26,906 research outputs found
The X-ray Line Emission from the Supernova Remnant W49B
The Galactic supernova remnant W49B has one of the most impressive X-ray
emission line spectra obtained with the Advanced Satellite for Cosmology and
Astronomy (ASCA). We use both plasma line diagnostics and broadband model fits
to show that the Si and S emission lines require multiple spectral components.
The spectral data do not necessarily require individual elements to be
spatially stratified, as suggested by earlier work, although when ASCA line
images are considered, it is possible that Fe is stratified with respect to Si
and S. Most of the X-ray emitting gas is from ejecta, based on the element
abundances required, but is surprisingly close to being in collisional
ionization equilibrium. A high ionization age implies a high internal density
in a young remnant. The fitted emission measure for W49B indicates a minimum
density of 2 cm^-3, with the true density likely to be significantly higher.
W49B probably had a Type Ia progenitor, based on the relative element
abundances, although a low-mass Type II progenitor is still possible. We find
persuasive evidence for Cr and possibly Mn emission in the ASCA spectrum--the
first detection of these elements in X-rays from a cosmic source.Comment: 22 pages incl 8 postscript figures, to appear in Ap
Theory of the Three Dimensional Quantum Hall Effect in Graphite
We predict the existence of a three dimensional quantum Hall effect plateau
in a graphite crystal subject to a magnetic field. The plateau has a Hall
conductivity quantized at with the
c-axis lattice constant. We analyze the three-dimensional Hofstadter problem of
a realistic tight-binding Hamiltonian for graphite, find the gaps in the
spectrum, and estimate the critical value of the magnetic field above which the
Hall plateau appears. When the Fermi level is in the bulk Landau gap, Hall
transport occurs through the appearance of chiral surface states. We estimate
the magnetic field necessary for the appearance of the three dimensional
quantum Hall Effect to be T for electron carriers and T for hole
carriers.Comment: Several new references adde
Products and Ratios of Characteristic Polynomials of Random Hermitian Matrices
We present new and streamlined proofs of various formulae for products and
ratios of characteristic polynomials of random Hermitian matrices that have
appeared recently in the literature.Comment: 18 pages, LaTe
Deep Chandra Observation of the Pulsar Wind Nebula Powered by the Pulsar J1846-0258 in the Supernova Remnant Kes 75
We present the results of detailed spatial and spectral analysis of the
pulsar wind nebula (PWN) in supernova remnant Kes 75 (G29.7-0.3) using a deep
exposure with Chandra X-ray observatory. The PWN shows a complex morphology
with clear axisymmetric structure. We identified a one-sided jet and two bright
clumps aligned with the overall nebular elongation, and an arc-like feature
perpendicular to the jet direction. Further spatial modeling with a torus and
jet model indicates a position angle 207\arcdeg\pm8 \arcdeg for the PWN
symmetry axis. We interpret the arc as an equatorial torus or wisp and the
clumps could be shock interaction between the jets and the surrounding medium.
The lack of any observable counter jet implies a flow velocity larger than
0.4c. Comparing to an archival observation 6 years earlier, some small-scale
features in the PWN demonstrate strong variability: the flux of the inner jet
doubles and the peak of the northern clump broadens and shifts 2" outward. In
addition, the pulsar flux increases by 6 times, showing substantial spectral
softening from =1.1 to 1.9 and an emerging thermal component which was
not observed in the first epoch. The changes in the pulsar spectrum are likely
related to the magnetar-like bursts of the pulsar that occurred 7 days before
the Chandra observation, as recently reported from RXTE observations.Comment: Accepted by ApJ, 8 figures, some of them have been scaled down in
resolutio
Nonparametric and Semiparametric Analysis of Current Status Data Subject to Outcome Misclassification
In this article, we present nonparametric and semiparametric methods to analyze current status data subject to outcome misclassification. Our methods use nonparametric maximum likelihood estimation (NPMLE) to estimate the distribution function of the failure time when sensitivity and specificity may vary among subgroups. A nonparametric test is proposed for the two sample hypothesis testing. In regression analysis, we apply the Cox proportional hazard model and likelihood ratio based confidence intervals for the regression coefficients are proposed. Our methods are motivated and demonstrated by data collected from an infectious disease study in Seattle, WA
Dilation Angle and Liquefaction Potential
Most of our understandin9 of the liquefaction phenomenon has come from laboratory tests. It would be desirable to express liquefaction resistance in terms of a parameter which can be measured both in the laboratory and in the field. It is proposed that the dilation angle or expansion rate of the sand is such a parameter. It is readily measured in the laboratory from drained simple shear or triaxial tests and in the field from self boring pressuremeter tests. Based on laboratory tests on Ottawa sand a chart is presented for estimating the liquefaction resistance of saturated sands in terms of dilation angle in addition to the usual parameters relative density and blow count. When the chart was used in conjunction with pressuremeter tests, a conservative estimate of liquefaction resistance of a hydraulic fill dam was obtained
0103-72.6: A New Oxygen-Rich Supernova Remnant in the Small Magellanic Cloud
010372.6, the second brightest X-ray supernova remnant (SNR) in the Small
Magellanic Cloud (SMC), has been observed with the {\it Chandra X-Ray
Observatory}. Our {\it Chandra} observation unambiguously resolves the X-ray
emission into a nearly complete, remarkably circular shell surrounding bright
clumpy emission in the center of the remnant. The observed X-ray spectrum for
the central region is evidently dominated by emission from reverse shock-heated
metal-rich ejecta. Elemental abundances in this ejecta material are
particularly enhanced in oxygen and neon, while less prominent in the heavier
elements Si, S, and Fe. We thus propose that 010372.6 is a new
``oxygen-rich'' SNR, making it only the second member of the class in the SMC.
The outer shell is the limb-brightened, soft X-ray emission from the swept-up
SMC interstellar medium. The presence of O-rich ejecta and the SNR's location
within an H{\small II} region attest to a massive star core-collapse origin for
010372.6. The elemental abundance ratios derived from the ejecta suggest an
18 M progenitor star.Comment: 6 pages (ApJ emulator format), including 5 figures and 2 tables. For
high quality Figs.1,2, & 3, contact [email protected]. Accepted by the ApJ
Letter
Dielectric behaviour of graded spherical cells with an intrinsic dispersion
The dielectric properties of single-shell spherical cells with an intrinsic
dielectric dispersion has been investigated. By means of the dielectric
dispersion spectral representation (DDSR) for the Clausius-Mossotti (CM)
factor, we express the dispersion strengths as well as the characteristic
frequencies of the CM factor analytically in terms of the parameters of the
cell model. These analytic expressions enable us to assess the influence of
various model parameters on the electrokinetics of cells. Various interesting
behaviours have been reported. We extend our considerations to a more realistic
cell model with a graded core, which can have spatial gradients in the
conductivity and/or permittivity. To this end, we address the effects of a
graded profile in a small-gradient expansion in the framework of DDSR.Comment: accepted by European Physical Journal
Proof of the Thin Sandwich Conjecture
We prove that the Thin Sandwich Conjecture in general relativity is valid,
provided that the data satisfy certain geometric
conditions. These conditions define an open set in the class of possible data,
but are not generically satisfied. The implications for the ``superspace''
picture of the Einstein evolution equations are discussed.Comment: 8 page
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