391 research outputs found
X-ray emission from PSR J1809-1917 and its pulsar wind nebula, possibly associated with the TeV gamma-ray source HESS J1809-193
We detected X-ray emission from the 50-kyr-old pulsar J1809-1917 and resolved
its pulsar wind nebula (PWN) with Chandra. The pulsar spectrum fits PL+BB model
with the photon index of 1.2 and the BB temperature of 2 MK for n_{H}=0.7\times
10^{22} cm^{-2}. The luminosities are(4\pm 1)\times 10^{31} ergs s^{-1} for the
PL component (in the 0.5-8 keV band) and ~1\times 10^{32} ergs s^{-1} for the
BB component (bolometric) at a plausible distance of 3.5 kpc. The bright inner
PWN component of a 3''\times12'' size is elongated in the north-south
direction, with the pulsar close to its south end. This component is immersed
in a larger (20''\times40''), similarly elongated outer PWN component of lower
surface brightness. The elongated shape of the compact PWN can be explained by
the ram pressure confinement of the pulsar wind due to the supersonic motion of
the pulsar. The PWN spectrum fits a PL model with photon index of 1.4\pm0.1 and
0.5-8 keV luminosity of 4\times10^{32} ergs s^{-1}. The compact PWN appears to
be inside a large-scale (~4'\times4') emission more extended to the south of
the pulsar, i.e. in the direction of the alleged pulsar motion. To explain the
extended X-ray emission ahead of the moving pulsar, one has to invoke strong
intrinsic anisotropy of the pulsar wind or assume that this emission comes from
a relic PWN swept by the asymmetrical reverse SNR shock. The pulsar and its PWN
are located within the extent of the unidentified TeV source HESS J1809-193
whose brightest part is offset by ~8' to the south of the pulsar, i.e. in the
same direction as the large-scale X-ray emission. Although the association
between J1809-1917 and HESS J1809-193 is plausible, an alternative source of
relativistic electrons powering HESS J1809-193 might be the serendipitously
discovered X-ray source CXOU J180940.7-192544.Comment: 13 pages, 10 figures and 3 tables, submitted to ApJ. Version with the
high-resolution figures is available at
http://www.astro.psu.edu/users/green/J1809/ms_astroph.pd
Compressibility and structural stability of ultra-incompressible bimetallic interstitial carbides and nitrides
We have investigated by means of high-pressure x-ray diffraction the
structural stability of Pd2Mo3N, Ni2Mo3C0.52N0.48, Co3Mo3C0.62N0.38, and
Fe3Mo3C. We have found that they remain stable in their ambient-pressure cubic
phase at least up to 48 GPa. All of them have a bulk modulus larger than 330
GPa, being the least compressible material Fe3Mo3C, B0 = 374(3) GPa. In
addition, apparently a reduction of compressibility is detected as the carbon
content increased. The equation of state for each material is determined. A
comparison with other refractory materials indicates that interstitial nitrides
and carbides behave as ultra-incompressible materials.Comment: 14 pages, 3 figures, 1 tabl
Structure and Stability of Si(114)-(2x1)
We describe a recently discovered stable planar surface of silicon, Si(114).
