arXiv subjects
Publications and source records attributed to K. Wu.
We present the symplectic algorithm in the Lagrangian formalism for the Hamiltonian systems by virtue of the noncommutative differential calculus with respect to the discrete time and the Euler--Lagrange cohomological concepts. We also show that the trapezoidal integrator is symplectic in certain sense.
From study of the Kosterlitz-Thouless-Berezinskii (KTB) transition in the superfluid density, n_s(T), of ultrathin c-axis oriented YBa_{2}Cu_{3}O_{7-δ} (YBCO) films, we find that interlayer coupling is unexpectedly strong. The KTB transition occurs at a high temperature, as if the films were isotropic rather than quasi-two-dimensional. This result agrees with a comparison of the superfluid density of YBCO with Bi_{2}Sr_{2}CaCu_{2}O_{8} and with numerical simulations of Josephson junction arrays, and challenges the thermal phase fluctuation interpretation of critical behavior near T_c in YBCO.
We study the difference discrete variational principle in the framework of multi-parameter differential approach by regarding the forward difference as an entire geometric object in view of noncomutative differential geometry. By virtue of this variational principle, we get the difference discrete Euler-Lagrange equations and canonical ones for the difference discrete versions of the classical mechanics and classical field theory. We also explore the difference discrete versions for the Euler-Lagrange cohomology and apply them to the symplectic or multisymplectic geometry and their preserving properties in both Lagrangian and Hamiltonian formalism. In terms of the difference discrete Euler-Lagrange cohomological concepts, we show that the symplectic or multisymplectic geometry and their difference discrete structure preserving properties can always be established not only in the solution spaces of the discrete Euler-Lagrange/canonical equations derived by the difference discrete variational principle but also in the function space in each case if and only if the relevant closed Euler-Lagrange cohomological conditions are satisfied. We also apply the difference discrete variational principle and cohomological approach directly to the symplectic and multisymplectic algorithms.
We study the Euler-Lagrange cohomology and explore the symplectic or multisymplectic geometry and their preserving properties in classical mechanism and classical field theory in Lagrangian and Hamiltonian formalism in each case respectively. By virtue of the Euler-Lagrange cohomology that is nontrivial in the configuration space, we show that the symplectic or multisymplectic geometry and related preserving property can be established not only in the solution space but also in the function space if and only if the relevant closed Euler-Lagrange cohomological condition is satisfied in each case. We also apply the cohomological approach directly to Hamiltonian-like ODEs and Hamiltonian-like PDEs no matter whether there exist known Lagrangian and/or Hamiltonian associated with them.
Sharp dips observed in the pulse profiles of three X-ray pulsars (GX 1+4, RX J0812.4-3114 and A 0535+26) have previously been suggested to arise from partial eclipses of the emission region by the accretion column occurring once each rotation period. We present pulse-phase spectroscopy from Rossi X-ray Timing Explorer satellite observations of GX 1+4 and RX J0812.4-3114 which for the first time confirms this interpretation. The dip phase corresponds to the closest approach of the column axis to the line of sight, and the additional optical depth for photons escaping from the column in this direction gives rise to both the decrease in flux and increase in the fitted optical depth measured at this phase. Analysis of the arrival time of individual dips in GX~1+4 provides the first measurement of azimuthal wandering of a neutron star accretion column. The column longitude varies stochastically with standard deviation 2-6 degrees depending on the source luminosity. Measurements of the phase width of the dip both from mean pulse profiles and individual eclipses demonstrates that the dip width is proportional to the flux. The variation is consistent with that expected if the azimuthal extent of the accretion column depends only upon the Keplerian velocity at the inner disc radius, which varies as a consequence of the accretion rate Mdot.
We report multifrequency radio observations of XTE J1550-564 obtained with the Molonglo Observatory Synthesis Telescope and the Australia Telescope Compact Array at the time of its discovery and subsequent hard and soft X-ray outburst in 1998 September. A large radio flare was observed, peaking about 1.8 days after the X-ray flare. In addition, we present Australian Long Baseline Array images obtained shortly after the maximum of the radio flare which show evolving structure. The apparent separation velocity of the two outermost components is v>2c.
