5,347 research outputs found
Radio Emission from the Composite Supernova Remnant G326.3-1.8 (MSH15-56)
High resolution radio observations of the composite supernova remnant (SNR)
G326.3-1.8 or MSH 15-56 with the Australia Telescope Compact Array show details
of both the shell and the bright plerion which is offset about 1/3 of the
distance from the center of the SNR to the shell. The shell appears to be
composed of thin filaments, typical of older shell SNRs. The central part of
the elongated plerion is composed of a bundle of parallel ridges which bulge
out at the ends and form a distinct ring structure on the northwestern end. The
magnetic field with a strength of order 45 microGauss, is directed along the
axis of the ridges but circles around the northwestern ring. This plerion is
large and bright in the radio but is not detected in x-ray or optical
wavelengths. There is, however, a faint hard x-ray feature closer to the shell
outside the plerion. Perhaps if the supernova explosion left a rapidly moving
magnetar with large energy input but initially rapid decay of both relativistic
particles and magnetic field, the observed differences with wavelength could be
explained.Comment: 15 pages, 10 figures, accepted by Ap
Mining Missing Hyperlinks from Human Navigation Traces: A Case Study of Wikipedia
Hyperlinks are an essential feature of the World Wide Web. They are
especially important for online encyclopedias such as Wikipedia: an article can
often only be understood in the context of related articles, and hyperlinks
make it easy to explore this context. But important links are often missing,
and several methods have been proposed to alleviate this problem by learning a
linking model based on the structure of the existing links. Here we propose a
novel approach to identifying missing links in Wikipedia. We build on the fact
that the ultimate purpose of Wikipedia links is to aid navigation. Rather than
merely suggesting new links that are in tune with the structure of existing
links, our method finds missing links that would immediately enhance
Wikipedia's navigability. We leverage data sets of navigation paths collected
through a Wikipedia-based human-computation game in which users must find a
short path from a start to a target article by only clicking links encountered
along the way. We harness human navigational traces to identify a set of
candidates for missing links and then rank these candidates. Experiments show
that our procedure identifies missing links of high quality
Transition from a phase-segregated state to single-phase incommensurate sodium ordering in Na_xCoO_2 with x \approx 0.53
Synchrotron X-ray diffraction investigations of two single crystals of
Na_xCoO_2 from different batches with composition x = 0.525-0.530 reveal
homogeneous incommensurate sodium ordering with propagation vector (0.53 0.53
0) at room-temperature. The incommensurate (qq0) superstructure exists between
220 K and 430 K. The value of q varies between q = 0.514 and 0.529, showing a
broad plateau at the latter value between 260 K and 360 K. On cooling, unusual
reversible phase segregation into two volume fractions is observed. Below 220
K, one volume fraction shows the well-known commensurate orthorhombic x = 0.50
superstructure, while a second volume fraction with x = 0.55 exhibits another
commensurate superstructure, presumably with a 6a x 6a x c hexagonal supercell.