This high-index surface, oriented 19.5 degrees away from (001) toward (111),
undergoes a 2x1 reconstruction. We propose a complete model for the
reconstructed surface based on scanning tunneling microscopy images and
first-principles total-energy calculations. The structure and stability of
Si(114)-(2x1) arises from a balance between surface dangling bond reduction and
surface stress relief, and provides a key to understanding the morphology of a
family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures
is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published
in Phys. Rev. Let
Separable Dual Space Gaussian Pseudo-potentials
We present pseudo-potential coefficients for the first two rows of the
periodic table. The pseudo potential is of a novel analytic form, that gives
optimal efficiency in numerical calculations using plane waves as basis set. At
most 7 coefficients are necessary to specify its analytic form. It is separable
and has optimal decay properties in both real and Fourier space. Because of
this property, the application of the nonlocal part of the pseudo-potential to
a wave-function can be done in an efficient way on a grid in real space. Real
space integration is much faster for large systems than ordinary multiplication
in Fourier space since it shows only quadratic scaling with respect to the size
of the system. We systematically verify the high accuracy of these
pseudo-potentials by extensive atomic and molecular test calculations.Comment: 16 pages, 4 postscript figure
A hard metallic material: Osmium Diboride
We calculate the structural and electronic properties of OsB2 using density
functional theory with or without taking into account spin-orbit (SO)
interaction. Our results show that the bulk modulus with and without SO
interaction are 364 and 365 Gpa respectively, both are in good agreement with
experiment (365-395 Gpa). The evidence of covalent bonding of Os-B, which plays
an important role to form a hard material, is indicated both in charge density,
atoms in molecules analysis, and density of states analysis. The good
metallicity and hardness of OsB2 might suggest its potential application as
hard conductors.Comment: Figures improve
Recommended from our members
DISSIMILAR WELDS FOR THE APT SUPERCONDUCTING CAVITY'S CRYOGENIC PLUMBING SYSTEM
Potential super-hard Osmium di-nitride with fluorite structure: First-principles calculations
We have performed systematic first-principles calculations on di-carbide,
-nitride, -oxide and -boride of platinum and osmium with the fluorite
structure. It is found that only PtN, OsN and OsO are
mechanically stable. In particular OsN has the highest bulk modulus of
360.7 GPa. Both the band structure and density of states show that the new
phase of OsN is metallic. The high bulk modulus is owing to the strong
covalent bonding between Os 5\textit{d} and N 2\textit{p} states and the dense
packed fluorite structure.Comment: Phys. Rev. B 74,125118 (2006
Recommended from our members
Method for the prediction of the hydriding thermodynamics of ternary PD-based alloys.
A method has been developed to calculate the hydriding thermodynamics of ternary Pd-X-Y systems, where X and Y are substitutional alloying elements, by using the properties of the binary Pd-X and Pd-Y systems. Experimental data was collected on the Pd-Rh-Co system to test the validity of this method. Hydrogen pressure-composition isotherms of several binary Pd-Rh and Pd-Co alloys and Pd-Rh-Co ternary alloys were measured to determine the thermodynamics of hydrogen absorption, hydride formation and decomposition, and hydrogen capacity. Good agreement between the calculated and measured values for the ternary Pd-Rh-Co system, in the dilute alloying regime (< 10 at.% total alloying additions), was obtained using our method. Examining literature results on other ternary Pd-X-Y systems checked the universality of this method. Again, the method succeeds in predicting the hydriding thermodynamics for both lattice contracted and lattice expanded alloy systems, Pd-Ni-Rh and Pd-Ag-Y respectively
Discovery of absorption features in the X-ray spectrum of an isolated neutron star
We observed 1E 1207.4--5209, a neutron star in the center of the supernova
remnant PKS 1209--51/52, with the ACIS detector aboard the Chandra X-ray
observatory and detected two absorption features in the source spectrum. The
features are centered near 0.7 keV and 1.4 keV, their equivalent widths are
about 0.1 keV. We discuss various possible interpretations of the absorption
features and exclude some of them. A likely interpretation is that the features
are associated with atomic transitions of once-ionized helium in the neutron
star atmosphere with a strong magnetic field. The first clear detection of
absorption features in the spectrum of an isolated neutron star provides an
opportunity to measure the mass-to-radius ratio and constrain the equation of
state of the superdense matter.Comment: 11 pages, including 1 figure and 1 table, to be published in ApJ
Singular Structure and Enhanced Friedel Oscillations in the Two-Dimensional Electron Gas
We calculate the leading order corrections (in ) to the static
polarization , with dynamically screened interactions, for the
two-dimensional electron gas. The corresponding diagrams all exhibit singular
logarithmic behavior in their derivatives at and provide significant
enhancement to the proper polarization particularly at low densities. At a
density of , the contribution from the leading order {\em fluctuational}
diagrams exceeds both the zeroth order (Lindhard) response and the self-energy
and exchange contributions. We comment on the importance of these diagrams in
two-dimensions and make comparisons to an equivalent three-dimensional electron
gas; we also consider the impact these finding have on computed
to all orders in perturbation theory
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