Polarization and light curve variability: the "patchy shell" model

Recent advances in early detection and detailed monitoring of GRB afterglows have revealed variability in some afterglow light curves. One of the leading models for this behavior is the patchy shell model. This model attributes the variability to random angular fluctuations in the relativistic jet energy. These an-axisymmetric fluctuations should also impose variations in the degree and angle of polarization that are correlated to the light curve variability. In this letter we present a solution of the light curve and polarization resulting from a given spectrum of energy fluctuations. We compare light curves produced using this solution to the variable light curve of GRB 021004 and we show that the main features in both the light curve and the polarization fluctuations are very well reproduced by this model. We use our results to draw constraints on the characteristics of the energy fluctuations that might have been present in GRB 021004.Comment: 10 pages, 2 figures, Final version to appear in ApJ

Ion thermal effects in oscillating multi-ion plasma sheath theory

The effects of ion temperature are discussed in a two-ion electron plasma and for a model applicable to the oscillating sheath theory that has recently been much in the focus of researchers. The differences between the fluid and kinetic models have been pointed out, as well as the differences between the approximative kinetic description (which involves the expansion of the plasma dispersion function), and the exact kinetic description. It is shown that the approximative kinetic description, first, can not describe the additional acoustic mode which naturally exists in the plasma with an additional ion population with a finite temperature, and, second, it yields an inaccurate Landau damping of the bulk ion acoustic mode. The reasons for these two failures are described. In addition to this, a fluid model is presented that is capable of capturing both of these features that are missing in the approximative kinetic description, i.e., two (fast and slow) ion acoustic modes, and the corresponding Landau damping of both modes

Dissociative multi-photon ionization of isolated uracil and uracil-adenine complexes

Recent multi-photon ionization (MPI) experiments on uracil revealed a fragment ion at m/z 84 that was proposed as a potential marker for ring opening in the electronically excited neutral molecule. The present MPI measurements on deuterated uracil identify the fragment as C3H4N2O+ (uracil+ less CO), a plausible dissociative ionization product from the theoretically predicted open-ring isomer. Equivalent measurements on thymine do not reveal an analogous CO loss channel, suggesting greater stability of the excited DNA base. MPI and electron impact ionization experiments have been carried out on uracil-adenine clusters in order to better understand the radiation response of uracil within RNA. Evidence for C3H4N2O+ production from multi-photon-ionized uracil-adenine clusters is tentatively attributed to a significant population of π-stacked configurations in the neutral beam

Electro-Deposition of Polymer Chains on an Adsorbing Wall: Density Profiles and Wall Coverage

Growth of polymer density in an electro-deposition model of polymer chains on an impenetrable wall is studied on a two dimensional discrete lattice using a Monte Carlo simulation. Polymer-polymer repulsion and polymer-wall attraction for the adsorbing wall (along with the neutral and repulsive interactions) are considered in an external field. Effects of the field strength (B), temperature (T), and chain length (L-c) on the density profile of the polymer chains and wall coverage are investigated. The spatial density profile shows onset of oscillation near the wall at a characteristic field (B-c) which depends on chain length and temperature, In low field, adsorption-co-desorption transition at the wall appear on increasing the temperature (unlike neutral and repulsive walls). In high field regime, on the other hand, a non-monotonic dependence of coverage on temperature is observed with a maximum at a temperature (T-m) which increases on increasing B. The equilibrium value of the polymer density (P-d) shows a power-law decay with the chain length, p(d) similar to L-c(-alpha), at the wall and in the bulk with corresponding values of the exponent alpha(w) and alpha(B); these exponents differ substantially and depend on B, T, and L-c. The coverage decays monotonically with the chain length at a constant temperature and field. (C) 1997 American Institute of Physics. [S0021-9606(97)51347-X]

Discrete-to-Continuum Simulation Approach to Polymer Chain Systems: Subdiffusion, Segregation, and Chain Folding

