978 - 1980 localizations from the interplanetary network

Eighty-two gamma ray bursts were detected between 1978 September 14 and 1980 February 13 by the experiments of the interplanetary network (Prognoz 7, Venera 11 and 12 SIGNE experiments, Pioneer Venus Orbiter, International Sun-Earth Explorer 3, Helios 2, and Vela). Sixty-five of these events have been localized to annuli or error boxes by the method of arrival time analysis. The distribution of sources is consistent with isotropy, and there is no statistically convincing evidence for the detection of more than one burst from any source position. The localizations are compared with those of two previous catalogs

Limits to the burster repetition rate as deduced from the 2nd catalog of the interplanetary network

The burster repetition rate is an important parameter in many gamma ray burst models. The localizations of the interplanetary network, which have a relatively small combined surface area, may be used to estimate the average repetition rate. The method consists of (1) estimating the number of random overlaps between error boxes expected in the catalog and comparing this number to that actually observed; (2) modeling the response of the detectors in the network, so that the probability of detecting a burst can be estimated; and (3) simulating the arrival of bursts at the network assuming that burster repetition is governed by a Poisson process. The application of this method for many different burster luminosity functions shows that (1) the lower limit to the burster repetition rate depends strongly upon the assumed luminosity function; (2) the best lower limit to the repetition period obtainable from the data of the network is about 100 months; and (3) that a luminosity function for all bursters similar to that of the 1979 Mar 5 burster is inconsistent with the data

Three precise gamma-ray burst source locations

The precise source regions of three moderately intense gamma ray bursts are derived. These events were observed with the first interplanetary burst sensor network. The optimum locations of the detectors, widely separated throughout the inner solar system, allowed for high accuracy, over-determined source fields of size 0.7 to 7.0 arc-min(2). All three locations are at fairly high galactic latitude in regions of low source confusion; none can be identified with a steady source object. Archived photographs were searched for optical transients that are able to be associated with these source fields; one such association was made

Effect of ambient temperature variations on an indigenous microalgae-nitrifying bacteria culture dominated by Chlorella

[EN] Two outdoor photobioreactors were operated to evaluate the effect of variable ambient temperature on an indigenous microalgae-nitrifying bacteria culture dominated by Chlorella. Four experiments were carried out in different seasons, maintaining the temperature-controlled PBR at around 25¿°C (by either heating or cooling), while the temperature in the non-temperature-controlled PBR was allowed to vary with the ambient conditions. Temperatures in the range of 15¿30¿°C had no significant effect on the microalgae cultivation performance. However, when the temperature rose to 30¿35¿°C microalgae viability was significantly reduced. Sudden temperature rises triggered AOB growth in the indigenous microalgae culture, which worsened microalgae performance, especially when AOB activity made the system ammonium-limited. Microalgae activity could be recovered after a short temperature peak over 30¿°C once the temperature dropped, but stopped when the temperature was maintained around 28¿30¿°C for several days.This research work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO, Projects CTM2014-54980-C2-1-R and CTM2014-54980-C2-2-R) jointly with the European Regional Development Fund (ERDF), both of which are gratefully acknowledged. It also received support from the Spanish Ministry of Education, Culture and Sport via a pre-doctoral FPU fellowship to authors J. González-Camejo (FPU14/05082) and S. Aparicio (FPU/15/02595).Gonzalez-Camejo, J.; Aparicio Antón, SE.; Ruano, M.; Borrás, L.; Barat, R.; Ferrer, J. (2019). Effect of ambient temperature variations on an indigenous microalgae-nitrifying bacteria culture dominated by Chlorella. Bioresource Technology. 290:1-10. https://doi.org/10.1016/j.biortech.2019.121788S11029

Flow, viscoelastic and masticatory properties of tailor made thickened pea cream for people with swallowing problems

[EN] This study evaluated the flow, viscoelastic and masticatory properties of tailor made pea cream using different hydrocolloids, and the potential effect that saliva could have while chewing. The creams thickened with pectin, carboxymethyl cellulose, tara gum and konjac gum showed the weakest internal gel structure with highest values of loss tangent at 1Hz (0.49 +/- 0.07), slope of log elastic modulus versus log frequency (0.34 +/- 0.05) and maximum capacitance (0.07 +/- 0.02 Pa-1). The samples thickened with commercial hydrocolloids were affected the most by the presence of saliva, with a significant reduction in apparent viscosity at 10 s(-1), that ranged from 1500 mPas until 11000 mPas. These results demonstrate that it is possible to optimize the structure of thickened pea cream for dysphagia by using, for example, carboxymethyl cellulose, tara gum and konjac gum, because they provide a greater viscous component, and, therefore, a bolus that is more easy to swallow.The authors thank the "Ministerio Espanol de Ciencia e Innovaci.on" for the financial support provided through the RTI2018-098842-B-I00 Project.Talens Oliag, P.; Castells, ML.; Verdú, S.; Barat Baviera, JM.; Grau Meló, R. (2021). Flow, viscoelastic and masticatory properties of tailor made thickened pea cream for people with swallowing problems. Journal of Food Engineering. 292:1-10. https://doi.org/10.1016/j.jfoodeng.2020.110265S11029