890 research outputs found
Spin Gap of S=1/2 Heisenberg Model on Distorted Diamond Chain
We study the spin gap of the S=1/2 Heisenberg model on the distorted diamond
chain, which is recently proposed to represent magnetic properties of Cu_3 Cl_6
(H_2 O)_2 2H_8 C_4 SO_2. This model is composed of stacked trimers and has
three kinds of exchange interactions J_1, J_2 and J_3. Using the numerical
diagonalization, we obtain a contour map of the spin gap in the J_2/J_1-J_3/J_1
plane. We argue possible values of the exchange constants based on the contour
map and the observed value of the spin gap.Comment: 2 pages, 4 figure
Long-Term X-ray Spectral Variability in Seyfert 1 Galaxies
Direct time-resolved spectral fitting has been performed on continuous RXTE
monitoring of seven Seyfert 1 galaxies in order to study their broadband
spectral variability and Fe K alpha variability characteristics on time scales
of days to years. Variability in the Fe K alpha line is not detected in some
objects but is present in others, e.g., in NGC 3516, NGC 4151 and NGC 5548
there are systematic decreases in line flux by factors of ~2-5 over 3-4 years.
The Fe K alpha line varies less strongly than the broadband continuum, but,
like the continuum, exhibits stronger variability towards longer time scales.
Relatively less model-dependent broadband fractional variability amplitude
(Fvar) spectra also show weaker line variability compared to the continuum
variability. Comparable systematic long-term decreases in the line and
continuum are present in NGC 5548. Overall, however, there is no evidence for
correlated variability between the line and continuum, severely challenging
models in which the line tracks continuum variations modified only by a
light-travel time delay. Local effects such as the formation of an ionized skin
at the site of line emission may be relevant. The spectral fitting and Fvar
spectra both support spectral softening as continuum flux increases.Comment: Accepted for publication in ApJ. 29 page
Chandra observations of the millisecond X-ray pulsar IGR J00291+5934 in quiescence
In this Paper we report on our analysis of three Chandra observations of the
accretion-powered millisecond X-ray pulsar IGR J00291+5934 obtained during the
late stages of the 2004 outburst. We also report the serendipitous detection of
the source in quiescence by ROSAT during MJD 48830-48839. The detected 0.3-10
keV source count rates varied significantly between the Chandra observations
from (7.2+-1.2)x10^-3, (6.8+-0.9)x10^-3, and (1.4+-0.1)x10^-2 counts per second
for the 1st, 2nd, and 3rd Chandra observation, on MJD 53371.88, 53383.99, and
53407.57, respectively. The count rate for the 3rd observation is 2.0+-0.4
times as high as that of the average of the first two observations. The
unabsorbed 0.5-10 keV source flux for the best-fit power-law model to the
source spectrum was (7.9+-2.5)x10^-14, (7.3+-2.0)x10^-14, and
(1.17+-0.22)x10^-13 erg cm^-2 s^-1 for the 1st, 2nd, and 3rd Chandra
observation, respectively. We find that this source flux is consistent with
that found by ROSAT [~(5.4+-2.4)x10^-14 erg cm^-2 s^-1]. Under the assumption
that the interstellar extinction, N_H, does not vary between the observations,
we find that the blackbody temperature during the 2nd Chandra observation is
significantly higher than that during the 1st and 3rd observation. Furthermore,
the effective temperature of the neutron star derived from fitting an absorbed
blackbody or neutron star atmosphere model to the data is rather high in
comparison with many other neutron star soft X-ray transients in quiescence,
even during the 1st and 3rd observation. If we assume that the source quiescent
luminosity is similar to that measured for two other accretion powered
millisecond pulsars in quiescence, the distance to IGR J00291+5934 is 2.6-3.6
kpc.Comment: 7 pages, 3 Figures, accepted for publication in MNRA
Spin Dressed Relaxation and Frequency Shifts from Field Imperfections
Critical dressing, the simultaneous dressing of two spin species to the same
effective Larmor frequency, is a technique that can, in principle, improve the
sensitivity to small frequency shifts. The benefits of spin dressing and thus
critical dressing are achieved at the expense of generating a large (relative
to the holding field ,) homogeneous oscillating field. Due to inevitable
imperfections of the fields generated, the benefits of spin dressing may be
lost from the additional relaxation and noise generated by the dressing field
