74,692 research outputs found
On the Disappearance of Kilohertz Quasi-Periodic Oscillations at a High Mass Accretion Rate in Low-Mass X-ray Binaries
For all sources in which the phenomenon of kilo-Hertz quasi-periodic
oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the
inferred mass accretion rate exceeds a certain threshold. Although the
threshold cannot at present be accurately determined (or even quantified)
observationally, it is clearly higher for bright Z sources than for faint atoll
sources. Here we propose that the observational manifestation of kHz QPOs {\em
requires} direct interaction between the neutron star magnetosphere and the
Keplerian accretion disk and that the cessation of kHz QPOs at high accretion
rate is due to the lack of such an interact when the Keplerian disk terminates
at the last stable orbit and yet the magnetosphere is pushed farther inward.
The threshold is therefore dependent of the magnetic field strength -- the
stronger the magnetic field the higher the threshold. This is certainly in
agreement with the atoll/Z paradigm, but we argue that it is also generally
true, even for individual sources within each (atoll or Z) category. For atoll
sources, the kHz QPOs also seem to vanish at low accretion rate. Perhaps the
``disengagement'' between the magnetosphere and the Keplerian disk also takes
place under such circumstances, because of, for instance, the presence of
quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron
star. Unfortunately, in this case, the estimation of the accretion rate
threshold would require a knowledge of the physical mechanisms that cause the
disengagement. If the ADAF is responsible, the threshold is likely dependent of
the magnetic field of the neutron star.Comment: Minor revisions to match the published versio
Optimal design application on the advanced aeroelastic rotor blade
The vibration and performance optimization procedure using regression analysis was successfully applied to an advanced aeroelastic blade design study. The major advantage of this regression technique is that multiple optimizations can be performed to evaluate the effects of various objective functions and constraint functions. The data bases obtained from the rotorcraft flight simulation program C81 and Myklestad mode shape program are analytically determined as a function of each design variable. This approach has been verified for various blade radial ballast weight locations and blade planforms. This method can also be utilized to ascertain the effect of a particular cost function which is composed of several objective functions with different weighting factors for various mission requirements without any additional effort
Quantum phase estimation algorithms with delays: effects of dynamical phases
The unavoidable finite time intervals between the sequential operations
needed for performing practical quantum computing can degrade the performance
of quantum computers. During these delays, unwanted relative dynamical phases
are produced due to the free evolution of the superposition wave-function of
the qubits. In general, these coherent "errors" modify the desired quantum
interferences and thus spoil the correct results, compared to the ideal
standard quantum computing that does not consider the effects of delays between
successive unitary operations. Here, we show that, in the framework of the
quantum phase estimation algorithm, these coherent phase "errors", produced by
the time delays between sequential operations, can be avoided by setting up the
delay times to satisfy certain matching conditions.Comment: 10 pages, no figur
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