A discrete-to-continuum approach is introduced to study the static and dynamic properties of polymer chain systems with a bead-spring chain model in two dimensions. A finitely extensible nonlinear elastic potential is used for the bond between the consecutive beads with the Lennard-Jones (LJ) potential with smaller (Rc=21/6σ=0.95) and larger (Rc=2.5σ=2.1) values of the upper cutoff for the nonbonding interaction among the neighboring beads. We find that chains segregate at temperature T =1.0 with Rc=2.1 and remain desegregated with Rc=0.95. At low temperature (T=0.2), chains become folded, in a ribbonlike conformation, unlike random and self-avoiding walk conformations at T=1.0. The power-law dependence of the rms displacements of the center of mass (Rc.m.) of the chains and their center node (Rcn) with time are nonuniversal, with the range of exponents v1≃045−0.25 and v2≃0.30−0.10, respectively. Both radius of gyration (Rg) and average bond length (⧼I⧽) decrease on increasing the range of interaction (Rc), consistent with the extended state in good solvent to collapsed state in poor solvent description of the polymer chains. Analysis of the radial distribution function supports these observations

Computer Simulation Study of the Permeability of Driven Polymers Through Porous Media

A computer simulation model is used to study the permeability of polymer chains driven by a biased flow field through a porous medium in two dimensions. The chains are modeled by constrained self-avoiding walks, which reptate through the heterogeneous medium with a biased probability imposed by the driven field. A linear response description is used to evaluate an effective permeability. The permeability σ shows an unusual decay behavior on reducing the porosity ps. We find that the permeability decreases on increasing the bias above a characteristic value Bc. This characteristic bias shows a logarithmic decay on reducing the porosity, Bc∼−γ(1−ps), with γ≃0.35. The permeability decays with the length (Lc) of the chains; at low polymer concentration it shows a power-law decay, σ∼L−αc, the exponent α is nonuniversal and depends on both the porosity as well as the biased field (α≃1.64–3.73). We find that the biased field B and porosity ps affect the conformation of the chains. The radius of gyration Rg of the chains increases with increasing biased field in high porosity, while it decreases on decreasing the porosity at high field bias. In high porosity and low polymer concentrations, the radius of gyration shows a power-law dependence on the chain length, Rg∼Lvc, with ν depending on the biased field (ν≃0.84–0.94). In order to explain the deviations from the Darcy Law for the polymer flow, a plausible nonlinear response theory via a power-law response formula is suggested; we point out the associated complexities involved in addressing the flow problems in driven polymers

Nonuniversal Scaling and Conformational Crossover of Polymer Chains in an Electrophoretic Deposition

A computer simulation model of electrodeposition of polymer chains on an impenetrable wall is used to evaluate the power-law scaling exponents (νx(y)) for the longitudinal and transverse spread, Rgx(y)∼Lcνx(y); we find that the exponents νx(y) depend on the field strength, i.e., they are nonuniversal. A conformational crossover is observed for the transverse spread from the bulk with νy≃1/3-2/3 to the wall with νy≃2/3-1. A similar crossover also occurs for the longitudinal component of Rg but with an opposite trend, i.e., magnitude of νx is larger in bulk than at the wall

Conformation of Interacting Polymer Chains: Effects of Temperature, Bias, Polymer Concentration, and Porosity

The conformations of interacting polymer chains driven by a biased field in heterogeneous media are studied using Monte Carlo simulations in three dimensions. In addition to excluded volume, a nearest-neighbor interaction is considered with polymer-polymer repulsion and polymer-solvent attraction. Two types of heterogeneous media are considered: (i) a homogeneous-annealed system consisting of mobile polymer chains and solvents and (ii) quenched porous media, generated by adding a random distribution of quenched barriers. Effects of polymer concentration (p), bias (B), temperature (T), and porosity (ps) on the magnitude of the radius of gyration (Rg) of the chains and its scaling with the chain length (Lc) are studied. In an annealed system, we observe a crossover in power-law variation of the radius of gyration with the chain length, Rg∼Lyc, from an extended conformation with γ≃0.7 at low bias (B=0.2), low p, and high T to a collapsed conformation with γ∼0.20-0.31 at high bias (B⩾0.5) and low T. In a quenched porous medium, we observe a somewhat lower value of the power-law exponent, γ∼0.60-0.70, from its annealed value at high T and low bias. At low temperatures, in contrast, the magnitude of γ∼0.39-0.47 is enhanced with respect to its annealed value. Various nonlinear responses of Rg to bias are observed in different ranges of B, Lc, ps and T. In particular, we find that the response is nonmonotonic at low temperatures (T≃0.1) in the annealed system and at high temperatures (T≃100.0) in a porous medium with a relatively high barrier concentration (pb⩾0.3