3,918 research outputs found
Micro & strong lensing with the Square Kilometer Array: The mass--function of compact objects in high--redshift galaxies
We present the results from recent VLA 8.5-GHz and WSRT 1.4 and 4.9-GHz
monitoring campaigns of the CLASS gravitational lens B1600+434 and show how the
observed variations argue strongly in favor of microlensing by MACHOs in the
halo of a dark-matter dominated edge-on disk galaxy at z=0.4. The population of
flat-spectrum radio sources with micro-Jy flux-densities detected with the
Square-Kilometer-Array is expected to have dimensions of micro-arcsec. They
will therefore vary rapidly as a result of Galactic scintillation (diffractive
and refractive). However, when positioned behind distant galaxies they will
also show variations due to microlensing, even more strongly than in the case
of B1600+434. Relativistic or superluminal motion in these background sources
typically leads to temporal variations on time scales of days to weeks.
Scintillation and microlensing can be distinguished, and separated, by their
different characteristic time scales and the frequency dependence of their
modulations. Monitoring studies with Square-Kilometer-Array at GHz frequencies
will thus probe both microscopic and macroscopic properties of dark matter and
its mass-function as a function of redshift, information very hard to obtain by
any other method.Comment: 8 pages, 5 figures, to appear in Perspectives in Radio Astronomy:
Scientific Imperatives at cm and m Wavelengths (Dwingeloo: NFRA), Edited by:
M.P. van Haarlem & J.M. van der Huls
The application of NASTRAN at Sperry Univac Holland
Very divergent problems arising with different calculations indicate that NASTRAN is not always accessible for common use. Problems with engineering, modelling, and use of the program system are analysed and a way of solution is outlined. Related to this, some supplementary modifications are made at Sperry Univac Holland to facilitate the program for the less skilled user. The implementation of a new element also gives an insight into the use of NASTRAN at Sperry Univac Holland. As the users of Univac computers are from very different kinds of industries like shipbuilders, petrochemical industries, and building industries, the variety of problems coming from these users is very large. This variety results in experience not with one special kind of calculation nor one special kind of construction, but with a wide area of problems arising in the use of NASTRAN. These problems can roughly be divided into three different groups: (1) recognition of what is to be calculated and how, (2) construction of a model, and (3) handling the NASTRAN program. These are the basic problems for every less skilled user of NASTRAN and the Application/Research Department of Sperry Univac has to give reasonable answers to these questions
A time-delay determination from VLA light curves of the CLASS gravitational lens B1600+434
We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images
of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find
a nearly linear decrease of 18-19% in the flux densities of both lens images
over a period of eight months (February-October) in 1998. Additionally, the
brightest image A shows modulations up to 11% peak-to-peak on scales of days to
weeks over a large part of the observing period. Image B varies significantly
less on this time scale. We conclude that most of the short-term variability in
image A is not intrinsic source variability, but is most likely caused by
microlensing in the lens galaxy. The alternative, scintillation by the ionized
Galactic ISM, is shown to be implausible based on its strong opposite frequency
dependent behavior compared with results from multi-frequency WSRT monitoring
observations (Koopmans & de Bruyn 1999). From these VLA light curves we
determine a median time delay between the lens images of 47^{+5}_{-6} d (68%)
or 47^{+12}_{-9} d (95%). We use two different methods to derive the time
delay; both give the same result within the errors. We estimate an additional
systematic error between -8 and +7 d. If the mass distribution of lens galaxy
can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998),
this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11}
(95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and
Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give
H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For
Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED)Comment: 14 pages, 6 figures, accepted for publication in Astronomy &
Astrophysics (Figs 1 and 3 with degraded resolution
The efficacy of prolonged release oxycodone/naloxone for the treatment of Opioid Induced Constipation; from clinical trial to daily practice
Over the years opioid receptor antagonists like methylnaltrexone, naloxegol and naloxone are increasingly being used for the pathophysiological treatment of opioid induced constipation (OIC). The efficacy of laxative treatment and pathophysiological treatment with a prolonged release combination of oxycodone/naloxone (PR OXN) in daily practice is not clear.
With this thesis we aimed to gain insights in the efficacy PR OXN treatment for OIC, with a focus on patients with laxative refractory OIC. Moreover, since data are lacking on the efficacy of the current Dutch laxative regime a pilot study was performed aiming to obtain insights in the efficacy of the current Dutch laxative regime in daily practice.
