7,394 research outputs found
Extended Source Diffraction Effects Near Gravitational Lens Fold Caustics
Calculations are presented detailing the gravitational lens diffraction due
to the steep brightness gradient of the limb of a stellar source. The lensing
case studied is the fold caustic crossing. The limb diffraction signal greatly
exceeds that due to the disk as a whole and should be detectable for white
dwarf sources in our Galaxy and it's satellites with existing telescopes.
Detection of this diffraction signal would provide an additional mathematical
constraint, reducing the degeneracy among models of the lensing geometry. The
diffraction pattern provides pico-arcsecond resolution of the limb profile.Comment: 19 pages including 17 figures, Accepted for publication in ApJ, Minor
conceptual change from previous versio
Exotic Statistics for Ordinary Particles in Quantum Gravity
Objects exhibiting statistics other than the familiar Bose and Fermi ones are
natural in theories with topologically nontrivial objects including geons,
strings, and black holes. It is argued here from several viewpoints that the
statistics of ordinary particles with which we are already familiar are likely
to be modified due to quantum gravity effects. In particular, such
modifications are argued to be present in loop quantum gravity and in any
theory which represents spacetime in a fundamentally piecewise-linear fashion.
The appearance of unusual statistics may be a generic feature (such as the
deformed position-momentum uncertainty relations and the appearance of a
fundamental length scale) which are to be expected in any theory of quantum
gravity, and which could be testable.Comment: Awarded an honourable mention in the 2008 Gravity Research Foundation
Essay Competitio
A Characteristic Planetary Feature in Double-Peaked, High-Magnification Microlensing Events
A significant fraction of microlensing planets have been discovered in
high-magnification events, and a significant fraction of these events exhibit a
double-peak structure at their peak. However, very wide or very close binaries
can also produce double-peaked high-magnification events, with the same gross
properties as those produced by planets. Traditionally, distinguishing between
these two interpretations has relied upon detailed modeling, which is both
time-consuming and generally does not provide insight into the observable
properties that allow discrimination between these two classes of models. We
study the morphologies of these two classes of double-peaked high-magnification
events, and identify a simple diagnostic that can be used to immediately
distinguish between perturbations caused by planetary and binary companions,
without detailed modeling. This diagnostic is based on the difference in the
shape of the intra-peak region of the light curves. The shape is smooth and
concave for binary lensing, while it tends to be either boxy or convex for
planetary lensing. In planetary lensing this intra-peak morphology is due to
the small, weak cusp of the planetary central caustic located between the two
stronger cusps. We apply this diagnostic to five observed double-peaked
high-magnification events to infer their underlying nature. A corollary of our
study is that good coverage of the intra-peak region of double-peaked
high-magnification events is likely to be important for their unique
interpretation.Comment: 6 pages, 3 figure
The distribution of microlensed light curve derivatives: the relationship between stellar proper motions and transverse velocity
We present a method for computing the probability distribution of microlensed
light curve derivatives both in the case of a static lens with a transverse
velocity, and in the case of microlensing that is produced through stellar
proper motions. The distributions are closely related in form, and can be
considered equivalent after appropriate scaling of the input transverse
velocity. The comparison of the distributions in this manner provides a
consistent way to consider the relative contribution to microlensing (both
large and small fluctuations) of the two classes of motion, a problem that is
otherwise an extremely expensive computational exercise. We find that the
relative contribution of stellar proper motions to the microlensing rate is
independent of the mass function assumed for the microlenses, but is a function
of optical depth and shear. We find that stellar proper motions produce a
higher overall microlensing rate than a transverse velocity of the same
magnitude. This effect becomes more pronounced at higher optical depth. With
the introduction of shear, the relative rates of microlensing become dependent
on the direction of the transverse velocity. This may have important
consequences in the case of quadruply lensed quasars such as Q2237+0305, where
the alignment of the shear vector with the source trajectory varies between
images.Comment: 12 pages, including 9 figures. Submitted to M.N.R.A.S. Revised
version includes a short section on the applicability of the metho
Large-D Expansion from Variational Perturbation Theory
We derive recursively the perturbation series for the ground-state energy of
the D-dimensional anharmonic oscillator and resum it using variational
perturbation theory (VPT). From the exponentially fast converging approximants,
we extract the coefficients of the large-D expansion to higher orders. The
calculation effort is much smaller than in the standard field-theoretic
approach based on the Hubbard-Stratonovich transformation.Comment: Author Information under http://hbar.wustl.edu/~sbrandt and
http://www.theo-phys.uni-essen.de/tp/ags/pelster_di
Gravitational Microlensing Near Caustics I: Folds
We study the local behavior of gravitational lensing near fold catastrophes.