We argue that the commensurate-to-incommensurate transition is an intrinsic
feature of samples with Na concentrations x = 0.5 + d with d ~ 0.03.Comment: Corrected/improved versio
Summation and transformation formulas for elliptic hypergeometric series
Using matrix inversion and determinant evaluation techniques we prove several
summation and transformation formulas for terminating, balanced,
very-well-poised, elliptic hypergeometric series.Comment: 21 pages, AMS-LaTe
Far-flung Filaments of Ejecta in the Young Supernova Remnant G292.0+1.8
New optical images of the young SNR G292.0+1.8, obtained from the 0.9-m
telescope at CTIO, show a more extensive network of filaments than had been
known previously. Filaments emitting in [O III] are distributed throughout much
of the 8 arcmin diameter shell seen in X-ray and radio images, including a few
at the very outermost shell limits. We have also detected four small complexes
of filaments that show [S II] emission along with [OIII]. In a single long-slit
spectrum we find variations of almost an order of magnitude in the relative
strengths of oxygen and sulfur lines, which must result from abundance
variations. None of the filaments, with or without [S II], shows any evidence
for hydrogen, so all appear to be fragments of pure SN ejecta. The [S II]
filaments provide the first evidence for undiluted products of oxygen burning
in the ejecta from the supernova that gave rise to G292.0+1.8. Some oxygen
burning must have occurred, but the paucity of [S II]-emitting filaments
suggests that either the oxygen burning was not extensive or that most of its
products have yet to be excited. Most of the outer filaments exhibit radial,
pencil-like morphologies that suggest an origin as Rayleigh-Taylor fingers of
ejecta, perhaps formed during the explosion. Simulations of core-collapse
supernovae predict such fingers, but these have never before been so clearly
observed in a young SNR. The total flux from the SNR in [OIII] 5007 is 5.4 *
10**-12 ergs/cm**2/s. Using a distance of 6 kpc and an extinction correction
corresponding to E(B-V) = 0.6 (lower than previous values but more consistent
both with our data and radio and X-ray estimates of NH), this leads to a
luminosity of 1.6 * 10**35 ergs/s in the 5007 Ang. line.Comment: 32 pages including 10 figures, and 3 tables, accepted for publication
in AJ. Vol 132, July 2006. Higher resolution versions of the figures and a
pdf of the manuscript can be found at
http://www-int.stsci.edu/~long/papers/g292_optical
Brauer group of moduli spaces of pairs
We show that the Brauer group of any moduli space of stable pairs with fixed
determinant over a curve is zero.Comment: 12 pages. Final version, accepted in Communications in Algebr
Nonlinear Band Structure in Bose Einstein Condensates: The Nonlinear Schr\"odinger Equation with a Kronig-Penney Potential
All Bloch states of the mean field of a Bose-Einstein condensate in the
presence of a one dimensional lattice of impurities are presented in closed
analytic form. The band structure is investigated by analyzing the stationary
states of the nonlinear Schr\"odinger, or Gross-Pitaevskii, equation for both
repulsive and attractive condensates. The appearance of swallowtails in the
bands is examined and interpreted in terms of the condensates superfluid
properties. The nonlinear stability properties of the Bloch states are
described and the stable regions of the bands and swallowtails are mapped out.
We find that the Kronig-Penney potential has the same properties as a
sinusoidal potential; Bose-Einstein condensates are trapped in sinusoidal
optical lattices. The Kronig-Penney potential has the advantage of being
analytically tractable, unlike the sinusoidal potential, and, therefore, serves
as a good model for experimental phenomena.Comment: Version 2. Fixed typos, added referenc
Parallax of PSR J1744-1134 and the Local Interstellar Medium
We present the annual trigonometric parallax of PSR J1744-1134 derived from
an analysis of pulse times of arrival. The measured parallax, pi = 2.8+/-0.3
mas ranks among the most precisely determined distances to any pulsar. The
parallax distance of 357+/-39 pc is over twice that derived from the dispersion
measure using the Taylor & Cordes model for the Galactic electron distribution.
The mean electron density in the path to the pulsar, n_e = (0.0088 +/- 0.0009)
cm^{-3}, is the lowest for any disk pulsar. We have compared the n_e for PSR
J1744-1134 with those for another 11 nearby pulsars with independent distance
estimates. We conclude that there is a striking asymmetry in the distribution
of electrons in the local interstellar medium. The electron column densities
for pulsars in the third Galactic quadrant are found to be systematically
higher than for those in the first. The former correlate with the position of
the well known local HI cavity in quadrant three. The excess electrons within
the cavity may be in the form of HII clouds marking a region of interaction
between the local hot bubble and a nearby superbubble.Comment: revised version accepted for publication in ApJ Letters; reanalysis
of uncertainty in parallax measure and changes to fig
Cohomological Hasse principle and motivic cohomology for arithmetic schemes
In 1985 Kazuya Kato formulated a fascinating framework of conjectures which
generalizes the Hasse principle for the Brauer group of a global field to the
so-called cohomological Hasse principle for an arithmetic scheme. In this paper
we prove the prime-to-characteristic part of the cohomological Hasse principle.
We also explain its implications on finiteness of motivic cohomology and
special values of zeta functions.Comment: 47 pages, final versio
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