97 research outputs found
Astrometric Detection of Double Gravitational Microlensing Events
If a gravitational microlensing event is caused by a widely separated binary
lens and the source approaches both lens components, the source flux is
successively magnified by the individual lenses: double microlensing events. If
events are observed astrometrically, double lensing events are expected to
occur with an increased frequency due to the long range astrometric effect of
the companion. We find that although the trajectory of the source star image
centroid shifts of an astrometric double lensing event has a distorted shape
from both of the elliptical ones induced by the individual single lens
components, event duplication can be readily identified by the characteristic
loop in the trajectory formed during the source's passage close to the
companion. We determine and compare the probabilities of detecting double
lensing events from both photometric and astrometric lensing observations by
deriving analytic expressions for the relations between binary lensing
parameters to become double lensing events. From this determination, we find
that for a given set of the binary separation and the mass ratio the
astrometric probability is roughly an order higher than the photometric
probability. Therefore, we predict that a significant fraction of events that
will be followed up by using future high precision interferometeric instruments
will be identified as double lensing events.Comment: total 6 pages, including 4 figures and no table, ApJ, submitte
Radiative transfer in disc galaxies V. The accuracy of the KB approximation
We investigate the accuracy of an approximate radiative transfer technique
that was first proposed by Kylafis & Bahcall (hereafter the KB approximation)
and has been popular in modelling dusty late-type galaxies. We compare
realistic galaxy models calculated with the KB approximation with those of a
three-dimensional Monte Carlo radiative transfer code SKIRT. The SKIRT code
fully takes into account of the contribution of multiple scattering whereas the
KB approximation calculates only single scattered intensity and multiple
scattering components are approximated. We find that the KB approximation gives
fairly accurate results if optically thin, face-on galaxies are considered.
However, for highly inclined () and/or optically thick
(central face-on optical depth ) galaxy models, the approximation can
give rise to substantial errors, sometimes, up to . Moreover, it
is also found that the KB approximation is not always physical, sometimes
producing infinite intensities at lines of sight with high optical depth in
edge-on galaxy models. There is no "simple recipe" to correct the errors of the
KB approximation that is universally applicable to any galaxy models.
Therefore, it is recommended that the full radiative transfer calculation be
used, even though it's slower than the KB approximation.Comment: 10 pages, 6 figures, accepted for publication in MNRA
Photometric defocus observations of transiting extrasolar planets
We have carried out photometric follow-up observations of bright transiting
extrasolar planets using the CbNUOJ 0.6m telescope. We have tested the
possibility of obtaining high photometric precision by applying the telescope
defocus technique allowing the use of several hundred seconds in exposure time
for a single measurement. We demonstrate that this technique is capable of
obtaining a root-mean-square scatter of order sub-millimagnitude over several
hours for a V 10 host star typical for transiting planets detected from
ground-based survey facilities. We compare our results with transit
observations with the telescope operated in in-focus mode. High photometric
precision is obtained due to the collection of a larger amount of photons
resulting in a higher signal compared to other random and systematic noise
sources. Accurate telescope tracking is likely to further contribute to
lowering systematic noise by probing the same pixels on the CCD. Furthermore, a
longer exposure time helps reducing the effect of scintillation noise which
otherwise has a significant effect for small-aperture telescopes operated in
in-focus mode. Finally we present the results of modelling four light-curves
for which a root-mean-square scatter of 0.70 to 2.3 milli-magnitudes have been
achieved.Comment: 12 pages, 11 figures, 5 tables. Submitted to Journal of Astronomy and
Space Sciences (JASS
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