915 research outputs found

### Parameter degeneracy in models of the quadruple lens system Q2237+0305

The geometry of the quadruple lens system Q2237+0305 is modeled with a simple
astigmatic lens: a power-law mass distribution: m \propto r^\beta with an
external shear \gamma. The image positions can be reproduced with an accuracy
better than 0.01 arcsec for any 0.0 < \beta < 1.85 and the corresponding value
of \gamma = 0.1385 - 0.0689 \beta. This is a factor of about 4 more precise
than what can be achieved by the best constant M/L lens models. The image
intensity ratios and the time delay ratios are almost constant along our one
parameter family of models, but the total magnification varies from 8 to >
1000, and the maximum time delay (between leading image B and trailing image C)
for H_0 of 75 km/sec/Mpc varies from more than 20 hours to about 1.5 hours,
while \beta increases from 0.0 to 1.85.Comment: 15 pages uuencoded compressed postscript text plus 4 postscript
figures; MPI Astrophysik report MPA-821; to appear in Astronomical Journa

### Probing The Gravity Induced Bias with Weak Lensing: Test of Analytical results Against Simulations

Future weak lensing surveys will directly probe the density fluctuation in
the universe. Recent studies have shown how the statistics of the weak lensing
convergence field is related to the statistics of collapsed objects. Extending
earlier analytical results on the probability distribution function of the
convergence field we show that the bias associated with the convergence field
can directly be related to the bias associated with the statistics of
underlying over-dense objects. This will provide us a direct method to study
the gravity induced bias in galaxy clustering. Based on our analytical results
which use the hierarchical {\em ansatz} for non-linear clustering, we study how
such a bias depends on the smoothing angle and the source red-shift. We compare
our analytical results against ray tracing experiments through N-body
simulations of four different realistic cosmological scenarios and found a very
good match. Our study shows that the bias in the convergence map strongly
depends on the background geometry and hence can help us in distinguishing
different cosmological models in addition to improving our understanding of the
gravity induced bias in galaxy clustering.Comment: 17 pages including 8 figures and 1 table, MNRAS, submitte

### The multiple quasar Q2237+0305 under a microlensing caustic

We use the high magnification event seen in the 1999 OGLE campaign light
curve of image C of the quadruply imaged gravitational lens Q2237+0305 to study
the structure of the quasar engine. We have obtained g'- and r'-band photometry
at the Apache Point Observatory 3.5m telescope where we find that the event has
a smaller amplitude in the r'-band than in the g'- and OGLE V-bands. By
comparing the light curves with microlensing simulations we obtain constraints
on the sizes of the quasar regions contributing to the g'- and r'-band flux.
Assuming that most of the surface mass density in the central kiloparsec of the
lensing galaxy is due to stars and by modeling the source with a Gaussian
profile, we obtain for the Gaussian width 1.20 x 10^15 sqrt(M/0.1M_sun)cm <
sigma_g' < 7.96 x 10^15 sqrt(M/0.1Msun) cm, where M is the mean microlensing
mass, and a ratio sigma_r'/sigma_g'=1.25^{+0.45}_{-0.15}. With the limits on
the velocity of the lensing galaxy from Gil-Merino et al. (2005) as our only
prior, we obtain 0.60 x 10^15 sqrt(M/0.1Msun) cm < sigma_g' < 1.57 x 10^15
sqrt(M/0.1Msun) cm and a ratio sigma_r'/sigma_g'=1.45^{+0.90}_{-0.25} (all
values at 68 percent confidence). Additionally, from our microlensing
simulations we find that, during the chromatic microlensing event observed, the
continuum emitting region of the quasar crossed a caustic at >72 percent
confidence.Comment: Accepted for publication in A&A, 8 pages, 4 figures. Slightly
modified compared to the original version: qualitative results unchanged,
constraints on the r'/g' source size ratio now tighter due to correction of
an error in the numerical treatment of the simulated light curve

### Detectability of extrasolar moons as gravitational microlenses

We evaluate gravitational lensing as a technique for the detection of
extrasolar moons. Since 2004 gravitational microlensing has been successfully
applied as a detection method for extrasolar planets. In principle, the method
is sensitive to masses as low as an Earth mass or even a fraction of it. Hence
it seems natural to investigate the microlensing effects of moons around
extrasolar planets. We explore the simplest conceivable triple lens system,
containing one star, one planet and one moon. From a microlensing point of
view, this system can be modelled as a particular triple with hierarchical mass
ratios very different from unity. Since the moon orbits the planet, the
planet-moon separation will be small compared to the distance between planet
and star. Such a configuration can lead to a complex interference of caustics.
We present detectability and detection limits by comparing triple-lens light
curves to best-fit binary light curves as caused by a double-lens system
consisting of host star and planet -- without moon. We simulate magnification
patterns covering a range of mass and separation values using the inverse ray
shooting technique. These patterns are processed by analysing a large number of
light curves and fitting a binary case to each of them. A chi-squared criterion
is used to quantify the detectability of the moon in a number of selected
triple-lens scenarios. The results of our simulations indicate that it is
feasible to discover extrasolar moons via gravitational microlensing through
frequent and highly precise monitoring of anomalous Galactic microlensing
events with dwarf source stars.Comment: 14 pages, 11 figures. Updated to A&A published version: updated
references, 1 additional illustration (Fig. 10), further analogies to solar
system and extended discussio

### Error Estimates for Measurements of Cosmic Shear

In the very near future, weak lensing surveys will map the projected density
of the universe in an unbiased way over large regions of the sky. In order to
interpret the results of studies it is helpful to develop an understanding of
the errors associated with quantities extracted from the observations. In a
generalization of one of our earlier works, we present estimators of the
cumulants and cumulant correlators of the weak lensing convergence field, and
compute the variance associated with these estimators. By restricting ourselves
to so-called compensated filters we are able to derive quite simple expressions
for the errors on these estimates. We also separate contributions from cosmic
variance, shot noise and intrinsic ellipticity of the source galaxies.Comment: 12 pages, including 5 figures, uses mn.sty. Substantially revised
version accepted by MNRA

### Expectations on the mass determination using astrometric microlensing by Gaia

Context. Astrometric gravitational microlensing can be used to determine the
mass of a single star (the lens) with an accuracy of a few percent. To do so,
precise measurements of the angular separations between lens and background
star with an accuracy below 1 milli-arcsecond at different epochs are needed.
Hence only the most accurate instruments can be used. However, since the
timescale is in the order of months to years, the astrometric deflection might
be detected by Gaia, even though each star is only observed on a low cadence.
Aims. We want to show how accurately Gaia can determine the mass of the lensing
star. Methods. Using conservative assumptions based on the results of the
second Gaia Data release, we simulated the individual Gaia measurements for 501
predicted astrometric microlensing events during the Gaia era (2014.5 -
2026.5). For this purpose we use the astrometric parameters of Gaia DR2, as
well as an approximative mass based on the absolute G magnitude. By fitting the
motion of lens and source simultaneously we then reconstruct the 11 parameters
of the lensing event. For lenses passing by multiple background sources, we
also fit the motion of all background sources and the lens simultaneously.
Using a Monte-Carlo simulation we determine the achievable precision of the
mass determination. Results. We find that Gaia can detect the astrometric
deflection for 114 events. Further, for 13 events Gaia can determine the mass
of the lens with a precision better than 15% and for 13 + 21 = 34 events with a
precision of 30% or better.Comment: 13 pages; 10 figures; 3 tables; accepted by A&A (April. 28th 2020)
The Python-based code for our simulation is made publicly available
https://github.com/jkluter/ML

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