602 research outputs found
Constraints on the Equation-of-State of neutron stars from nearby neutron star observations
We try to constrain the Equation-of-State (EoS) of supra-nuclear-density
matter in neutron stars (NSs) by observations of nearby NSs. There are seven
thermally emitting NSs known from X-ray and optical observations, the so-called
Magnificent Seven (M7), which are young (up to few Myrs), nearby (within a few
hundred pc), and radio-quiet with blackbody-like X-ray spectra, so that we can
observe their surfaces. As bright X-ray sources, we can determine their
rotational (pulse) period and their period derivative from X-ray timing. From
XMM and/or Chandra X-ray spectra, we can determine their temperature. With
precise astrometric observations using the Hubble Space Telescope, we can
determine their parallax (i.e. distance) and optical flux. From flux, distance,
and temperature, one can derive the emitting area - with assumptions about the
atmosphere and/or temperature distribution on the surface. This was recently
done by us for the two brightest M7 NSs RXJ1856 and RXJ0720. Then, from
identifying absorption lines in X-ray spectra, one can also try to determine
gravitational redshift. Also, from rotational phase-resolved spectroscopy, we
have for the first time determined the compactness (mass/radius) of the M7 NS
RBS1223. If also applied to RXJ1856, radius (from luminosity and temperature)
and compactness (from X-ray data) will yield the mass and radius - for the
first time for an isolated single neutron star. We will present our
observations and recent results.Comment: refereed NPA5 conference proceedings, in pres
Small Angle Scattering by Fractal Aggregates: A Numerical Investigation of the Crossover Between the Fractal Regime and the Porod Regime
Fractal aggregates are built on a computer using off-lattice cluster-cluster
aggregation models. The aggregates are made of spherical particles of different
sizes distributed according to a Gaussian-like distribution characterised by a
mean and a standard deviation . The wave vector dependent
scattered intensity is computed in order to study the influence of the
particle polydispersity on the crossover between the fractal regime and the
Porod regime. It is shown that, given , the location of the
crossover decreases as increases. The dependence of on
can be understood from the evolution of the shape of the center-to-center
interparticle-distance distribution function.Comment: RevTex, 4 pages + 6 postscript figures, compressed using "uufiles",
published in Phys. Rev. B 50, 1305 (1994
Exploring the error characteristics of thin ice cloud property retrievals using a Markov chain Monte Carlo algorithm
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95385/1/jgrd15111.pd
Modeling Single Electron Transfer in Si:P Double Quantum Dots
Solid-state systems such as P donors in Si have considerable potential for
realization of scalable quantum computation. Recent experimental work in this
area has focused on implanted Si:P double quantum dots (DQDs) that represent a
preliminary step towards the realization of single donor charge-based qubits.
This paper focuses on the techniques involved in analyzing the charge transfer
within such DQD devices and understanding the impact of fabrication parameters
on this process. We show that misalignment between the buried dots and surface
gates affects the charge transfer behavior and identify some of the challenges
posed by reducing the size of the metallic dot to the few donor regime.Comment: 11 pages, 7 figures, submitted to Nanotechnolog
An Efficient Molecular Dynamics Scheme for the Calculation of Dopant Profiles due to Ion Implantation
We present a highly efficient molecular dynamics scheme for calculating the
concentration depth profile of dopants in ion irradiated materials. The scheme
incorporates several methods for reducing the computational overhead, plus a
rare event algorithm that allows statistically reliable results to be obtained
over a range of several orders of magnitude in the dopant concentration.
We give examples of using this scheme for calculating concentration profiles
of dopants in crystalline silicon. Here we can predict the experimental profile
over five orders of magnitude for both channeling and non-channeling implants
at energies up to 100s of keV.
