4,892 research outputs found
Precision Measurement of Orthopositronium Decay Rate Using SiO_2 Powder
The intrinsic decay rate of orthopositronium formed in powder
is measured using the direct correction method such that the time
dependence of the pick-off annihilation rate is precisely determined using high
energy-resolution germanium detectors. As a systematic test, two different
types of powder are used with consistent findings. The intrinsic
decay rate of orthopositronium is found to be , which is consistent with previous measurements using powder with about twice the accuracy. Results agree well with a recent
QED prediction, varying experimental standard
deviations from other measurements.Comment: 16 pages, 7 figures included. To be published in Physics Letters
Solution of Orthopositronium lifetime Puzzle
The intrinsic decay rate of orthopositronium formed in powder
is measured using the direct correction method such that the time
dependence of the pick-off annihilation rate is precisely determined. The decay
rate of orthopositronium is found to be , which is consistent with our previous measurements with
about twice the accuracy. Results agree well with the QED
prediction, and also with a result reported very recently using nanoporous
film
Chandra observations of the accretion-driven millisecond X-ray pulsars XTE J0929-314 and XTE J1751-305 in quiescence
(Abridge) We observed the accreting millisecond X-ray pulsars XTE J0929-314
and XTE J1751-305 in their quiescent states using Chandra. From XTE J0929-314
we detected 22 photons (0.3-8 keV) in 24.4 ksec, resulting in a count rate of 9
x 10^{-4} c/s. The small number of photons detected did not allow for a
detailed spectral analysis, but we can demonstrate that the spectrum is harder
than simple thermal emission which is what is usually presumed to arise from a
cooling neutron star that has been heated during the outbursts. Assuming a
power-law model for the spectrum, we obtain a power-law index of ~1.8 and an
unabsorbed flux of 6 x 10^{-15} ergs/s/cm^2 (0.5-10 keV), resulting in a
luminosity of 7 x 10^{31} (d/10 kpc)^2 ergs/s, with d in kpc. No thermal
component could be detected; such a component contributed at most 30% to the
0.5-10 keV flux. Variability in the count rate of XTE J0929-314 was observed at
the 95% confidence level. We did not conclusively detect XTE J1751-305 in our
43 ksec observation, with 0.5-10 keV flux upper limits between 0.2 and 2.7 x
10^{-14} ergs/s/cm^2 depending on assumed spectral shape, resulting in
luminosity upper limits of 0.2 - 2 x 10^{32} (d/8 kpc)^2 ergs/s. We compare our
results with those obtained for other neutron-star X-ray transients in their
quiescent state. Using simple accretion disk physics in combination with our
measured quiescent luminosity of XTE J0929-314 and the luminosity upper limits
of XTE J1751-305, and the known spin frequency of the neutron stars, we could
constrain the magnetic field of the neutron stars in XTE J0929-314 and XTE
J1751-305 to be less than 3 x 10^9 (d/10 kpc) and 3 - 7 x 10^8 (d/8 kpc) Gauss
(depending on assumed spectral shape of the quiescent spectrum), respectively.Comment: Accepted for publication in ApJ, 29 September 2004. Added spectral
variability search for the data of XTE J0929-314 and added the non-detection
with Chandra of XTE J1751-30
Precise measurement of positronium hyperfine splitting using the Zeeman effect
Positronium is an ideal system for the research of the quantum
electrodynamics (QED) in bound state. The hyperfine splitting (HFS) of
positronium, , gives a good test of the bound state
calculations and probes new physics beyond the Standard Model. A new method of
QED calculations has revealed the discrepancy by 15\,ppm (3.9) of
between the QED prediction and the experimental
average. There would be possibility of new physics or common systematic
uncertainties in the previous all experiments. We describe a new experiment to
reduce possible systematic uncertainties and will provide an independent check
of the discrepancy. We are now taking data and the current result of
has been obtained so far. A measurement with a precision of (ppm) is
expected within a year.Comment: 8 pages, 8 figures, 2 tables, proceeding of LEAP2011, accepted by
Hyperfine Interaction
Novel mechanism of photoinduced reversible phase transitions in molecule-based magnets
A novel microscopic mechanism of bi-directional structural changes is
proposed for the photo-induced magnetic phase transition in Co-Fe Prussian blue
analogues on the basis of ab initio quantum chemical cluster calculations. It
is shown that the local potential energies of various spin states of Co are
sensitive to the number of nearest neighbor Fe vacancies. As a result, the
forward and backward structural changes are most readily initiated by
excitation of different local regions by different photons. This mechanism
suggests an effective strategy to realize photoinduced reversible phase
transitions in a general system consisting of two local components.Comment: 4 pages, LaTex, 3 figures, to appear in Phys. Rev. Let
The Properties of X-ray Cold Fronts in a Statistical Sample of Simulated Galaxy Clusters
We examine the incidence of cold fronts in a large sample of galaxy clusters
extracted from a (512h^-1 Mpc) hydrodynamic/N-body cosmological simulation with
adiabatic gas physics computed with the Enzo adaptive mesh refinement code.
