9,571 research outputs found
The Quiescent Optical and Infrared Counterpart to EXO 0748-676 = UY Vol
We present optical and infrared photometry of the low-mass X-ray binary EXO
0748-676 in quiescence for the first time in 24 years since it became X-ray
active in 1985. We find the counterpart at average magnitudes of R=22.4 and
J=21.3. We monitored the source approximately nightly through 2008 November to
2009 January. During this time there was considerable night-to-night optical
variability but no long term trends were apparent. The night-to-night
variability reveals a periodicity of P=0.159331+/-0.000012d, consistent with
the X-ray orbital period to within 0.01%. This indicates that the quiescent
optical modulation is indeed orbital in nature rather than a superhump.
Interestingly, the modulation remains single-peaked with a deep minimum
coincident with the times of X-ray eclipse, and there is no indication of a
double-peaked ellipsoidal modulation. This indicates that even in `quiescence'
emission from the accretion disk and/or X-ray heated inner face of the
companion star dominate the optical emission, and implies that obtaining an
accurate dynamical mass estimate in quiescence will be challenging.Comment: Accepted for publication by the Astrophysical Journal Letter
A 20 kiloHertz space station power system
The space station represents the next major U.S. commitment in space. The efficient delivery of power to multiple user loads is key to that success. In 1969, NASA Lewis Research Center began a series of studies with component and circuit developments that led to the high frequency, bi-directional, four quadrant resonant driven converter. Additional studies and subsequent developments into the early 1980's have shown how the high frequency ac power system could provide overall advantages to many aerospace power systems. Because of its wide versatility, it also has outstanding advantages for the Space Station Program and its wide range of users. High frequency ac power provides higher efficiency, lower cost, and improved safety. The 20 kHz power system has exceptional flexibility, is inherently user friendly, and is compatible with all types of energy sources - photovoltaic, solar dynamic, rotating machines or nuclear. Lewis has recently completed development under contract a 25 kW, 20 kHz ac power distribution system testbed. The testbed demonstrates flexibility, versatility, and transparency to user technology as well as high efficiency, low mass, and reduced volume
Spectral Formation in X-Ray Pulsar Accretion Columns
We present the first self-consistent model for the dynamics and the radiative
transfer occurring in bright X-ray pulsar accretion columns, with a special
focus on the role of the shock in energizing the emerging X-rays. The pressure
inside the accretion column of a luminous X-ray pulsar is dominated by the
photons, and consequently the equations describing the coupled
radiative-dynamical structure must be solved simultaneously. Spectral formation
in these sources is therefore a complex, nonlinear phenomenon. We obtain the
analytical solution for the Green's function describing the upscattering of
monochromatic radiation injected into the column from the thermal mound located
near the base of the flow. The Green's function is convolved with a Planck
distribution to model the X-ray spectrum resulting from the reprocessing of
blackbody photons produced in the thermal mound. These photons diffuse through
the infalling gas and eventually escape out the walls of the column, forming
the observed X-ray spectrum. We show that the resulting column-integrated,
phase-averaged spectrum has a power-law shape at high energies and a blackbody
shape at low energies, in agreement with the observational data for many X-ray
pulsars.Comment: Accepted for publication in ApJ Letters. Several typos noticed during
the proof review were correcte
The Anomalous Hall effect in re-entrant AuFe alloys and the real space Berry phase
The Hall effect has been studied in a series of AuFe samples in the
re-entrant concentration range, as well as in the spin glass range. The data
demonstrate that the degree of canting of the local spins strongly modifies the
anomalous Hall effect, in agreement with theoretical predictions associating
canting, chirality and a real space Berry phase. The canonical parametrization
of the Hall signal for magnetic conductors becomes inappropriate when local
spins are canted.Comment: 4 pages, 1 eps figur
Running quark mass in two flavor QCD
We present first results for the step scaling function sigma_P of the
renormalization factor Z_P of the pseudoscalar density. The simulations are
performed within the framework of the Schroedinger functional with two flavors
of O(a) improved Wilson fermions. The knowledge of sigma_P is required to
compute the renormalization group invariant quark masses. We also study the
performance of a variant of the HMC algorithm using two pseudofermion fields.Comment: 3 pages, 2 figures, Lattice2002(spectrum
