4,236 research outputs found
HEAO-A nominal scanning observation schedule
The HEAO-A observatory, scheduled for launch in late June 1977, will spend most of its orbital lifetime in a scanning mode, spining from 0.03 to 0.1 rpm about an axis aligned with the sun. The dates of availability in the scan band are given for a list of 248 X-ray sources. Celestial maps of source locations and scan planes, and examples of the nighttime elevation of available sources are presented. This document is intended to aid ground-based observers in planning coordinated observations with HEAO-A
Brownian Motion Model of Quantization Ambiguity and Universality in Chaotic Systems
We examine spectral equilibration of quantum chaotic spectra to universal
statistics, in the context of the Brownian motion model. Two competing time
scales, proportional and inversely proportional to the classical relaxation
time, jointly govern the equilibration process. Multiplicity of quantum systems
having the same semiclassical limit is not sufficient to obtain equilibration
of any spectral modes in two-dimensional systems, while in three-dimensional
systems equilibration for some spectral modes is possible if the classical
relaxation rate is slow. Connections are made with upper bounds on
semiclassical accuracy and with fidelity decay in the presence of a weak
perturbation.Comment: 13 pages, 6 figures, submitted to Phys Rev
The onset of the GeV afterglow of GRB 090510
We study the emission of the short/hard GRB 090510 at energies > 0.1 GeV as
observed by the Large Area Telescope (LAT) onboard the Fermi satellite. The GeV
flux rises in time as t^2 and decays as t^-1.5 up to 200 s. The peak of the
high energy flux is delayed by 0.2 s with respect to the main ~MeV pulse
detected by the Fermi Gamma Burst Monitor (GBM). Its energy spectrum is
consistent with F(E)=E^-1. The time behavior and the spectrum of the high
energy LAT flux are strong evidences of an afterglow origin. We then interpret
it as synchrotron radiation produced by the forward shock of a fireball having
a bulk Lorentz factor Gamma ~ 2000. The afterglow peak time is independent of
energy in the 0.1-30 GeV range and coincides with the arrival time of the
highest energy photon (~ 30 GeV). Since the flux detected by the GBM and the
LAT have different origins, the delay between these two components is not
entirely due to possible violation of the Lorentz invariance. It is the LAT
component by itself that allows to set a stringent lower limit on the
quantum-gravity mass of 4.7 times the Planck mass.Comment: 4 pages, 3 figures, submitted to ApJ Letter
A numerical and symbolical approximation of the Nonlinear Anderson Model
A modified perturbation theory in the strength of the nonlinear term is used
to solve the Nonlinear Schroedinger Equation with a random potential. It is
demonstrated that in some cases it is more efficient than other methods.
Moreover we obtain error estimates. This approach can be useful for the
solution of other nonlinear differential equations of physical relevance.Comment: 21 pages and 7 figure
Spin Diffusion in Double-Exchange Manganites
The theoretical study of spin diffusion in double-exchange magnets by means
of dynamical mean-field theory is presented. We demonstrate that the
spin-diffusion coefficient becomes independent of the Hund's coupling JH in the
range of parameters JH*S >> W >> T, W being the bandwidth, relevant to colossal
magnetoresistive manganites in the metallic part of their phase diagram. Our
study reveals a close correspondence as well as some counterintuitive
differences between the results on Bethe and hypercubic lattices. Our results
are in accord with neutron scattering data and with previous theoretical work
for high temperatures.Comment: 4.0 pages, 3 figures, RevTeX 4, replaced with the published versio
Massively Parallel Construction of Radix Tree Forests for the Efficient Sampling of Discrete Probability Distributions
We compare different methods for sampling from discrete probability
distributions and introduce a new algorithm which is especially efficient on
massively parallel processors, such as GPUs. The scheme preserves the
distribution properties of the input sequence, exposes constant time complexity
on the average, and significantly lowers the average number of operations for
certain distributions when sampling is performed in a parallel algorithm that
requires synchronization afterwards. Avoiding load balancing issues of na\"ive
approaches, a very efficient massively parallel construction algorithm for the
required auxiliary data structure is complemented
Neutron and X-ray diffraction study of cubic [111] field cooled Pb(Mg1/3Nb2/3)O3
Neutron and x-ray diffraction techniques have been used to study the
competing long and short-range polar order in the relaxor ferroelectric
Pb(MgNb)O (PMN) under a [111] applied electric field.
Despite reports of a structural transition from a cubic phase to a rhombohedral
phase for fields E 1.7 kV/cm, we find that the bulk unit cell remains cubic
(within a sensitivity of 90- =0.03)for fields up to
8 kV/cm. Furthermore, we observe a structural transition confined to the near
surface volume or `skin' of the crystal where the cubic cell is transformed to
a rhombohedral unit cell at T=210 K for E 4 kV/cm, for which
90-=0.08 0.03 below 50 K. While the bulk unit
cell remains cubic, a suppression of the diffuse scattering and concomitant
enhancement of the Bragg peak intensity is observed below T=210 K,
indicating a more ordered structure with increasing electric field yet an
absence of a long-range ferroelectric ground state in the bulk. The electric
field strength has little effect on the diffuse scattering above T,
however below T the diffuse scattering is reduced in intensity and adopts
an asymmetric lineshape in reciprocal space. The absence of hysteresis in our
neutron measurements (on the bulk) and the presence of two distinct temperature
scales suggests that the ground state of PMN is not a frozen glassy phase as
suggested by some theories but is better understood in terms of random fields
introduced through the presence of structural disorder. Based on these results,
we also suggest that PMN represents an extreme example of the two-length scale
problem, and that the presence of a distinct skin maybe necessary for a relaxor
ground state.Comment: 12 pages, 9 figure
Arnol'd Tongues and Quantum Accelerator Modes
The stable periodic orbits of an area-preserving map on the 2-torus, which is
formally a variant of the Standard Map, have been shown to explain the quantum
accelerator modes that were discovered in experiments with laser-cooled atoms.
We show that their parametric dependence exhibits Arnol'd-like tongues and
perform a perturbative analysis of such structures. We thus explain the
arithmetical organisation of the accelerator modes and discuss experimental
implications thereof.Comment: 20 pages, 6 encapsulated postscript figure
Gamma-ray burst spectroscopy capabilities of the BATSE/GRO experiment
A scintillation spectrometer is included in each of the eight BATSE/GRO detector modules, resulting in all-sky coverage for gamma-ray bursts. The scientific motivation, design and capabilities of these spectrometers for performing spectral observations over a wide range of gamma-ray energies and burst intensities are described
Quantum and classical chaos for a single trapped ion
In this paper we investigate the quantum and classical dynamics of a single
trapped ion subject to nonlinear kicks derived from a periodic sequence of
Guassian laser pulses. We show that the classical system exhibits diffusive
growth in the energy, or 'heating', while quantum mechanics suppresses this
heating. This system may be realized in current single trapped-ion experiments
with the addition of near-field optics to introduce tightly focussed laser
pulses into the trap.Comment: 8 pages, REVTEX, 8 figure
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