24,999 research outputs found
Optical counterparts of ROSAT X-ray sources in two selected fields at low vs. high Galactic latitudes
The optical identification of large number of X-ray sources such as those
from the ROSAT All-Sky Survey is challenging with conventional spectroscopic
follow-up observations. We investigate two ROSAT All-Sky Survey fields of size
10 * 10 degrees each, one at galactic latitude b = 83 deg (Com), the other at b
= -5 deg (Sge), in order to optically identify the majority of sources. We used
optical variability, among other more standard methods, as a means of
identifying a large number of ROSAT All-Sky Survey sources. All objects fainter
than about 12 mag and brighter than about 17 mag, in or near the error circle
of the ROSAT positions, were tested for optical variability on hundreds of
archival plates of the Sonneberg field patrol.
The present paper contains probable optical identifications of altogether 256
of the 370 ROSAT sources analysed. In particular, we found 126 AGN (some of
them may be misclassified CVs), 17 likely clusters of galaxies, 16 eruptive
double stars (mostly CVs), 43 chromospherically active stars, 65 stars brighter
than about 13 mag, 7 UV Cet stars, 3 semiregular resp. slow irregular variable
stars of late spectral type, 2 DA white dwarfs, 1 Am star, 1 supernova remnant
and 1 planetary nebula.
X-ray emission is, expectedly, tightly correlated with optical variability,
and thus our new method for optically identifying X-ray sources is demonstrated
to be feasible.Comment: 92 pages, 521 figures, A&A (accepted
Random Matrices and Chaos in Nuclear Physics: Nuclear Reactions
The application of random-matrix theory (RMT) to compound-nucleus (CN)
reactions is reviewed. An introduction into the basic concepts of nuclear
scattering theory is followed by a survey of phenomenological approaches to CN
scattering. The implementation of a random-matrix approach into scattering
theory leads to a statistical theory of CN reactions. Since RMT applies
generically to chaotic quantum systems, that theory is, at the same time, a
generic theory of quantum chaotic scattering. It uses a minimum of input
parameters (average S-matrix and mean level spacing of the CN). Predictions of
the theory are derived with the help of field-theoretical methods adapted from
condensed-matter physics and compared with those of phenomenological
approaches. Thorough tests of the theory are reviewed, as are applications in
nuclear physics, with special attention given to violation of symmetries
(isospin, parity) and time-reversal invariance.Comment: 50 pages, 26 figure
Semiclassical Theory of Chaotic Quantum Transport
We present a refined semiclassical approach to the Landauer conductance and
Kubo conductivity of clean chaotic mesoscopic systems. We demonstrate for
systems with uniformly hyperbolic dynamics that including off-diagonal
contributions to double sums over classical paths gives a weak-localization
correction in quantitative agreement with results from random matrix theory. We
further discuss the magnetic field dependence. This semiclassical treatment
accounts for current conservation.Comment: 4 pages, 1 figur
Design and fabrication of a low-specific-weight parabolic dish solar concentrator
A segmented design and fabrication and assembly techniques were developed for a 1.8 m (6 ft) diameter parabolic concentrator for space application. This design and these techniques were adaptable to a low cost, mass-produced concentrator. Minimal machining was required. Concentrator segments of formed magnesium were used. The concentrator weighed only 1.6 kg sq m (0.32 lbm/sq ft)
Prevalence of marginally unstable periodic orbits in chaotic billiards
The dynamics of chaotic billiards is significantly influenced by coexisting
regions of regular motion. Here we investigate the prevalence of a different
fundamental structure, which is formed by marginally unstable periodic orbits
and stands apart from the regular regions. We show that these structures both
{\it exist} and {\it strongly influence} the dynamics of locally perturbed
billiards, which include a large class of widely studied systems. We
demonstrate the impact of these structures in the quantum regime using
microwave experiments in annular billiards.Comment: 6 pages, 5 figure
Mechanical Mixing in Nonlinear Nanomechanical Resonators
Nanomechanical resonators, machined out of Silicon-on-Insulator wafers, are
operated in the nonlinear regime to investigate higher-order mechanical mixing
at radio frequencies, relevant to signal processing and nonlinear dynamics on
nanometer scales. Driven by two neighboring frequencies the resonators generate
rich power spectra exhibiting a multitude of satellite peaks. This nonlinear
response is studied and compared to -order perturbation theory and
nonperturbative numerical calculations.Comment: 5 pages, 7 figure
- …