imperfections. In this analysis the subject of relaxation and frequency shifts
are approached with simulations and theory. Analytical predictions are made
from a new quasi-quantum model that includes gradients in the holding field
and dressing field
where is oscillating at frequency . The results are compared
with a Monte Carlo simulation coupled with a 5 order Runge-Kutta
integrator. Comparisons of the two methods are presented as well as a set of
optimized parameters that produce stable critical dressing at a range for
oscillating frequencies as well as pulsed frequency modulation
parameters for maximum sensitivity
Frustrated trimer chain model and Cu3Cl6(H2O)2 2H8C4SO2 in a magnetic field
Recent magnetization and susceptibility measurements on Cu3Cl6(H2O)2 2H8C4SO2
by Ishii et.al. [J. Phys. Soc. Jpn. 69, 340 (2000)] have demonstrated the
existence of a spin gap. In order to explain the opening of a spin gap in this
copper-trimer system, Ishii et.al. have proposed a frustrated trimer chain
model. Since the exchange constants for this model have not yet been
determined, we develop a twelfth-order high-temperature series for the magnetic
susceptibility and fit it to the experimentally measured one. We find that some
of the coupling constants are likely to be ferromagnetic. The combination of
several arguments does not provide any evidence for a spin gap in the parameter
region with ferromagnetic coupling constants, but further results e.g. for the
magnetization process are in qualitative agreement with the experimental
observations.Comment: 9 pages REVTeX, 4 PostScript figures included using psfig.sty; for
series and supplementary material see http://www.tu-bs.de/~honecker/3mer/ or
http://www.itp.phys.ethz.ch/staff/laeuchli/3mer ; substantial reorganization
including shifting part of the discussion to WWW pages with "supplementary
material"; new appendix shows that antiferromagnetic J_i > 0 are not
compatible with the experimental data for the magnetic susceptibilit
Timing Noise in SGR 1806-20
We have phase connected a sequence of RXTE PCA observations of SGR 1806-20
covering 178 days. We find a simple secular spin-down model does not adequately
fit the data. The period derivative varies gradually during the observations
between 8.1 and 11.7 * 10^-11 s/s (at its highest, ~40% larger than the long
term trend), while the average burst rate as seen with BATSE drops throughout
the time interval. The phase residuals give no compelling evidence for
periodicity, but more closely resemble timing noise as seen in radio pulsars.
The magnitude of the timing noise, however, is large relative to the noise
level typically found in radio pulsars. Combining these results with the noise
levels measured for some AXPs, we find all magnetar candidates have \Delta_8
values larger than those expected from a simple extrapolation of the
correlation found in radio pulsars. We find that the timing noise in SGR
1806-20 is greater than or equal to the levels found in some accreting systems
(e.g., Vela X-1, 4U 1538-52 and 4U 1626-67), but the spin-down of SGR 1806-20
has thus far maintained coherence over 6 years. Alternatively, an orbital model
with a period P_orb = 733 days provides a statistically acceptable fit to the
data. If the phase residuals are created by Doppler shifts from a
gravitationally bound companion, then the allowed parameter space for the mass
function (small) and orbital separation (large) rule out the possibility of
accretion from the companion sufficient to power the persistent emission from
the SGR.Comment: 11 pages, accepted for publication in ApJ Letter
High-Sensitivity Measurement of 3He-4He Isotopic Ratios for Ultracold Neutron Experiments
Research efforts ranging from studies of solid helium to searches for a
neutron electric dipole moment require isotopically purified helium with a
ratio of 3He to 4He at levels below that which can be measured using
traditional mass spectroscopy techniques. We demonstrate an approach to such a
measurement using accelerator mass spectroscopy, reaching the 10e-14 level of
sensitivity, several orders of magnitude more sensitive than other techniques.
Measurements of 3He/4He in samples relevant to the measurement of the neutron
lifetime indicate the need for substantial corrections. We also argue that
there is a clear path forward to sensitivity increases of at least another
order of magnitude.Comment: 11 pages, 10 figure
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