1,789 research outputs found
Multi-Satellite Observations of Cygnus X-1 to Study the Focused Wind and Absorption Dips
High-mass X-ray binary systems are powered by the stellar wind of their donor
stars. The X-ray state of Cygnus X-1 is correlated with the properties of the
wind which defines the environment of mass accretion. Chandra-HETGS
observations close to orbital phase 0 allow for an analysis of the photoionzed
stellar wind at high resolution, but because of the strong variability due to
soft X-ray absorption dips, simultaneous multi-satellite observations are
required to track and understand the continuum, too. Besides an earlier joint
Chandra and RXTE observation, we present first results from a recent campaign
which represents the best broad-band spectrum of Cyg X-1 ever achieved: On 2008
April 18/19 we observed this source with XMM-Newton, Chandra, Suzaku, RXTE,
INTEGRAL, Swift, and AGILE in X- and gamma-rays, as well as with VLA in the
radio. After superior conjunction of the black hole, we detect soft X-ray
absorption dips likely due to clumps in the focused wind covering >95 % of the
X-ray source, with column densities likely to be of several 10^23 cm^-2, which
also affect photon energies above 20 keV via Compton scattering.Comment: 10 pages, contributed talk at the 7th Microquasar Workshop, Foca,
Turkey, Sept. 1-5, 200
Asymptotic analysis of a secondary bifurcation of the one-dimensional Ginzburg-Landau equations of superconductivity
The bifurcation of asymmetric superconducting solutions from the normal solution is considered for the one-dimensional Ginzburg--Landau equations by the methods of formal asymptotics. The behavior of the bifurcating branch depends on the parameters d, the size of the superconducting slab, and , the Ginzburg--Landau parameter. The secondary bifurcation in which the asymmetric solution branches reconnect with the symmetric solution branch is studied for values of for which it is close to the primary bifurcation from the normal state. These values of form a curve in the -plane, which is determined. At one point on this curve, called the quintuple point, the primary bifurcations switch from being subcritical to supercritical, requiring a separate analysis. The results answer some of the conjectures of [A. Aftalion and W. C. Troy, Phys. D, 132 (1999), pp. 214--232]
Entanglement-assisted quantum low-density parity-check codes
This article develops a general method for constructing entanglement-assisted quantum low-density parity-check (LDPC) codes, which is based on combinatorial design theory. Explicit constructions are given for entanglement-assisted quantum error-correcting codes with many desirable properties. These properties include the requirement of only one initial entanglement bit, high error-correction performance, high rates, and low decoding complexity. The proposed method produces several infinite families of codes with a wide variety of parameters and entanglement requirements. Our framework encompasses the previously known entanglement-assisted quantum LDPC codes having the best error-correction performance and many other codes with better block error rates in simulations over the depolarizing channel. We also determine important parameters of several well-known classes of quantum and classical LDPC codes for previously unsettled cases
Measurement of Length and Position with Frequency Combs
We show two different absolute distance measurement methods with micrometer accuracy based on frequency combs, and we discuss possible applications. Using a mode- locked laser and MEMS-based tracking optics, we measure the 3D position of a retroreflector within 10 ms and with a 24 μm volumetric accuracy. We also investigate modulator-based combs and show that they enable highly sensitive surface topography measurements with microsecond acquisition times and micrometer precision. Moreover, the potential for photonic integration of frequency comb sources is explored
Skipping orbits and enhanced resistivity in large-diameter InAs/GaSb antidot lattices
We investigated the magnetotransport properties of high-mobility InAs/GaSb
antidot lattices. In addition to the usual commensurability features at low
magnetic field we found a broad maximum of classical origin around 2.5 T. The
latter can be ascribed to a class of rosetta type orbits encircling a single
antidot. This is shown by both a simple transport calculation based on a
classical Kubo formula and an analysis of the Poincare surface of section at
different magnetic field values. At low temperatures we observe weak
1/B-periodic oscillations superimposed on the classical maximum.Comment: 4 pages, 4 Postscript figures, REVTeX, submitted to Phys Rev
Transition to turbulence in Hunt's flow in a moderate magnetic field
Pressure-driven magnetohydrodynamic duct flow in a transverse uniform magnetic field is studied by direct numerical simulation. The electric boundary conditions correspond to Hunt's flow with perfectly insulating walls parallel to the magnetic field (sidewalls) and perfectly conducting walls perpendicular to the magnetic field (Hartmann walls). The velocity distribution exhibits strong jets at the sidewalls, which are susceptible to instability even at low Reynolds numbers Re. We explore the onset of time-dependent flow and transition to states with evolved turbulence for a moderate Hartmann number . At low Re time-dependence appears in the form of elongated Ting-Walker vortices at the sidewalls of the duct, which, upon increasing Re, develop into more complex structures with higher energy and then the sidewall jets partially detach from the walls. At high values of Re jet detachments disappear and the flow consists of two turbulent jets and nearly laminar core. It is also demonstrated that, there is a range of Re, where Hunt's flow exhibits a pronounced hysteresis behavior, so that different unsteady states can be observed for the same flow parameters. In this range multiple states may develop and co-exist, depending on the initial conditions
Spatially resolved ultrafast precessional magnetization reversal
Spatially resolved measurements of quasi-ballistic precessional magnetic
switching in a microstructure are presented. Crossing current wires allow
detailed study of the precessional switching induced by coincident longitudinal
and transverse magnetic field pulses. Though the response is initially
spatially uniform, dephasing occurs leading to nonuniformity and transient
demagnetization. This nonuniformity comes in spite of a novel method for
suppression of end domains in remanence. The results have implications for the
reliability of ballistic precessional switching in magnetic devices.Comment: 17 pages (including 4 figures), submitted to Phys. Rev. Let
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