545 research outputs found
Solitons in a chain of PT-invariant dimers
Dynamics of a chain of interacting parity-time invariant nonlinear dimers is
investigated. A dimer is built as a pair of coupled elements with equal gain
and loss. A relation between stationary soliton solutions of the model and
solitons of the discrete nonlinear Schrodinger (DNLS) equation is demonstrated.
Approximate solutions for solitons whose width is large in comparison to the
lattice spacing are derived, using a continuum counterpart of the discrete
equations. These solitons are mobile, featuring nearly elastic collisions.
Stationary solutions for narrow solitons, which are immobile due to the pinning
by the effective Peierls-Nabarro potential, are constructed numerically,
starting from the anti-continuum limit. The solitons with the amplitude
exceeding a certain critical value suffer an instability leading to blowup,
which is a specific feature of the nonlinear PT-symmetric chain, making it
dynamically different from DNLS lattices. A qualitative explanation of this
feature is proposed. The instability threshold drops with the increase of the
gain-loss coefficient, but it does not depend on the lattice coupling constant,
nor on the soliton's velocity.Comment: 9 pages, 9 figure
Open questions in the study of population III star formation
The first stars were key drivers of early cosmic evolution. We review the
main physical elements of the current consensus view, positing that the first
stars were predominantly very massive. We continue with a discussion of
important open questions that confront the standard model. Among them are
uncertainties in the atomic and molecular physics of the hydrogen and helium
gas, the multiplicity of stars that form in minihalos, and the possible
existence of two separate modes of metal-free star formation.Comment: 15 pages, 2 figures. To appear in the conference proceedings for IAU
Symposium 255: Low-Metallicity Star Formation: From the First Stars to Dwarf
Galaxie
Definition of Time Induction of Self-Ignition of the Substance on the Prognostic Extrapolation Depending on the Basis of Indicators Fire and Explosion Hazard
In this article the research directed on development of a technique of definition of time of induction of the self-ignition of substances and materials which is an indicator of the beginning of development of an emergency is conducted. The experiment consisting in supervision over process of self-ignition of coal and oil deposits was the basis for research. On the basis of experimental data the curve expressing analytic - expected dependence of size of temperature of ignition on induction time was constructed. Proceeding from graphical representation of process, functional dependence of time of induction on a temperature indicator was received: y=16920•x{0.537}. By means of known indicators of such substances as bitumen oil oxidized (the combustible solid substance received by oxidation of residual product of oil refining) and tar oil (the combustible solid substance which is residual product of oil refining) and the received algorithm, verification of reliability of the received dependence and a technique of definition of time of induction of spontaneous ignition of deposits of oil in general was carried out. The practical importance of the conducted research is that having data on time of induction of process of self-ignition, by means of preventive measures becomes possible to avoid and prevent accidents in oil and oil processing branches, at the same time loss of property and loss of human life
Dark Matter Search Perspectives with GAMMA-400
GAMMA-400 is a future high-energy gamma-ray telescope, designed to measure
the fluxes of gamma-rays and cosmic-ray electrons + positrons, which can be
produced by annihilation or decay of dark matter particles, and to survey the
celestial sphere in order to study point and extended sources of gamma-rays,
measure energy spectra of Galactic and extragalactic diffuse gamma-ray
emission, gamma-ray bursts, and gamma-ray emission from the Sun. GAMMA-400
covers the energy range from 100 MeV to ~3000 GeV. Its angular resolution is
~0.01 deg(Eg > 100 GeV), and the energy resolution ~1% (Eg > 10 GeV). GAMMA-400
is planned to be launched on the Russian space platform Navigator in 2019. The
GAMMA-400 perspectives in the search for dark matter in various scenarios are
presented in this paperComment: 4 pages, 4 figures, submitted to the Proceedings of the International
Cosmic-Ray Conference 2013, Brazil, Rio de Janeir
A separation of electrons and protons in the GAMMA-400 gamma-ray telescope
The GAMMA-400 gamma-ray telescope is intended to measure the fluxes of gamma
rays and cosmic-ray electrons and positrons in the energy range from 100 MeV to
several TeV. Such measurements concern with the following scientific goals:
search for signatures of dark matter, investigation of gamma-ray point and
extended sources, studies of the energy spectra of Galactic and extragalactic
diffuse emission, studies of gamma-ray bursts and gamma-ray emission from the
