1,866 research outputs found
Minimal Absorption Measurements
We show that it is not possible to discriminate two close transparencies
without a certain number of photons being absorbed. We extend this to the
discrimination of patterns of transparency (images).Comment: 11 pages (latex
Atomic X-ray Spectroscopy of Accreting Black Holes
Current astrophysical research suggests that the most persistently luminous
objects in the Universe are powered by the flow of matter through accretion
disks onto black holes. Accretion disk systems are observed to emit copious
radiation across the electromagnetic spectrum, each energy band providing
access to rather distinct regimes of physical conditions and geometric scale.
X-ray emission probes the innermost regions of the accretion disk, where
relativistic effects prevail. While this has been known for decades, it also
has been acknowledged that inferring physical conditions in the relativistic
regime from the behavior of the X-ray continuum is problematic and not
satisfactorily constraining. With the discovery in the 1990s of iron X-ray
lines bearing signatures of relativistic distortion came the hope that such
emission would more firmly constrain models of disk accretion near black holes,
as well as provide observational criteria by which to test general relativity
in the strong field limit. Here we provide an introduction to this phenomenon.
While the presentation is intended to be primarily tutorial in nature, we aim
also to acquaint the reader with trends in current research. To achieve these
ends, we present the basic applications of general relativity that pertain to
X-ray spectroscopic observations of black hole accretion disk systems, focusing
on the Schwarzschild and Kerr solutions to the Einstein field equations. To
this we add treatments of the fundamental concepts associated with the
theoretical and modeling aspects of accretion disks, as well as relevant topics
from observational and theoretical X-ray spectroscopy.Comment: 63 pages, 21 figures, Einstein Centennial Review Article, Canadian
Journal of Physics, in pres
An Improved Upper Bound for the Ground State Energy of Fermion Lattice Models
We present an improved upper bound for the ground state energy of lattice
fermion models with sign problem. The bound can be computed by numerical
simulation of a recently proposed family of deformed Hamiltonians with no sign
problem. For one dimensional models, we expect the bound to be particularly
effective and practical extrapolation procedures are discussed. In particular,
in a model of spinless interacting fermions and in the Hubbard model at various
filling and Coulomb repulsion we show how such techniques can estimate ground
state energies and correlation function with great accuracy.Comment: 5 pages, 5 figures; to appear in Physical Review
Phase Separation Based on U(1) Slave-boson Functional Integral Approach to the t-J Model
We investigate the phase diagram of phase separation for the hole-doped two
dimensional system of antiferromagnetically correlated electrons based on the
U(1) slave-boson functional integral approach to the t-J model. We show that
the phase separation occurs for all values of J/t, that is, whether or with J, the Heisenberg coupling constant and t, the hopping
strength. This is consistent with other numerical studies of hole-doped two
dimensional antiferromagnets. The phase separation in the physically
interesting J region, is examined by introducing
hole-hole (holon-holon) repulsive interaction. We find from this study that
with high repulsive interaction between holes the phase separation boundary
tends to remain robust in this low region, while in the high J region, J/t
> 0.4, the phase separation boundary tends to disappear.Comment: 4 pages, 2 figures, submitted to Phys. Rev.
Magnetic Fields in the 3C 129 Cluster
We present multi-frequency VLA observations of the two radio galaxies 3C 129
and 3C 129.1 embedded in a luminous X-ray cluster. These radio observations
reveal a substantial difference in the Faraday Rotation Measures (RMs) toward
3C 129.1 at the cluster center and 3C 129 at the cluster periphery. After
deriving the density profile from available X-ray data, we find that the RM
structure of both radio galaxies can be fit by a tangled cluster magnetic field
with strength 6 microGauss extending at least 3 core radii (450 kpc) from the
cluster center. The magnetic field makes up a small contribution to the total
pressure (5%) in the central regions of the cluster. The radio morphology of 3C
129.1 appears disturbed on the southern side, perhaps by the higher pressure
environment. In contrast with earlier claims for the presence of a moderately
strong cooling flow in the 3C 129 cluster, our analysis of the X-ray data
places a limit on the mass deposition rate from any such flow of <1.2 Msun/yr.Comment: in press at MNRA
Issues and Observations on Applications of the Constrained-Path Monte Carlo Method to Many-Fermion Systems
We report several important observations that underscore the distinctions
between the constrained-path Monte Carlo method and the continuum and lattice
versions of the fixed-node method. The main distinctions stem from the
differences in the state space in which the random walk occurs and in the
manner in which the random walkers are constrained. One consequence is that in
the constrained-path method the so-called mixed estimator for the energy is not
an upper bound to the exact energy, as previously claimed. Several ways of
producing an energy upper bound are given, and relevant methodological aspects
are illustrated with simple examples.Comment: 28 pages, REVTEX, 5 ps figure
Fluctuating Elastic Rings: Statics and Dynamics
We study the effects of thermal fluctuations on elastic rings. Analytical
expressions are derived for correlation functions of Euler angles, mean square
distance between points on the ring contour, radius of gyration, and
probability distribution of writhe fluctuations. Since fluctuation amplitudes
diverge in the limit of vanishing twist rigidity, twist elasticity is essential
for the description of fluctuating rings. We find a crossover from a small
scale regime in which the filament behaves as a straight rod, to a large scale
regime in which spontaneous curvature is important and twist rigidity affects
the spatial configurations of the ring. The fluctuation-dissipation relation
between correlation functions of Euler angles and response functions, is used
to study the deformation of the ring by external forces. The effects of inertia
and dissipation on the relaxation of temporal correlations of writhe
fluctuations, are analyzed using Langevin dynamics.Comment: 43 pages, 9 Figure
The processing, properties and use of the pyrotechnic mixture-titanium subhydride/potassium perchlorate
Development of this pyrotechnic occurred because of the need for a static insensitive material to meet personnel safety requirements and related system safety issues in nuclear weapon energetic material component designs. Ti subhydride materials are made by the thermal dehydrding of commercial Ti hydride powder to the desired equivalent hydrogen composition in the Ti lattice. These Ti subhydrides, when blended with K perchlorate, meet the static insensitivity requirement of not being initiated from an equivalent human body electrostatic discharge. Individual material and blend qualification requirements provide a reproducible material from lot to lot. These pyrotechnic formulations meet the high reliability requirements (0.9995) for initiation and performance parameters and have the necessary stability and compatibility to meet long lived requirements of more than 25 years. Various experiences and problems are also discussed that have led to a mature technology for Ti subhydride/K perchlorate during its use in energetic material component designs
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