In 1998 September, the X-ray transient XTE J1550-564 underwent a major outburst in soft and hard X-rays, followed by a radio flare. Australian Long Baseline Array images obtained shortly after the peak in the radio flare showed evolving structure. The components observed have an apparent separation velocity of >2c.
Block-spin transformations from a fine lattice to a coarse one are shown to give rise to a one-to-one correspondence between the zero-modes of the Ginsparg-Wilson Dirac operators. The index is then preserved under the blocking process. Such a one-to-one correspondence is violated and the block-spin transformation becomes necessarily ill-defined when the absolute value of the index is larger than 2rN, where N is the number of the sites on the coarse lattice and r is the dimension of the gauge group representation of the fermion variables.
We report multifrequency radio observations of GRO J1655-40 obtained with the Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope and the Hartebeesthoek Radio Astronomy Observatory at the time of the major hard X-ray and radio outbursts in 1994 August-September. The radio emission reached levels of the order of a few Jy and was found to be linearly polarized by up to 10%, indicating a synchrotron origin. The light curves are in good agreement with those measured with the VLA, but our closer time sampling has revealed two new short-lived events and significant deviations from a simple exponential decay. The polarization data show that the magnetic field is well ordered and aligned at right angles to the radio jets for most of the monitoring period. The time evolution of the polarization cannot be explained solely in terms of a simple synchrotron bubble model, and we invoke a hybrid `core-lobe' model with a core which contributes both synchrotron and free-free emission and `lobes' which are classical synchrotron emitters.
Configuration space of abelian gauge theory on a periodic lattice becomes topologically disconnected by excising exceptional gauge field configurations. It is possible to define a U(1) bundle from the nonexceptional link variables by a smooth interpolation of the transition functions. The lattice analogue of Chern character obtained by a cohomological technique based on the noncommutative differential calculus is shown to give a topological charge related to the topological winding number of the U(1) bundle.
Pulse profiles from X-ray pulsars often exhibit strong energy dependence and both periodic and aperiodic variations with time. The great variety of profiles observed in various sources, and even from individual sources, makes it difficult to separate the numerous factors influencing the phase-dependence of the X-ray emission. These factors include the system geometry and particularly the photon energy and angle dependence of emission about the neutron star poles. Comptonisation may play an important role in determining beam patterns and hence pulse profiles in X-ray pulsars. A Monte Carlo simulation is used to investigate the beaming due to Comptonisation in a simple accretion column geometry. We apply the model to the extremely variable pulse profiles of the high-magnetic field pulsar GX 1+4.
We have analyzed the kinematical parameters of Cir X-1 to constrain the nature of its companion star, the eccentricity of the binary and the pre-supernova parameter space. We argue that the companion is most likely to be a low-mass (< 2.0 M_sun) unevolved star and that the eccentricity of the orbit is 0.94 +/- 0.04. We have evaluated the dynamical effects of the supernova explosion and we find it must have been asymmetric. On average, we find that a kick of 740 km/s is needed to account for the recently measured radial velocity of +430 km/s (Johnston, Fender & Wu) for this extreme system. The corresponding minimum kick velocity is 500 km/s. This is the largest kick needed to explain the motion of any observed binary system. If Cir X-1 is associated with the supernova remnant G321.9-0.3 then we find a limiting minimum age of this remnant of 60000 yr. Furthermore, we predict that the companion star has lost 10% of its mass as a result of stripping and ablation from the impact of the supernova shell shortly after the explosion.
We use the super version of RS construction method and Gauss decomposition to obtain Drinfeld's currents realization of super Yangian double $DY(gl(1|1))$ with center extension
Using super RS construction method and Gauss decomposition, we obtain Drinfeld's currents realization of two-parameter quantum affine superalgebra $U_{p,q}\widehat{(gl(1|1))}$ and get co-product structure for these currents.
We obtain Drinfel'd's realization of quantum affine superalgebra $U_q\hat{(gl(1|1))}$ based on the super version of RS construction method and Gauss decomposition.
We extend Yangian double to super (or graded) case and give its Drinfel'd generators realization by Gauss decomposition.
On the basis of graded RTT formalism,the defining relation of the super-Yangian Y(gl(1|1)) is derived and its oscillator realization is constructed.