This simulation contains a sample of roughly 4000 galaxy clusters with M >
10^14 M_sun at z=0. For each simulated galaxy cluster, we have created mock
0.3-8.0 keV X-ray observations and spectroscopic-like temperature maps. We have
searched these maps with a new automated algorithm to identify the presence of
cold fronts in projection. Using a threshold of a minimum of 10 cold front
pixels in our images, corresponding to a total comoving length L_cf > 156h^-1
kpc, we find that roughly 10-12% of all projections in a mass-limited sample
would be classified as cold front clusters. Interestingly, the fraction of
clusters with extended cold front features in our synthetic maps of a
mass-limited sample trends only weakly with redshift out to z=1.0. However,
when using different selection functions, including a simulated flux limit, the
trending with redshift changes significantly. The likelihood of finding cold
fronts in the simulated clusters in our sample is a strong function of cluster
mass. In clusters with M>7.5x10^14 M_sun the cold front fraction is 40-50%. We
also show that the presence of cold fronts is strongly correlated with
disturbed morphology as measured by quantitative structure measures. Finally,
we find that the incidence of cold fronts in the simulated cluster images is
strongly dependent on baryonic physics.Comment: 16 pages, 21 figures, Accepted to Ap
Resolving the Hard X-ray Emission of GX 5-1 with INTEGRAL
We present the study of one year of INTEGRAL data on the neutron star low
mass X-ray binary GX 5-1. Thanks to the excellent angular resolution and
sensitivity of INTEGRAL, we are able to obtain a high quality spectrum of GX
5-1 from ~5 keV to ~100 keV, for the first time without contamination from the
nearby black hole candidate GRS 1758-258 above 20 keV. During our observations,
GX 5-1 is mostly found in the horizontal and normal branch of its hardness
intensity diagram. A clear hard X-ray emission is observed above ~30 keV which
exceeds the exponential cut-off spectrum expected from lower energies. This
spectral flattening may have the same origin of the hard components observed in
other Z sources as it shares the property of being characteristic to the
horizontal branch. The hard excess is explained by introducing Compton
up-scattering of soft photons from the neutron star surface due to a thin hot
plasma expected in the boundary layer. The spectral changes of GX 5-1 downward
along the "Z" pattern in the hardness intensity diagram can be well described
in terms of monotonical decrease of the neutron star surface temperature. This
may be a consequence of the gradual expansion of the boundary layer as the mass
accretion rate increases.Comment: 10 pages, 17 figures, accepted for publication in A&
Ferromagnetism and Superconductivity in the multi-orbital Hubbard Model: Hund's Rule Coupling versus Crystal-Field Splitting
The multi-orbital Hubbard model in one dimension is studied using the
numerical diagonalization method. Due to the effect of the crystal-field
splitting , the fully polarized ferromagnetism which is observed in the
strong coupling regime becomes unstable against the partially polarized
ferromagnetism when the Hund's rule coupling is smaller than a certain
critical value of order of . In the vicinity of the partially polarized
ferromagnetism, the orbital fluctuation develops due to the competition between
the Hund's rule coupling and the crystal-field splitting. The superconducting
phase with the Luttinger liquid parameter is observed for the
singlet ground state in this region.Comment: 4 pages,5 figures,submitted to J.Phys.Soc.Jp
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