151 research outputs found
A Method to Extract Potentials from the Temperature Dependence of Langmuir Constants for Clathrate-Hydrates
It is shown that the temperature dependence of Langmuir constants contains
all the information needed to determine spherically averaged intermolecular
potentials. An analytical ``inversion'' method based on the standard
statistical model of van der Waals and Platteeuw is presented which extracts
cell potentials directly from experimental data. The method is applied to
ethane and cyclopropane clathrate-hydrates, and the resulting potentials are
much simpler and more meaningful than those obtained by the usual method of
numerical fitting with Kihara potentials.Comment: 33 pages, 7 figures, to appear in Physica
Subtraction of Bright Point Sources from Synthesis Images of the Epoch of Reionization
Bright point sources associated with extragalactic AGN and radio galaxies are
an important foreground for low frequency radio experiments aimed at detecting
the redshifted 21cm emission from neutral hydrogen during the epoch of
reionization. The frequency dependence of the synthesized beam implies that the
sidelobes of these sources will move across the field of view as a function of
observing frequency, hence frustrating line-of-sight foreground subtraction
techniques. We describe a method for subtracting these point sources from dirty
maps produced by an instrument such as the MWA. This technique combines matched
filters with an iterative centroiding scheme to locate and characterize point
sources in the presence of a diffuse background. Simulations show that this
technique can improve the dynamic range of EOR maps by 2-3 orders of magnitude.Comment: 11 pages, 8 figures, 1 table, submitted to PAS
PMN J1632-0033: A new gravitationally lensed quasar
We report the discovery of a gravitationally lensed quasar resulting from our
survey for lenses in the southern sky. Radio images of PMN J1632-0033 with the
VLA and ATCA exhibit two compact, flat-spectrum components with separation
1.47" and flux density ratio 13.2. Images with the HST reveal the optical
counterparts to the radio components and also the lens galaxy. An optical
spectrum of the bright component, obtained with the first Magellan telescope,
reveals quasar emission lines at redshift 3.42. Deeper radio images with MERLIN
and the VLBA reveal a faint third radio component located near the center of
the lens galaxy, which is either a third image of the background quasar or
faint emission from the lens galaxy.Comment: 21 pp., including 4 figures; thoroughly revised in light of new
MERLIN/HST data; accepted for publication in A
The Sloan Digital Sky Survey Quasar Lens Search. I. Candidate Selection Algorithm
We present an algorithm for selecting an uniform sample of gravitationally
lensed quasar candidates from low-redshift (0.6<z<2.2) quasars brighter than
i=19.1 that have been spectroscopically identified in the SDSS. Our algorithm
uses morphological and color selections that are intended to identify small-
and large-separation lenses, respectively. Our selection algorithm only relies
on parameters that the SDSS standard image processing pipeline generates,
allowing easy and fast selection of lens candidates. The algorithm has been
tested against simulated SDSS images, which adopt distributions of field and
quasar parameters taken from the real SDSS data as input. Furthermore, we take
differential reddening into account. We find that our selection algorithm is
almost complete down to separations of 1'' and flux ratios of 10^-0.5. The
algorithm selects both double and quadruple lenses. At a separation of 2'',
doubles and quads are selected with similar completeness, and above (below) 2''
the selection of quads is better (worse) than for doubles. Our morphological
selection identifies a non-negligible fraction of single quasars: To remove
these we fit images of candidates with a model of two point sources and reject
those with unusually small image separations and/or large magnitude differences
between the two point sources. We estimate the efficiency of our selection
algorithm to be at least 8% at image separations smaller than 2'', comparable
to that of radio surveys. The efficiency declines as the image separation
increases, because of larger contamination from stars. We also present the
magnification factor of lensed images as a function of the image separation,
which is needed for accurate computation of magnification bias.Comment: 15 pages, 17 figures, 4 tables, accepted for publication in A
Determining the Lensing Fraction of SDSS Quasars: Methods and Results from the EDR
We present an algorithm for selecting gravitational lens candidates from
amongst Sloan Digital Sky Survey (SDSS) quasars. In median Early Data Release
(EDR) conditions, the algorithm allows for the recovery of pairs of equal flux
point sources down to separations of \sim 0{\farcs}7 or with flux ratios up
to 10:1 at a separation of 1\farcs5. The algorithm also recovers a
wide variety of plausible quad geometries. We also present a method for
determining the selection function of this algorithm through the use of
simulated SDSS images and introduce a method for calibrating our simulated
images through truth-testing with real SDSS data. Finally, we apply our
algorithm and selection function to SDSS quasars from the EDR to get an upper
bound on the lensing fraction. We find 13 candidates among 5120 z 0.6 SDSS
quasars, implying an observed lensing fraction of not more than 4 . There is one likely lens system in our final sample, implying an
observed lensing fraction of not less than (95% confidence
levels).Comment: Accepted for publication in The Astronomical Journal, 48 pages, 15
figure
SDSS J115517.35+634622.0: A Newly Discovered Gravitationally Lensed Quasar
We report the discovery of SDSSJ115517.35+634622.0, a previously unknown
gravitationally lensed quasar. The lens system exhibits two images of a quasar, with an image separation of 1{\farcs}832 \pm 0.007 . Near-IR
imaging of the system reveals the presence of the lensing galaxy between the
two quasar images. Based on absorption features seen in the Sloan Digital Sky
Survey (SDSS) spectrum, we determine a lens galaxy redshift of .
