11,602 research outputs found
Field-enhancement calculations for a field-distortion triggered spark gap
We present the results of numerical field calculation which supplement a recent article in which we described a new design concept for field-distortion triggered spark gaps. The calculations verify the shielding and field enhancement assumptions made in the article, and they provide insight into the interaction of the design tradeoffs associated with simultaneously maximizing the holdoff voltage and the triggering capability of the gap
Visualizing elements of Sha[3] in genus 2 jacobians
Mazur proved that any element xi of order three in the Shafarevich-Tate group
of an elliptic curve E over a number field k can be made visible in an abelian
surface A in the sense that xi lies in the kernel of the natural homomorphism
between the cohomology groups H^1(k,E) -> H^1(k,A). However, the abelian
surface in Mazur's construction is almost never a jacobian of a genus 2 curve.
In this paper we show that any element of order three in the Shafarevich-Tate
group of an elliptic curve over a number field can be visualized in the
jacobians of a genus 2 curve. Moreover, we describe how to get explicit models
of the genus 2 curves involved.Comment: 12 page
Gamma-Ray Burster Counterparts: HST Blue and Ultraviolet Data
The surest solution of the Gamma Ray Burst (GRB) mystery is to find an
unambiguous low-energy quiescent counterpart. However, to date no reasonable
candidates have been identified in the x-ray, optical, infrared, or radio
ranges. The Hubble Space Telescope (HST) has now allowed for the first deep
ultraviolet searches for quiescent counterparts. This paper reports on
multiepoch ultraviolet searches of five GRB positions with HST. We found no
sources with significant ultraviolet excesses, variability, parallax, or proper
motion in any of the burst error regions. In particular, we see no sources
similar to that proposed as a counterpart to the GRB970228. While this negative
result is disappointing, it still has good utility for its strict limits on the
no-host-galaxy problem in cosmological models of GRBs. For most cosmological
models (with peak luminosity 6X10^50 erg/s), the absolute B magnitude of any
possible host galaxy must be fainter than -15.5 to -17.4. These smallest boxes
for some of the brightest bursts provide the most critical test, and our limits
are a severe problem for all published cosmological burst models.Comment: 15 pages, 2 ps figures, accepted for publication in the Astrophysical
Journa
Low-momentum Hyperon-Nucleon Interactions
We present a first exploratory study for hyperon-nucleon interactions using
renormalization group techniques. The effective two-body low-momentum potential
V_low-k is obtained by integrating out the high-momentum components from
realistic Nijmegen YN potentials. A T-matrix equivalence approach is employed,
so that the low-energy phase shifts are reproduced by V_low-k up to a momentum
scale Lambda ~ 500 MeV. Although the various bare Nijmegen models differ
somewhat from each other, the corresponding V_low-k interactions show
convergence in some channels, suggesting a possible unique YN interaction at
low momenta.Comment: 4 pages, 6 figure
Nitrogen mineralization and reorganization in casts of the geophagous tropical earthworm Pontoscolex corethrurus (Glossoscolecidae)
A lattice evaluation of four-quark operators in the nucleon
Nucleon matrix elements of various four-quark operators are evaluated in
quenched lattice QCD using Wilson fermions. Some of these operators give rise
to twist-four contributions to nucleon structure functions. Furthermore, they
bear valuable information about the diquark structure of the nucleon. Mixing
with lower-dimensional operators is avoided by considering appropriate
representations of the flavour group. We find that for a certain flavour
combination of baryon structure functions, twist-four contributions are very
small. This suggests that twist-four effects for the nucleon might be much
smaller than m_p^2/Q^2.Comment: 17 pages, 3 figure
Applied lattice gauge calculations: diquark content of the nucleon
As an example of an application of lattice QCD we describe a computation of
four-quark operators in the nucleon. The results are interpreted in a diquark
language.Comment: 6 pages, 1 figure, Invited talk given by M. G\"ockeler at the
European Workshop on the QCD Structure of the Nucleon (QCD - N'02), Ferrara,
Italy, 3-6 Apr 200
From X-ray dips to eclipse: Witnessing disk reformation in the recurrent nova USco
The 10th recorded outburst of the recurrent eclipsing nova USco was observed
simultaneously in X-ray, UV, and optical by XMM-Newton on days 22.9 and 34.9
after outburst. Two full passages of the companion in front of the nova ejecta
were observed, witnessing the reformation of the accretion disk. On day 22.9,
we observed smooth eclipses in UV and optical but deep dips in the X-ray light
curve which disappeared by day 34.9, then yielding clean eclipses in all bands.
X-ray dips can be caused by clumpy absorbing material that intersects the line
of sight while moving along highly elliptical trajectories. Cold material from
the companion could explain the absence of dips in UV and optical light. The
disappearance of X-ray dips before day 34.9 implies significant progress in the
formation of the disk. The X-ray spectra contain photospheric continuum
emission plus strong emission lines, but no clear absorption lines. Both
continuum and emission lines in the X-ray spectra indicate a temperature
increase from day 22.9 to day 34.9. We find clear evidence in the spectra and
light curves for Thompson scattering of the photospheric emission from the
white dwarf. Photospheric absorption lines can be smeared out during scattering
in a plasma of fast electrons. We also find spectral signatures of resonant
line scattering that lead to the observation of the strong emission lines.
Their dominance could be a general phenomenon in high-inclination systems such
as Cal87.Comment: Submitted to ApJ. 16 pages, 16 figure
- …