134,330 research outputs found
Theory of Pulsar Wind Nebulae
Our understanding of Pulsar Wind Nebulae (PWNe), has greatly improved in the
last years thanks to unprecedented high resolution images taken from the
HUBBLE, CHANDRA and XMM satellites. The discovery of complex but similar inner
features, with the presence of unexpected axisymmetric rings and jets, has
prompted a new investigation into the dynamics of the interaction of the pulsar
winds with the surrounding SNR, which, thanks to the improvement in the
computational resources, has let to a better understanding of the properties of
these objects. On the other hand the discovery of non-thermal emission from bow
shock PWNe, and of systems with a complex interaction between pulsar and SNR,
has led to the development of more reliable evolutionary models. I will review
the standard theory of PWNe, their evolution, and the current status in the
modeling of their emission properties, in particular I will show that our
evolutionary models are able to describe the observations, and that the X-ray
emission can now be reproduced with sufficient accuracy, to the point that we
can use these nebulae to investigate fundamental issues as the properties of
relativistic outflows and particle acceleration.Comment: 9 page, 5 figures, Proceeding of the conference "40 Years of
Pulsars", 12-17 August 2007, Montreal, Canada. (figures are not properly
displayed in .ps or .pdf version please download archive for them
The Parkes Pulsar Timing Array
Detection and study of gravitational waves from astrophysical sources is a
major goal of current astrophysics. Ground-based laser-interferometer systems
such as LIGO and VIRGO are sensitive to gravitational waves with frequencies of
order 100 Hz, whereas space-based systems such as LISA are sensitive in the
millihertz regime. Precise timing observations of a sample of millisecond
pulsars widely distributed on the sky have the potential to detect
gravitational waves at nanohertz frequencies. Potential sources of such waves
include binary super-massive black holes in the cores of galaxies, relic
radiation from the inflationary era and oscillations of cosmic strings. The
Parkes Pulsar Timing Array (PPTA) is an implementation of such a system in
which 20 millisecond pulsars have been observed using the Parkes radio
telescope at three frequencies at intervals of two -- three weeks for more than
two years. Analysis of these data has been used to limit the gravitational wave
background in our Galaxy and to constrain some models for its generation. The
data have also been used to investigate fluctuations in the interstellar and
Solar-wind electron density and have the potential to investigate the stability
of terrestrial time standards and the accuracy of solar-system ephemerides.Comment: 9 pages, 6 figures, Proceedings of "40 Years of Pulsars: Millisecond
Pulsars, Magnetars and More", Montreal, August 2007. Corrected SKA detection
limi
Quakes in Solid Quark Stars
A starquake mechanism for pulsar glitches is developed in the solid quark
star model. It is found that the general glitch natures (i.e., the glitch
amplitudes and the time intervals) could be reproduced if solid quark matter,
with high baryon density but low temperature, has properties of shear modulus
\mu = 10^{30~34} erg/cm^3 and critical stress \sigma_c = 10^{18~24} erg/cm^3.
The post-glitch behavior may represent a kind of damped oscillations.Comment: 11 pages, 4 figures (but Fig.3 is lost), a complete version can be
obtained by http://vega.bac.pku.edu.cn/~rxxu/publications/index_P.htm, a new
version to be published on Astroparticle Physic
Van der Waals epitaxy of Bi2Se3 on Si(111) vicinal surface: An approach to prepare high-quality thin films of topological insulator
Epitaxial growth of topological insulator Bi2Se3 thin films on nominally flat
and vicinal Si(111) substrates is studied. In order to achieve planner growth
front and better quality epifilms, a two-step growth method is adopted for the
van der Waal epitaxy of Bi2Se3 to proceed. By employing vicinal Si(111)
substrate surfaces, the in-pane growth rate anisotropy of Bi2Se3 is explored to
achieve single crystalline Bi2Se3 epifilms, in which threading defects and
twins are effectively suppressed. Optimization of the growth parameters has
resulted in vicinal Bi2Se3 films showing a carrier mobility of ~ 2000 cm2V-1s-1
and the background doping of ~ 3 x 1018 cm-3 of the as-grown layers. Such
samples not only show relatively high magnetoresistance but also a linear
dependence on magnetic field.Comment: 18 pages, 4 figure
On the Wang-Landau Method for Off-Lattice Simulations in the "Uniform" Ensemble
We present a rigorous derivation for off-lattice implementations of the
so-called "random-walk" algorithm recently introduced by Wang and Landau [PRL
86, 2050 (2001)]. Originally developed for discrete systems, the algorithm
samples configurations according to their inverse density of states using
Monte-Carlo moves; the estimate for the density of states is refined at each
simulation step and is ultimately used to calculate thermodynamic properties.
We present an implementation for atomic systems based on a rigorous separation
of kinetic and configurational contributions to the density of states. By
constructing a "uniform" ensemble for configurational degrees of freedom--in
which all potential energies, volumes, and numbers of particles are equally
probable--we establish a framework for the correct implementation of simulation
acceptance criteria and calculation of thermodynamic averages in the continuum
case. To demonstrate the generality of our approach, we perform sample
calculations for the Lennard-Jones fluid using two implementation variants and
in both cases find good agreement with established literature values for the
vapor-liquid coexistence locus.Comment: 21 pages, 4 figure
Magneto-infrared spectroscopy of Landau levels and Zeeman splitting of three-dimensional massless Dirac Fermions in ZrTe
We present a magneto-infrared spectroscopy study on a newly identified
three-dimensional (3D) Dirac semimetal ZrTe. We observe clear transitions
between Landau levels and their further splitting under magnetic field. Both
the sequence of transitions and their field dependence follow quantitatively
the relation expected for 3D \emph{massless} Dirac fermions. The measurement
also reveals an exceptionally low magnetic field needed to drive the compound
into its quantum limit, demonstrating that ZrTe is an extremely clean
system and ideal platform for studying 3D Dirac fermions. The splitting of the
Landau levels provides a direct and bulk spectroscopic evidence that a
relatively weak magnetic field can produce a sizeable Zeeman effect on the 3D
Dirac fermions, which lifts the spin degeneracy of Landau levels. Our analysis
indicates that the compound evolves from a Dirac semimetal into a topological
line-node semimetal under current magnetic field configuration.Comment: Editors' Suggestio
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