The lens is rather unusual in that one of the quasar images is only
0{\farcs}22\pm0{\farcs}07 () from the center of the
lens galaxy and photometric modeling indicates that this image is significantly
brighter than predicted by a SIS model. This system was discovered in the
course of an ongoing search for strongly lensed quasars in the dataset from the
SDSS.Comment: 18 pages, 6 figures. Accepted for publication in A
Discovering Gravitational Lenses Through Measurements Of Their Time Delays
We consider the possibility that future wide-field time-domain optical
imaging surveys may be able to discover gravitationally lensed quasar pairs
through serendipitous measurements of their time delays. We discuss the merits
such a discovery technique would have relative to conventional lens searches.
Using simulated quasar lightcurves, we demonstrate that in a survey which
observes objects several times each lunar cycle over the course of five years,
it is possible to improve the efficiency of a gravitational lens search by 2-3
orders of magnitude through the use of time delay selection. In the most
advantageous scenario considered, we are able to improve efficiency by a factor
of 1000 with no loss of completeness. In the least advantageous scenario, we
are able to improve efficiency by a factor of 110 while reducing completeness
by a factor of 9. We show that window function effects associated with the
length of the observing season are more important than the total number of
datapoints in determining the effectiveness of this method. We also
qualitatively discuss several complications which might be relevant to a real
time delay search.Comment: 22 pages, 7 figures. Submitted to Ap
Modelling and peeling extended sources with shapelets: a Fornax A case study
To make a power spectrum (PS) detection of the 21 cm signal from the Epoch of
Reionisation (EoR), one must avoid/subtract bright foreground sources. Sources
such as Fornax A present a modelling challenge due to spatial structures
spanning from arc seconds up to a degree. We compare modelling with multi-scale
(MS) CLEAN components to 'shapelets', an alternative set of basis functions. We
introduce a new image-based shapelet modelling package, SHAMFI. We also
introduce a new CUDA simulation code (WODEN) to generate point source,
Gaussian, and shapelet components into visibilities. We test performance by
modelling a simulation of Fornax A, peeling the model from simulated
visibilities, and producing a residual PS. We find the shapelet method
consistently subtracts large-angular-scale emission well, even when the
angular-resolution of the data is changed. We find that when increasing the
angular-resolution of the data, the MS CLEAN model worsens at large
angular-scales. When testing on real MWA data, the expected improvement is not
seen in real data because of the other dominating systematics still present.
Through further simulation we find the expected differences to be lower than
obtainable through current processing pipelines. We conclude shapelets are
worthwhile for subtracting extended galaxies, and may prove essential for an
EoR detection in the future, once other systematics have been addressed.Comment: 17 pages, 11 Figures, accepted for publication in Publications of the
Astronomical Society of Australia (18/05/2020). "For the SHAMFI code, see:
https://github.com/JLBLine/SHAMFI" . "For the SHAMFI documentation, see:
https://shamfi.readthedocs.io/" . "For the WODEN code and documentation see:
https://github.com/JLBLine/WODEN
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