38,600 research outputs found
Discovery of kHz Fluctuations in Centaurus X-3: Evidence for Photon Bubble Oscillations (PBO) and Turbulence in a High Mass X-ray Binary Pulsar
We report the discovery of kHz fluctuations, including quasi-periodic
oscillations (QPO) at ~330 Hz and ~760 Hz and a broadband kHz continuum in the
power density spectrum of the high mass X-ray binary pulsar Centaurus X-3.
These observations of Cen X-3 were carried out with the Rossi X-ray Timing
Explorer (RXTE). The fluctuation spectrum is flat from mHz to a few Hz, then
steepens to behavior between a few Hz and ~100 Hz. Above a hundred Hz,
the spectrum shows the QPO features, plus a flat continuum extending to ~1200
Hz and then falling out to ~1800 Hz. These results, which required the
co-adding three days of observations of Cen X-3, are at least as fast as the
fastest known variations in X-ray emission from an accreting compact object
(kHz QPO in LMXB sources) and probably faster since extension to ~1800 Hz is
indicated by the most likely parameterization of the data.
Multi-dimensional radiation hydrodynamics simulations of optically thick
plasma flow onto the magnetic poles of an accreting neutron star show that the
fluctuations at frequencies above 100 Hz are consistent with photon bubble
turbulence and oscillations (PBO) previously predicted to be observable in this
source. For a polar cap opening angle of 0.25 radians, we show that the
spectral form above 100 Hz is reproduced by the simulations, including the
frequencies of the QPO and the relative power in the QPO and the kHz continuum.
This has resulted in the first model-dependent measurement of the polar cap
size of an X-ray pulsar.Comment: received ApJ: April 1, 1999 accepted ApJ: September 1, 199
Seven-fluorochrome mouse M-FISH for high-resolution analysis of interchromosomal rearrangements
The mouse has evolved to be the primary mammalian genetic model organism. Important applications include the modeling of human cancer and cloning experiments. In both settings, a detailed analysis of the mouse genome is essential. Multicolor karyotyping technologies have emerged to be invaluable tools for the identification of mouse chromosomes and for the deciphering of complex rearrangements. With the increasing use of these multicolor technologies resolution limits are critical. However, the traditionally used probe sets, which employ 5 different fluorochromes, have significant limitations. Here, we introduce an improved labeling strategy. Using 7 fluorochromes we increased the sensitivity for the detection of small interchromosomal rearrangements (700 kb or less) to virtually 100%. Our approach should be important to unravel small interchromosomal rearrangements in mouse models for DNA repair defects and chromosomal instability. Copyright (C) 2003 S. Karger AG, Basel
Voltage and temperature dependence of the grain boundary tunneling magnetoresistance in manganites
We have performed a systematic analysis of the voltage and temperature
dependence of the tunneling magnetoresistance (TMR) of grain boundaries (GB) in
the manganites. We find a strong decrease of the TMR with increasing voltage
and temperature. The decrease of the TMR with increasing voltage scales with an
increase of the inelastic tunneling current due to multi-step inelastic
tunneling via localized defect states in the tunneling barrier. This behavior
can be described within a three-current model for magnetic tunnel junctions
that extends the two-current Julliere model by adding an inelastic,
spin-independent tunneling contribution. Our analysis gives strong evidence
that the observed drastic decrease of the GB-TMR in manganites is caused by an
imperfect tunneling barrier.Comment: to be published in Europhys. Lett., 8 pages, 4 figures (included
Evidence for Diverging Barriers in the Disordered Vortex Solid in the K,Ba BiO3 Superconducting Oxide
International audienceVortex dynamics has been investigated in the cubic K, Ba BiO3 superconductor using ac susceptibility measurements on a large frequency range 0.03 Hz , v , 60 kHz . Power law diverging barriers have been obtained on both sides of the order-disorder transition line. The m exponent remains close to 5 2 (elastic creep value) in some part of the disordered phase and finally decreases at high temperature and/or high field, in good agreement with the recent plastic collective creep theory [J. Kierfeld, H. Nordborg, and V. M. Vinokur, Phys. Rev. Lett., 85, 4948 (2000)]
CLAS+FROST: new generation of photoproduction experiments at Jefferson Lab
A large part of the experimental program in Hall B of the Jefferson Lab is
dedicated to baryon spectroscopy. Photoproduction experiments are essential
part of this program. CEBAF Large Acceptance Spectrometer (CLAS) and
availability of circularly and linearly polarized tagged photon beams provide
unique conditions for this type of experiments. Recent addition of the Frozen
Spin Target (FROST) gives a remarkable opportunity to measure double and triple
polarization observables for different pseudo-scalar meson photoproduction
processes. For the first time, a complete or nearly complete experiment becomes
possible and will allow model independent extraction of the reaction amplitude.
An overview of the experiment and its current status is presented.Comment: 6 pages, 7 figures. Invited paper NSTAR 2009 conferenc
Rooted Spiral Trees on Hyper-cubical lattices
We study rooted spiral trees in 2,3 and 4 dimensions on a hyper cubical
lattice using exact enumeration and Monte-Carlo techniques. On the square
lattice, we also obtain exact lower bound of 1.93565 on the growth constant
. Series expansions give and on a square lattice. With Monte-Carlo simulations we get the
estimates as , and . These results
are numerical evidence against earlier proposed dimensional reduction by four
in this problem. In dimensions higher than two, the spiral constraint can be
implemented in two ways. In either case, our series expansion results do not
support the proposed dimensional reduction.Comment: replaced with published versio
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