Critical exponents of a three dimensional O(4) spin model
By Monte Carlo simulation we study the critical exponents governing the
transition of the three-dimensional classical O(4) Heisenberg model, which is
considered to be in the same universality class as the finite-temperature QCD
with massless two flavors. We use the single cluster algorithm and the
histogram reweighting technique to obtain observables at the critical
temperature. After estimating an accurate value of the inverse critical
temperature \Kc=0.9360(1), we make non-perturbative estimates for various
critical exponents by finite-size scaling analysis. They are in excellent
agreement with those obtained with the expansion method with
errors reduced to about halves of them.Comment: 25 pages with 8 PS figures, LaTeX, UTHEP-28
From Uncertainty Data to Robust Policies for Temporal Logic Planning
We consider the problem of synthesizing robust disturbance feedback policies
for systems performing complex tasks. We formulate the tasks as linear temporal
logic specifications and encode them into an optimization framework via
mixed-integer constraints. Both the system dynamics and the specifications are
known but affected by uncertainty. The distribution of the uncertainty is
unknown, however realizations can be obtained. We introduce a data-driven
approach where the constraints are fulfilled for a set of realizations and
provide probabilistic generalization guarantees as a function of the number of
considered realizations. We use separate chance constraints for the
satisfaction of the specification and operational constraints. This allows us
to quantify their violation probabilities independently. We compute disturbance
feedback policies as solutions of mixed-integer linear or quadratic
optimization problems. By using feedback we can exploit information of past
realizations and provide feasibility for a wider range of situations compared
to static input sequences. We demonstrate the proposed method on two robust
motion-planning case studies for autonomous driving
Does the CAMELS bank ratings system follow a procyclical pattern?
The financial crisis which erupted in 2007-8 has illustrated the disruptive effects of procyclicality. The phenomenon of procyclicality refers to the mutually reinforcing interactions between the financial system and the real economy that tend to amplify business cycle fluctuations. In this study, we empirically investigate the sensitivity of the CAMELS ratings system, which is used by the U.S. authorities to monitor the conditions in the banking market, to the fluctuations of the economic cycle. Our results suggest that the overall state of the U.S. economy and bank regulatory ratings are positively linked to each other: CAMELS increase during economic upturns and decrease during downturns. This is to say that the performance and risk-taking behaviour of banks is rated higher when the conditions in the economy are favourable and lower when the economic environment is weak. Along these lines, we document a positive relationship between CAMELS and the conditions in financial markets. This very important and rather unknown source of procyclicality should be taken into serious consideration by authorities
Long-term Dynamics of the Electron-nuclear Spin System of a Semiconductor Quantum Dot
A quasi-classical theoretical description of polarization and relaxation of
nuclear spins in a quantum dot with one resident electron is developed for
arbitrary mechanisms of electron spin polarization. The dependence of the
electron-nuclear spin dynamics on the correlation time of electron
spin precession, with frequency , in the nuclear hyperfine field is
analyzed. It is demonstrated that the highest nuclear polarization is achieved
for a correlation time close to the period of electron spin precession in the
nuclear field. For these and larger correlation times, the indirect hyperfine
field, which acts on nuclear spins, also reaches a maximum. This maximum is of
the order of the dipole-dipole magnetic field that nuclei create on each other.
This value is non-zero even if the average electron polarization vanishes. It
is shown that the transition from short correlation time to
does not affect the general structure of the equation for nuclear spin
temperature and nuclear polarization in the Knight field, but changes the
values of parameters, which now become functions of . For
correlation times larger than the precession time of nuclei in the electron
hyperfine field, it is found that three thermodynamic potentials (,
, ) characterize the polarized electron-nuclear spin
system. The values of these potentials are calculated assuming a sharp
transition from short to long correlation times, and the relaxation mechanisms
of these potentials are discussed. The relaxation of the nuclear spin potential
is simulated numerically showing that high nuclear polarization decreases
relaxation rate.Comment: RevTeX 4, 12 pages, 9 figure
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