Using a generic form for the lensing map near a fold, we determine the
observable properties of the lensed images, focusing on the case when the
individual images are unresolved, i.e., microlensing. Allowing for images not
associated with the fold, we derive analytic expressions for the photometric
and astrometric behavior near a generic fold caustic. We show how this form
reduces to the more familiar linear caustic, which lenses a nearby source into
two images which have equal magnification, opposite parity, and are equidistant
from the critical curve. In this case, the simplicity and high degree of
symmetry allows for the derivation of semi-analytic expressions for the
photometric and astrometric deviations in the presence of finite sources with
arbitrary surface brightness profiles. We use our results to derive some basic
properties of astrometric microlensing near folds, in particular we predict for
finite sources with uniform and limb darkening profiles, the detailed shape of
the astrometric curve as the source crosses a fold. We find that the
astrometric effects of limb darkening will be difficult to detect with the
currently planned accuracy of the Space Interferometry Mission. We verify our
results by numerically calculating the expected astrometric shift for the
photometrically well-covered Galactic binary lensing event OGLE-1999-BUL-23,
finding excellent agreement with our analytic expressions. Our results can be
applied to any lensing system with fold caustics, including Galactic binary
lenses and quasar microlensing.Comment: 37 pages, 7 figures. Revised version includes an expanded discussion
of applications. Accepted to ApJ, to appear in the August 1, 2002 issue
(v574
Imaging a Quasar Accretion Disk with Microlensing
We show how analysis of a quasar high-magnification microlensing event may be
used to construct a map of the frequency-dependent surface brightness of the
quasar accretion disk. The same procedure also allows determination of the disk
inclination angle, the black hole mass (modulo the caustic velocity), and
possibly the black hole spin. This method depends on the validity of one
assumption: that the optical and ultraviolet continuum of the quasar is
produced on the surface of an azimuthally symmetric, flat equatorial disk,
whose gas follows prograde circular orbits in a Kerr spacetime (and plunges
inside the marginally stable orbit). Given this assumption, we advocate using a
variant of first-order linear regularization to invert multi-frequency
microlensing lightcurves to obtain the disk surface brightness as a function of
radius and frequency. The other parameters can be found by minimizing
chi-square in a fashion consistent with the regularized solution for the
surface brightness.
We present simulations for a disk model appropriate to the Einstein Cross
quasar, an object uniquely well-suited to this approach. These simulations
confirm that the surface brightness can be reconstructed quite well near its
peak, and that there are no systematic errors in determining the other model
parameters. We also discuss the observational requirements for successful
implementation of this technique.Comment: accepted to ApJ for publicatio
Engineering report (conceptual design) PFP solution stabilization
This Engineering Report (Conceptual Design) addresses remediation of the plutonium-bearing solutions currently in inventory at the Plutonium Finishing Plant (PFP). The recommendation from the Environmental Impact Statement (EIS) is that the solutions be treated thermally and stabilized as a solid for long term storage. For solutions which are not discardable, the baseline plan is to utilize a denitration process to stabilize the solutions prior to packaging for storage
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