Asymptotic channels and gauge transformations of the time-dependent Dirac equation for extremely relativistic heavy-ion collisions

We discuss the two-center, time-dependent Dirac equation describing the dynamics of an electron during a peripheral, relativistic heavy-ion collision at extreme energies. We derive a factored form, which is exact in the high-energy limit, for the asymptotic channel solutions of the Dirac equation, and elucidate their close connection with gauge transformations which transform the dynamics into a representation in which the interaction between the electron and a distant ion is of short range. We describe the implications of this relationship for solving the time-dependent Dirac equation for extremely relativistic collisions.Comment: 12 pages, RevTeX, 2 figures, submitted to PR

Gas chemical investigation of hafnium and zirconium complexes with hexafluoroacetylacetone using preseparated short-lived radioisotopes

Volatile metal complexes of the group 4 elements Zr and Hf with hexafluoroacetylacetonate (hfa) have been studied using short-lived radioisotopes of the metals. The new technique of physical preseparation has been employed where reaction products from heavy-ion induced fusion reactions are isolated in a physical recoil separator - the Berkeley Gas-filled Separator in our work - and made available for chemistry experiments. Formation and decomposition of M(hfa)4 (M=Zr, Hf) has been observed and the interaction strength with a fluorinated ethylene propylene (FEP) Teflon surface has been studied. From the results of isothermal chromatography experiments, an adsorption enthalpy of -ΔHa=(57±3)kJ/mol was deduced. In optimization experiments, the time for formation of the complex and its transport to a counting setup installed outside of the irradiation cave was minimized and values of roughly one minute have been reached. The half-life of 165Hf, for which conflicting values appear in the literature, was measured to be (73.9±0.8)s. Provided that samples suitable for α-spectroscopy can be prepared, the investigation of rutherfordium (Rf), the transactinide member of group 4, appears possible. In the future, based on the studies presented here, it appears possible to investigate short-lived single atoms produced with low rates ( e.g. , transactinide isotopes) in completely new chemical systems, e.g. , as metal complexes with organic ligands as used here or as organometallic compound

Topical Issues for Particle Acceleration Mechanisms in Astrophysical Shocks

Particle acceleration at plasma shocks appears to be ubiquitous in the universe, spanning systems in the heliosphere, supernova remnants, and relativistic jets in distant active galaxies and gamma-ray bursts. This review addresses some of the key issues for shock acceleration theory that require resolution in order to propel our understanding of particle energization in astrophysical environments. These include magnetic field amplification in shock ramps, the non-linear hydrodynamic interplay between thermal ions and their extremely energetic counterparts possessing ultrarelativistic energies, and the ability to inject and accelerate electrons in both non-relativistic and relativistic shocks. Recent observational developments that impact these issues are summarized. While these topics are currently being probed by astrophysicists using numerical simulations, they are also ripe for investigation in laboratory experiments, which potentially can provide valuable insights into the physics of cosmic shocks.Comment: 13 pages, no figures. Invited review, accepted for publication in Astrophysics and Space Science, as part of the HEDLA 2006 conference proceeding

Magnetic fields in supernova remnants and pulsar-wind nebulae

We review the observations of supernova remnants (SNRs) and pulsar-wind nebulae (PWNe) that give information on the strength and orientation of magnetic fields. Radio polarimetry gives the degree of order of magnetic fields, and the orientation of the ordered component. Many young shell supernova remnants show evidence for synchrotron X-ray emission. The spatial analysis of this emission suggests that magnetic fields are amplified by one to two orders of magnitude in strong shocks. Detection of several remnants in TeV gamma rays implies a lower limit on the magnetic-field strength (or a measurement, if the emission process is inverse-Compton upscattering of cosmic microwave background photons). Upper limits to GeV emission similarly provide lower limits on magnetic-field strengths. In the historical shell remnants, lower limits on B range from 25 to 1000 microGauss. Two remnants show variability of synchrotron X-ray emission with a timescale of years. If this timescale is the electron-acceleration or radiative loss timescale, magnetic fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition arguments and dynamical modeling can be used to infer magnetic-field strengths anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field geometries often suggest a toroidal structure around the pulsar, but this is not universal. Viewing-angle effects undoubtedly play a role. MHD models of radio emission in shell SNRs show that different orientations of upstream magnetic field, and different assumptions about electron acceleration, predict different radio morphology. In the remnant of SN 1006, such comparisons imply a magnetic-field orientation connecting the bright limbs, with a non-negligible gradient of its strength across the remnant.Comment: 20 pages, 24 figures; to be published in SpSciRev. Minor wording change in Abstrac

The Science of Sungrazers, Sunskirters, and Other Near-Sun Comets

This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term near-Sun comets to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU). Sunskirters are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase sungrazers to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed sundivers. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics

Gamma rays from colliding winds of massive stars

Colliding winds of massive binaries have long been considered as potential sites of non-thermal high-energy photon production. This is motivated by the detection of non-thermal spectra in the radio band, as well as by correlation studies of yet unidentified EGRET gamma-ray sources with source populations appearing in star formation regions. This work re-considers the basic radiative processes and its properties that lead to high energy photon production in long-period massive star systems. We show that Klein-Nishina effects as well as the anisotropic nature of the inverse Compton scattering, the dominating leptonic emission process, likely yield spectral and variability signatures in the gamma-ray domain at or above the sensitivity of current or upcoming gamma ray instruments like GLAST-LAT. In addition to all relevant radiative losses, we include propagation (such as convection in the stellar wind) as well as photon absorption effects, which a priori can not be neglected. The calculations are applied to WR140 and WR147, and predictions for their detectability in the gamma-ray regime are provided. Physically similar specimen of their kind like WR146, WR137, WR138, WR112 and WR125 may be regarded as candidate sources at GeV energies for near-future gamma-ray experiments. Finally, we discuss several aspects relevant for eventually identifying this source class as a gamma-ray emitting population. Thereby we utilize our findings on the expected radiative behavior of typical colliding wind binaries in the gamma-ray regime as well as its expected spatial distribution on the gamma-ray sky

Integrating sequence and array data to create an improved 1000 Genomes Project haplotype reference panel

A major use of the 1000 Genomes Project (1000GP) data is genotype imputation in genome-wide association studies (GWAS). Here we develop a method to estimate haplotypes from low-coverage sequencing data that can take advantage of single-nucleotide polymorphism (SNP) microarray genotypes on the same samples. First the SNP array data are phased to build a backbone (or 'scaffold') of haplotypes across each chromosome. We then phase the sequence data 'onto' this haplotype scaffold. This approach can take advantage of relatedness between sequenced and non-sequenced samples to improve accuracy. We use this method to create a new 1000GP haplotype reference set for use by the human genetic community. Using a set of validation genotypes at SNP and bi-allelic indels we show that these haplotypes have lower genotype discordance and improved imputation performance into downstream GWAS samples, especially at low-frequency variants. © 2014 Macmillan Publishers Limited. All rights reserved

Monte Carlo simulation of collisionless shocks showing preferential acceleration of high A/Z particles

A collisionless, quasi-parallel shock is simulated using Monte Carlo techniques. In this kinetic approach, scattering of all velocity particles (from thermal to high energy) is assumed to occur such that the mean free path is directly proportional to velocity times the mass to charge ratio and inversely proportional to the plasma density. Within the constraints of this assumption, the shock profile and velocity spectra are obtained showing preferential acceleration of high A/Z particles relative to protons.           ARK: https://n2t.net/ark:/88439/y033533 Permalink: https://geophysicsjournal.com/article/152 &nbsp