active Sun, as well as high-precision measurements of spectra of high-energy
electrons and positrons, protons, and nuclei up to the knee. The main
components of cosmic rays are protons and helium nuclei, whereas the part of
lepton component in the total flux is ~10E-3 for high energies. In present
paper, the capability of the GAMMA-400 gamma-ray telescope to distinguish
electrons and positrons from protons in cosmic rays is investigated. The
individual contribution to the proton rejection is studied for each detector
system of the GAMMA-400 gamma-ray telescope. Using combined information from
all detector systems allow us to provide the proton rejection from electrons
with a factor of ~4x10E5 for vertical incident particles and ~3x10E5 for
particles with initial inclination of 30 degrees. The calculations were
performed for the electron energy range from 50 GeV to 1 TeV.Comment: 19 pages, 10 figures, submitted to Advances and Space Researc
The GAMMA-400 space observatory: status and perspectives
The present design of the new space observatory GAMMA-400 is presented in
this paper. The instrument has been designed for the optimal detection of gamma
rays in a broad energy range (from ~100 MeV up to 3 TeV), with excellent
angular and energy resolution. The observatory will also allow precise and high
statistic studies of the electron component in the cosmic rays up to the multi
TeV region, as well as protons and nuclei spectra up to the knee region. The
GAMMA-400 observatory will allow to address a broad range of science topics,
like search for signatures of dark matter, studies of Galactic and
extragalactic gamma-ray sources, Galactic and extragalactic diffuse emission,
gamma-ray bursts and charged cosmic rays acceleration and diffusion mechanism
up to the knee
New Approaches To Photometric Redshift Prediction Via Gaussian Process Regression In The Sloan Digital Sky Survey
Expanding upon the work of Way and Srivastava 2006 we demonstrate how the use
of training sets of comparable size continue to make Gaussian process
regression (GPR) a competitive approach to that of neural networks and other
least-squares fitting methods. This is possible via new large size matrix
inversion techniques developed for Gaussian processes (GPs) that do not require
that the kernel matrix be sparse. This development, combined with a
neural-network kernel function appears to give superior results for this
problem. Our best fit results for the Sloan Digital Sky Survey (SDSS) Main
Galaxy Sample using u,g,r,i,z filters gives an rms error of 0.0201 while our
results for the same filters in the luminous red galaxy sample yield 0.0220. We
also demonstrate that there appears to be a minimum number of training-set
galaxies needed to obtain the optimal fit when using our GPR rank-reduction
methods. We find that morphological information included with many photometric
surveys appears, for the most part, to make the photometric redshift evaluation
slightly worse rather than better. This would indicate that most morphological
information simply adds noise from the GP point of view in the data used
herein. In addition, we show that cross-match catalog results involving
combinations of the Two Micron All Sky Survey, SDSS, and Galaxy Evolution
Explorer have to be evaluated in the context of the resulting cross-match
magnitude and redshift distribution. Otherwise one may be misled into overly
optimistic conclusions.Comment: 32 pages, ApJ in Press, 2 new figures, 1 new table of comparison
methods, updated discussion, references and typos to reflect version in Pres
Three-Dimensional Simulations of a Starburst-Driven Galactic Wind
We have performed a series of three-dimensional simulations of a
starburst-driven wind in an inhomogeneous interstellar medium. The introduction
of an inhomogeneous disk leads to differences in the formation of a wind, most
noticeably the absence of the ``blow-out'' effect seen in homogeneous models. A
wind forms from a series of small bubbles that propagate into the tenuous gas
between dense clouds in the disk. These bubbles merge and follow the path of
least resistance out of the disk, before flowing freely into the halo.
Filaments are formed from disk gas that is broken up and accelerated into the
outflow. These filaments are distributed throughout a biconical structure
within a more spherically distributed hot wind. The distribution of the
inhomogeneous interstellar medium in the disk is important in determining the
morphology of this wind, as well as the distribution of the filaments. While
higher resolution simulations are required in order to ascertain the importance
of mixing processes, we find that soft X-ray emission arises from gas that has
been mass-loaded from clouds in the disk, as well as from bow shocks upstream
of clouds, driven into the flow by the ram pressure of the wind, and the
interaction between these shocks.Comment: 37 pages, 16 figures, mpg movie can be obtained at
http://www.mso.anu.edu.au/~jcooper/movie/video16.mpg, accepted for
publication in Ap
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