520 research outputs found
DIFFRACTION EFFECTS IN MICROLENSING OF Q2237+0305
Geometrical optics provides an excellent description for quasar images
crossing caustics which are formed by gravitational microlensing of objects
like Q2237+0305. Within this approximation the source size can be estimated
from the maximum magnification reached at caustic crossings. We evaluate the
limitations imposed by diffraction on caustics using the formalism developed by
Ulmer & Goodman (1995). Close to a caustic a new characteristic length, smaller
that the Fresnel length, enters the problem, limiting the angular resolution to
about 0.2 pico arcsecond, or equivalently about 3*10^9 cm at the source. To
achieve this resolution the brightness must be monitored at time intervals of a
few seconds. If a significant fraction of quasar luminosity comes from sources
smaller than those limits then interference effects would make the observed
intensity oscillate, in a close analogy with a two slit experiment. The
characteristic period of such oscillations is expected to be about one tenth of
a minute. If such oscillations are detected then photometry carried out at a
single site may permit the determination of the caustic transverse velocity,
and therefore may permit a direct conversion of the time units of brightness
variations to the linear units at the source. Subject headings: Gravitational
lensing - dark matter - quasars: structure -quasars: Q2237+0305Comment: 10 pages, plain TEX file, no figures. Submitted to ApJ postscript
available at by anonymous ftp at
ftp://astro.princeton.edu/library/preprint/pop615.ps.
Glass Transition in a Two-Dimensional Electron System in Silicon in a Parallel Magnetic Field
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional electron system (2DES) in Si in the vicinity of the
metal-insulator transition (MIT) persists in parallel magnetic fields B of up
to 9 T. At low B, both the glass transition density and , the
critical density for the MIT, increase with B such that the width of the
metallic glass phase () increases with B. At higher B, where the
2DES is spin polarized, and no longer depend on B. Our results
demonstrate that charge, as opposed to spin, degrees of freedom are responsible
for glassy ordering of the 2DES near the MIT.Comment: 4 pages, 5 figure
Determination of Stellar Ellipticities in Future Microlensing Surveys
We propose a method that can determine the ellipticities of source stars of
microlensing events produced by binary lenses. The method is based on the fact
that the products of the caustic-crossing timescale, , and the cosine
of the caustic incidence angle of the source trajectory, , of the
individual caustic crossings are different for events involving an elliptical
source, while the products are the same for events associated with a circular
source. The product corresponds to the
caustic-crossing timescale when the incidence angle of the source trajectory is
. For the unique determination of the source ellipticity, resolutions
of at least three caustic crossings are required. Although this requirement is
difficult to achieve under the current observational setup based on
alert/follow-up mode, it will be possible with the advent of future lensing
experiments that will survey wide fields continuously at high cadence. For
typical Galactic bulge events, the difference in between
caustic crossings is of the order of minutes depending on the source
orientations and ellipticities. Considering the monitoring frequency of the
future lensing surveys of times/hr and the improved photometry
especially of the proposed space-based survey, we predict that ellipticity
determinations by the proposed method will be possible for a significant
fraction of multiple caustic-crossing binary lens events involving source stars
having non-negligible ellipticities.Comment: 6 pages, 4 figures, ApJ, submitte
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