The scheme has advantages over binary collision approximation (BCA)
simulations, in that it does not rely on a large set of empirically fitted
parameters. Although our scheme has a greater computational overhead than the
BCA, it is far superior in the low ion energy regime, where the BCA scheme
becomes invalid.Comment: 17 pages, 21 figures, 2 tables. See: http://bifrost.lanl.gov/~reed
Gas metallicity diagnostics in star-forming galaxies
Generally the gas metallicity in distant galaxies can only be inferred by
using a few prominent emission lines. Various theoretical models have been used
to predict the relationship between emission line fluxes and metallicity,
suggesting that some line ratios can be used as diagnostics of the gas
metallicity in galaxies. However, accurate empirical calibrations of these
emission line flux ratios from real galaxy spectra spanning a wide metallicity
range are still lacking. In this paper we provide such empirical calibrations
by using the combination of two sets of spectroscopic data: one consisting of
low-metallicity galaxies with a measurement of [OIII]4363, and the other one
consisting of galaxies in the SDSS DR4 whose metallicity has been determined
from various strong emission lines in their spectra. This combined data set
constitutes the largest sample of galaxies with information on the gas
metallicity available so far and spanning the widest metallicity range. Our
empirical diagrams show that the line ratio [OIII]/[OII] is a useful tool to
break the degeneracy in the R_23 parameter when no information on the [NII]
line is available. The line ratio [NeIII]/[OII] also results to be a useful
metallicity indicator for high-z galaxies. Finally, we compare these empirical
relations with photoionization models. We find that the empirical
R_23-metallicity sequence is strongly discrepant with respect to the trend
expected by models with constant ionization parameter, which is interpreted as
a consequence of a strong metallicity dependence of the average ionization
parameter in galaxies. This result should warn about the use of theoretical
models with constant ionization parameter to infer metallicities from observed
line ratios. (abbreviated)Comment: 27 pages, 19 figures, Astronomy and Astrophysics, in press. (Replaced
with the accepted version.
Is there a compact companion orbiting the late O-type binary star HD 164816?
We present a multi-wavelength (X-ray, -ray, optical and radio) study
of HD 194816, a late O-type X-ray detected spectroscopic binary. X-ray spectra
are analyzed and the X-ray photon arrival times are checked for pulsation. In
addition, newly obtained optical spectroscopic monitoring data on HD 164816 are
presented. They are complemented by available radio data from several large
scale surveys as well as the \emph{FERMI} -ray data from its
\emph{Large Area Telescope}. We report the detection of a low energy excess in
the X-ray spectrum that can be described by a simple absorbed blackbody model
with a temperature of 50 eV as well as a 9.78 s pulsation of the X-ray
source. The soft X-ray excess, the X-ray pulsation, and the kinematical age
would all be consistent with a compact object like a neutron star as companion
to HD 164816. The size of the soft X-ray excess emitting area is consistent
with a circular region with a radius of about 7 km, typical for neutron stars,
while the emission measure of the remaining harder emission is typical for late
O-type single or binary stars. If HD 164816 includes a neutron star born in a
supernova, this supernova should have been very recent and should have given
the system a kick, which is consistent with the observation that the star HD
164816 has a significantly different radial velocity than the cluster mean. In
addition we confirm the binarity of HD 164816 itself by obtaining an orbital
period of 3.82 d, projected masses = 2.355(69) M,
= 2.103(62) M apparently seen at low inclination
angle, determined from high-resolution optical spectra.Comment: Accepted for publication by MNRAS, 11 pages, 6 figures, 4 table
Evidence for structural and electronic instabilities at intermediate temperatures in -(BEDT-TTF)X for X=Cu[N(CN)]Cl, Cu[N(CN)]Br and Cu(NCS): Implications for the phase diagram of these quasi-2D organic superconductors
We present high-resolution measurements of the coefficient of thermal
expansion of the quasi-twodimensional
(quasi-2D) salts -(BEDT-TTF)X with X = Cu(NCS), Cu[N(CN)]Br
and Cu[N(CN)]Cl. At intermediate temperatures (B), distinct anomalies
reminiscent of second-order phase transitions have been found at
K and 45 K for the superconducting X = Cu(NCS) and Cu[N(CN)]Br salts,
respectively. Most interestingly, we find that the signs of the uniaxial
pressure coefficients of are strictly anticorrelated with those of
. We propose that marks the transition to a spin-density-wave
(SDW) state forming on minor, quasi-1D parts of the Fermi surface. Our results
are compatible with two competing order parameters that form on disjunct
portions of the Fermi surface. At elevated temperatures (C), all compounds show
anomalies that can be identified with a kinetic, glass-like
transition where, below a characteristic temperature , disorder in the
orientational degrees of freedom of the terminal ethylene groups becomes frozen
in. We argue that the degree of disorder increases on going from the X =
Cu(NCS) to Cu[N(CN)]Br and the Cu[N(CN)]Cl salt. Our results
provide a natural explanation for the unusual time- and cooling-rate
dependencies of the ground-state properties in the hydrogenated and deuterated
Cu[N(CN)]Br salts reported in the literature.Comment: 22 pages, 7 figure
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