3,724 research outputs found
X-ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State
We report spectral and variability analysis of two quiescent low mass X-ray
binaries (X5 and X7, previously detected with the ROSAT HRI) in a Chandra
ACIS-I observation of the globular cluster 47 Tuc. X5 demonstrates sharp
eclipses with an 8.666+-0.01 hr period, as well as dips showing an increased
N_H column. The thermal spectra of X5 and X7 are well-modeled by unmagnetized
hydrogen atmospheres of hot neutron stars. No hard power law component is
required. A possible edge or absorption feature is identified near 0.64 keV,
perhaps an OV edge from a hot wind. Spectral fits imply that X7 is
significantly more massive than the canonical 1.4 \Msun neutron star mass, with
M>1.8 \Msun for a radius range of 9-14 km, while X5's spectrum is consistent
with a neutron star of mass 1.4 \Msun for the same radius range. Alternatively,
if much of the X-ray luminosity is due to continuing accretion onto the neutron
star surface, the feature may be the 0.87 keV rest-frame absorption complex (O
VIII & other metal lines) intrinsic to the neutron star atmosphere, and a mass
of 1.4 \Msun for X7 may be allowed.Comment: 16 pages, 7 figures, accepted by Ap
Configuration mixing of angular-momentum projected triaxial relativistic mean-field wave functions
The framework of relativistic energy density functionals is extended to
include correlations related to the restoration of broken symmetries and to
fluctuations of collective variables. The generator coordinate method is used
to perform configuration mixing of angular-momentum projected wave functions,
generated by constrained self-consistent relativistic mean-field calculations
for triaxial shapes. The effects of triaxial deformation and of -mixing is
illustrated in a study of spectroscopic properties of low-spin states in
Mg.Comment: 15 pages, 11 figures, 4 tables, accepted for publication in Phys.
Rev.
An exactly solvable limit of low energy QCD
Starting from the QCD Hamiltonian, we derive a schematic Hamiltonian for low
energy quark dynamics with quarks restricted to the lowest s-level. The
resulting eigenvalue problem can be solved analytically. Even though the
Hamiltonian exhibits explicit chiral symmetry the severe restriction of the
number of degrees of freedom breaks the pattern of chiral symmetry breaking for
finite quark masses.Comment: 7 page
Fatal Pulmonary Embolus After Uterine Artery Fibroid Embolisation
We report a 44-year-old woman who developed a fatal pulmonary embolus after uterine artery fibroid embolisation (UAE). Bilateral UAE was carried out through a single right-femoral artery puncture. The largest fibroid in the anterior fundal wall measured 4.5 cm, and the largest fibroid in the posterior fundal wall measured 6 cm. The appearances after UAE were satisfactory, and the procedure was apparently uneventful. No immediate complications were noted. The patient developed sudden-onset shortness of breath and went into cardiac arrest 19 h after the procedure. Postmortem autopsy confirmed that the cause of a death was a pulmonary embolism. To our knowledge this is the first reported case in the United Kingdom in which death occurred from a pulmonary embolus after UAE
Three Additional Quiescent Low-Mass X-ray Binary Candidates in 47 Tucanae
We identify through their X-ray spectra one certain (W37) and two probable
(W17 and X4) quiescent low-mass X-ray binaries (qLMXBs) containing neutron
stars in a long Chandra X-ray exposure of the globular cluster 47 Tucanae, in
addition to the two previously known qLMXBs. W37's spectrum is dominated by a
blackbody-like component consistent with radiation from the hydrogen atmosphere
of a 10 km neutron star. W37's lightcurve shows strong X-ray variability which
we attribute to variations in its absorbing column depth, and eclipses with a
probable 3.087 hour period. For most of our exposures, W37's blackbody-like
emission (assumed to be from the neutron star surface) is almost completely
obscured, yet some soft X-rays (of uncertain origin) remain. Two additional
candidates, W17 and X4, present X-ray spectra dominated by a harder component,
fit by a power-law of photon index ~1.6-3. An additional soft component is
required for both W17 and X4, which can be fit with a 10 km hydrogen-atmosphere
neutron star model. X4 shows significant variability, which may arise from
either its power-law or hydrogen-atmosphere spectral component. Both W17 and X4
show rather low X-ray luminosities, Lx(0.5-10 keV)~5*10^{31} ergs/s. All three
candidate qLMXBs would be difficult to identify in other globular clusters,
suggesting an additional reservoir of fainter qLMXBs in globular clusters that
may be of similar numbers as the group of previously identified objects. The
number of millisecond pulsars inferred to exist in 47 Tuc is less than 10 times
larger than the number of qLMXBs in 47 Tuc, indicating that for typical
inferred lifetimes of 10 and 1 Gyr respectively, their birthrates are
comparable.Comment: Accepted for publication in ApJ. 13 pages, 7 figures (2 color
Fault-tolerant quantum computation with high threshold in two dimensions
We present a scheme of fault-tolerant quantum computation for a local
architecture in two spatial dimensions. The error threshold is 0.75% for each
source in an error model with preparation, gate, storage and measurement
errors.Comment: 4 pages, 4 figures; v2: A single 2D layer of qubits (simple square
lattice) with nearest-neighbor translation-invariant Ising interaction
suffices. Slightly improved threshol
Contemplations on Dirac's equation in quaternionic coordinates
A formulation of Dirac's equation using complex-quaternionic coordinates
appears to yield an enormous gain in formal elegance, as there is no longer any
need to invoke Dirac matrices. This formulation, however, entails several
peculiarities, which we investigate and attempt to interpret
Solving a "Hard" Problem to Approximate an "Easy" One: Heuristics for Maximum Matchings and Maximum Traveling Salesman Problems
We consider geometric instances of the Maximum Weighted Matching Problem
(MWMP) and the Maximum Traveling Salesman Problem (MTSP) with up to 3,000,000
vertices. Making use of a geometric duality relationship between MWMP, MTSP,
and the Fermat-Weber-Problem (FWP), we develop a heuristic approach that yields
in near-linear time solutions as well as upper bounds. Using various
computational tools, we get solutions within considerably less than 1% of the
optimum.
An interesting feature of our approach is that, even though an FWP is hard to
compute in theory and Edmonds' algorithm for maximum weighted matching yields a
polynomial solution for the MWMP, the practical behavior is just the opposite,
and we can solve the FWP with high accuracy in order to find a good heuristic
solution for the MWMP.Comment: 20 pages, 14 figures, Latex, to appear in Journal of Experimental
Algorithms, 200
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