605 research outputs found
Pinpointing the massive black hole in the Galactic Center with gravitationally lensed stars
A new statistical method for pinpointing the massive black hole (BH) in the
Galactic Center on the IR grid is presented and applied to astrometric IR
observations of stars close to the BH. This is of interest for measuring the IR
emission from the BH, in order to constrain accretion models; for solving the
orbits of stars near the BH, in order to measure the BH mass and to search for
general relativistic effects; and for detecting the fluctuations of the BH away
from the dynamical center of the stellar cluster, in order to study the stellar
potential. The BH lies on the line connecting the two images of any background
source it gravitationally lenses, and so the intersection of these lines fixes
its position. A combined search for a lensing signal and for the BH shows that
the most likely point of intersection coincides with the center of acceleration
of stars orbiting the BH. This statistical detection of lensing by the BH has a
random probability of ~0.01. It can be verified by deep IR stellar
spectroscopy, which will determine whether the most likely lensed image pair
candidates (listed here) have identical spectra.Comment: 4 pages, 2 figures, submitted to ApJ
Reconstruction of Stellar Orbits Close to Sagittarius A*: Possibilities for Testing General Relativity
We have reconstructed possible orbits for a collection of stars located
within 0.5 arcsec of Sgr A*. These orbits are constrained by observed stellar
positions and angular proper motions. The construction of such orbits serves as
a baseline from which to search for possible deviations due to the unseen mass
distribution in the central 1000 AU of the Galaxy. We also discuss the
likelihood that some of these stars may eventually exhibit detectable
relativistic effects, allowing for interesting tests of general relativity
around the 2.6 x 10^6 solar mass central object.Comment: 20 pages, 5 figures submitted to Astrophysical Journal, substantial
changes and additions based on referee's comment
Investigating Binary Properties with Next-Generation Microlensing Surveys
We explore the usefulness of future gravitational microlensing surveys in the
study of binary properties such as the binary fraction and the distributions of
binary separation and mass ratio by using the binary sample detectable through
a channel of repeating events. For this, we estimate the rate of repeating
microlensing eventstoward the Galactic bulge field based on standard models of
dynamical and physical distributions of Galactic matter combined with models of
binary separation and mass function. From this, we find that the total number
of repeating events expected to be detected from -year space-based
surveys will be --400, that is --50 times higher than the
rate of current surveys. We find that the high detection rate is due to the
greatly improved sensitivity to events associated with faint source stars and
low-magnification events. We find that the separation range of the binaries to
be covered by the repeating events will extend up to 100 AU. Therefore, the
future lensing surveys will provide a homogeneous sample that will allow to
investigate the statistical properties of Galactic binaries unbiased by
brightness of the binary components.Comment: total 6 pages, including 4 figures, ApJ, in pres
Characterizing the Cluster Lens Population
We present a detailed investigation into which properties of CDM halos make
them effective strong gravitational lenses. Strong lensing cross sections of
878 clusters from an N-body simulation are measured by ray tracing through
13,594 unique projections. We measure concentrations, axis ratios,
orientations, and the amount of substructure of each cluster, and compare the
lensing weighted distribution of each quantity to that of the cluster
population as a whole. The concentrations of lensing clusters are on average
34% larger than the typical cluster in the Universe. Despite this bias, the
anomalously high concentrations (c >14) recently measured by several groups,
appear to be inconsistent with the concentration distribution in our
simulations, which predict < 2% of lensing clusters should have concentrations
this high. No correlation is found between lensing cross section and the amount
of substructure. We introduce several types of simplified dark matter halos,
and use them to isolate which properties of CDM clusters make them effective
lenses. Projections of halo substructure onto small radii and the large scale
mass distribution of clusters do not significantly influence cross sections.
The abundance of giant arcs is primarily determined by the mass distribution
within an average overdensity of ~ 10,000. A multiple lens plane ray tracing
algorithm is used to show that projections of large scale structure increase
the giant arc abundance by a modest amount <7%. We revisit the question of
whether there is an excess of giant arcs behind high redshift clusters in the
RCS survey and find that the number of high redshift (z > 0.6) lenses is in
good agreement with LCDM, although our simulations predict more low redshift (z
< 0.6) lenses than were observed. (abridged)Comment: 19 pages, 15 figures. Submitted to Ap
A kinetic model of radiating electrons
A kinetic theory is developed to describe radiating electrons whose motion is governed by the Lorentz-Dirac equation. This gives rise to a generalized Vlasov equation coupled to an equation for the evolution of the physical submanifold of phase space. The pathological solutions of the 1-particle theory may be removed by expanding the latter equation in powers of τ ≔ q 2/6πm. The radiation-induced change in entropy is explored and its physical origin is discussed. As a simple demonstration of the theory, the radiative damping rate of longitudinal plasma waves is calculated
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