Also, a systematic review was performed to gain insights on the current clinical and observational study evidence of the efficacy of peripherally acting mu-opioid receptor antagonists (PAMORAs) and PR OXN. Finally, a cost-utility analysis was performed for PR OXN use in patients with non-malignant moderate-to-severe pain, who are eligible for opioid treatment to obtain adequate analgesia and are experiencing laxative-refractory OIC
Hierarchical Bayesian inference of the Initial Mass Function in Composite Stellar Populations
The initial mass function (IMF) is a key ingredient in many studies of galaxy
formation and evolution. Although the IMF is often assumed to be universal,
there is continuing evidence that it is not universal. Spectroscopic studies
that derive the IMF of the unresolved stellar populations of a galaxy often
assume that this spectrum can be described by a single stellar population
(SSP). To alleviate these limitations, in this paper we have developed a unique
hierarchical Bayesian framework for modelling composite stellar populations
(CSPs). Within this framework we use a parameterized IMF prior to regulate a
direct inference of the IMF. We use this new framework to determine the number
of SSPs that is required to fit a set of realistic CSP mock spectra. The CSP
mock spectra that we use are based on semi-analytic models and have an IMF that
varies as a function of stellar velocity dispersion of the galaxy. Our results
suggest that using a single SSP biases the determination of the IMF slope to a
higher value than the true slope, although the trend with stellar velocity
dispersion is overall recovered. If we include more SSPs in the fit, the
Bayesian evidence increases significantly and the inferred IMF slopes of our
mock spectra converge, within the errors, to their true values. Most of the
bias is already removed by using two SSPs instead of one. We show that we can
reconstruct the variable IMF of our mock spectra for signal-to-noise ratios
exceeding 75.Comment: Accepted for publication in MNRAS, 16 pages, 8 figure
The Fundamental Surface of Quad Lenses
In a quadruply imaged lens system the angular distribution of images around
the lens center is completely described by three relative angles. We show
empirically that in the 3D space of these angles, spanning 180 x 180 x 90
degrees, quads from simple two-fold symmetric lenses of arbitrary radial
density profile and arbitrary radially dependent ellipticity or external shear
define a nearly invariant 2D surface. We give a fitting formula for the surface
using SIS+elliptical lensing potential. Various circularly symmetric mass
distributions with shear up to 0.4 deviate from it by typically, rms~0.1 deg,
while elliptical mass distributions with ellipticity of up 0.4 deviate from it
by rms~1.5 deg. The existence of a near invariant surface gives a new insight
into the lensing theory and provides a framework for studying quads. It also
allows one to gain information about the lens mass distribution from the image
positions alone, without any recourse to mass modeling. As an illustration, we
show that about 3/4 of observed galaxy-lens quads do not belong to this surface
within observational error, and so require additional external shear or
substructure to be modeled adequately.Comment: 24 pages, 9 figures, accepted to MNRA
Measuring gravitational lens time delays using low-resolution radio monitoring observations
Obtaining lensing time delay measurements requires long-term monitoring
campaigns with a high enough resolution (< 1 arcsec) to separate the multiple
images. In the radio, a limited number of high-resolution interferometer arrays
make these observations difficult to schedule. To overcome this problem, we
propose a technique for measuring gravitational time delays which relies on
monitoring the total flux density with low-resolution but high-sensitivity
radio telescopes to follow the variation of the brighter image. This is then
used to trigger high-resolution observations in optimal numbers which then
reveal the variation in the fainter image. We present simulations to assess the
efficiency of this method together with a pilot project observing radio lens
systems with the Westerbork Synthesis Radio Telescope (WSRT) to trigger Very
Large Array (VLA) observations. This new method is promising for measuring time
delays because it uses relatively small amounts of time on high-resolution
telescopes. This will be important because instruments that have high
sensitivity but limited resolution, together with an optimum usage of followup
high-resolution observations from appropriate radio telescopes may in the
future be useful for gravitational lensing time delay measurements by means of
this new method.Comment: 10 pages, 7 figures, accepted by MNRA
The very knotty lenser: Exploring the role of regularization in source and potential reconstructions using Gaussian process regression
Reconstructing lens potentials and lensed sources can easily become an underconstrained problem, even when the degrees of freedom are low, due to degeneracies, particularly when potential perturbations superimposed on a smooth lens are included. Regularization has traditionally been used to constrain the solutions where the data failed to do so, e.g. in unlensed parts of the source. In this exploratory work, we go beyond the usual choices of regularization and adopt observationally motivated priors for the source brightness. We also perform a similar comparison when reconstructing lens potential perturbations, which are assumed to be stationary, i.e. permeate the entire field of view. We find that physically motivated priors lead to lower residuals, avoid overfitting, and are decisively preferred within a Bayesian quantitative framework in all the examples considered. For the perturbations, choosing the wrong regularization can have a detrimental effect that even high-quality data cannot correct for, while using a purely smooth lens model can absorb them to a very high degree and lead to biased solutions. Finally, our new implementation of the semi-linear inversion technique provides the first quantitative framework for measuring degeneracies between the source and the potential perturbations
Testing Convolutional Neural Networks for finding strong gravitational lenses in KiDS
Convolutional Neural Networks (ConvNets) are one of the most promising
methods for identifying strong gravitational lens candidates in survey data. We
present two ConvNet lens-finders which we have trained with a dataset composed
of real galaxies from the Kilo Degree Survey (KiDS) and simulated lensed
sources. One ConvNet is trained with single \textit{r}-band galaxy images,
hence basing the classification mostly on the morphology. While the other
ConvNet is trained on \textit{g-r-i} composite images, relying mostly on
colours and morphology. We have tested the ConvNet lens-finders on a sample of
21789 Luminous Red Galaxies (LRGs) selected from KiDS and we have analyzed and
compared the results with our previous ConvNet lens-finder on the same sample.
The new lens-finders achieve a higher accuracy and completeness in identifying
gravitational lens candidates, especially the single-band ConvNet. Our analysis
indicates that this is mainly due to improved simulations of the lensed
sources. In particular, the single-band ConvNet can select a sample of lens
candidates with purity, retrieving 3 out of 4 of the confirmed
gravitational lenses in the LRG sample. With this particular setup and limited
human intervention, it will be possible to retrieve, in future surveys such as
Euclid, a sample of lenses exceeding in size the total number of currently
known gravitational lenses.Comment: 16 pages, 10 figures. Accepted for publication in MNRA
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