17,854 research outputs found
Qualitative Theory for Lensed QSOs
We show that some characteristics of multiply-imaged QSO systems are very
model-independent and can be deduced accurately by simply scrutinizing the
relative positions of images and galaxy-lens center. These include the
time-ordering of the images, the orientation of the lens potential, and the
rough morphology of any ring. Other features can differ considerably between
specific models; H_0 is an example. Surprisingly, properties inherited from a
circularly symmetric lens system are model-dependent, whereas features that
arise from the breaking of circular symmetry are model-independent. We first
develop these results from some abstract geometrical ideas, then illustrate
them for some well-known systems (the quads Q2237+030, H1413+117,
HST14113+5211, PG1115+080, MG0414+0534, B1608+656, B1422+231, and RXJ0911+0551,
and the ten-image system B1933+507), and finally remark on two systems
(B1359+154 and PMN J0134-0931) where the lens properties are more complex. We
also introduce a Java applet which produces simple lens systems, and helps
further illustrate the concepts.Comment: 26 pages, incl. 15 figs; accepted to AJ; java applet available at
http://ankh-morpork.maths.qmw.ac.uk/~saha/astron/lens
Pixelated Lenses and H_0 from Time-delay QSOs
Observed time delays between images of a lensed QSO lead to the determination
of the Hubble constant by Refsdal's method, provided the mass distribution in
the lensing galaxy is reasonably well known. Since the two or four QSO images
usually observed are woefully inadequate by themselves to provide a unique
reconstruction of the galaxy mass, most previous reconstructions have been
limited to simple parameterized models, which may lead to large systematic
errors in the derived H_0 by failing to consider enough possibilities for the
mass distribution of the lens. We use non-parametric modeling of galaxy lenses
to better explore physically plausible but not overly constrained galaxy mass
maps, all of which reproduce the lensing observables exactly, and derive the
corresponding distribution of H_0's. Blind tests - where one of us simulated
galaxy lenses, lensing observables, and a value for H_0, and the other applied
our modeling technique to estimate H_0 indicate that our procedure is reliable.
For four simulated lensed QSOs the distribution of inferred H_0 have an
uncertainty of \simeq 10% at 90% confidence. Application to published
observations of the two best constrained time-delay lenses, PG1115+080 and
B1608+656, yields H_0=61 +/- 11 km/s/Mpc at 68% confidence and 61 +/- 18
km/s/Mpc at 90% confidence.Comment: 27 pages, including 17 figs, LaTeX; accepted to A
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Suspension Design, Modeling, and Testing of a Thermo-Acoustic-Driven Linear Alternator
The Score-Stove™ generates electricity from a wood-burning cooking stove using a thermo-acoustic engine (TAE) that converts heat to sound through a linear alternator (LA). This paper introduces a prototype hemitoroidal suspension that was refined into a segmented trapezoidal shape that gave a higher cyclic life for the LA and includes a critical evaluation that compares a theoretical analysis with experimental results. The results show an improvement from the 40% efficiency of a standard loudspeaker used in reverse as an LA to 70–80% efficiency with the new suspension and a double Halbach array magnetic topology
New constraints on a triaxial model of the Galaxy
We determine the values of parameters of an N-body model for the Galaxy
developed by Fux via comparison with an unbiased, homogeneous sample of OH/IR
stars. Via Monte-Carlo simulation, we find the plausibilities of the
best-fitting models, as well as their errors. The parameters that are
constrained best by these projected data are the total mass of the model and
the viewing angle of the central Bar, although the distribution of the latter
has multiple maxima. The best model has a viewing angle of 44 degrees,
semi-major axis of 2.5 kpc, a bar mass of 1.7E10 solar masses and a tangential
velocity of the local standard of rest of 171 km/s . We argue that the lower
values that are commonly found from stellar data for the viewing angle (around
25 degrees) arise when too few coordinates are available, when the longitude
range is too narrow or when low latitudes are excluded from the fit. The new
constraints on the viewing angle of the galactic Bar from stellar line-of-sight
velocities decrease further the ability of the Bar's distribution to account
for the observed micro-lensing optical depth toward Baade's window : our model
reproduces only half the observed value. The signal of triaxiality diminishes
quickly with increasing latitude, fading within approximately one scaleheight.
This suggests that Baade's window is not a very appropriate region to sample
Bar properties.Comment: 10 pages, 8 figures, TeX, accepted for publication in MNRA
Non-parametric Reconstruction of Cluster Mass Distribution from Strong Lensing: Modelling Abell 370
We describe a new non-parametric technique for reconstructing the mass
distribution in galaxy clusters with strong lensing, i.e., from multiple images
of background galaxies. The observed positions and redshifts of the images are
considered as rigid constraints and through the lens (ray-trace) equation they
provide us with linear constraint equations. These constraints confine the mass
distribution to some allowed region, which is then found by linear programming.
Within this allowed region we study in detail the mass distribution with
minimum mass-to-light variation; also some others, such as the smoothest mass
distribution. The method is applied to the extensively studied cluster Abell
370, which hosts a giant luminous arc and several other multiply imaged
background galaxies. Our mass maps are constrained by the observed positions
and redshifts (spectroscopic or model-inferred by previous authors) of the
giant arc and multiple image systems. The reconstructed maps obtained for \a370
reveal a detailed mass distribution, with substructure quite different from the
light distribution. The method predicts the bimodal nature of the cluster and
that the projected mass distribution is indeed elongated along the axis defined
by the two dominant cD galaxies. But the peaks in the mass distribution appear
to be offset from the centres of the cDs. We also present an estimate for the
total mass of the central region of the cluster. This is in good agreement with
previous mass determinations. The total mass of the central region is
M=(2.0-2.7) 10^14 Msun/h50, depending on the solution chosen.Comment: 14 pages(19 postscript figures), minor corrections, MNRAS in pres
Radial density profiles of time-delay lensing galaxies
We present non-parametric radial mass profiles for ten QSO strong lensing
galaxies. Five of the galaxies have profiles close to ,
while the rest are closer to r^{-1}, consistent with an NFW profile. The former
are all relatively isolated early-types and dominated by their stellar light.
The latter --though the modeling code did not know this-- are either in
clusters, or have very high mass-to-light, suggesting dark-matter dominant
lenses (one is a actually pair of merging galaxies). The same models give
H_0^{-1} = 15.2_{-1.7}^{+2.5}\Gyr (H_0 = 64_{-9}^{+8} \legacy), consistent
with a previous determination. When tested on simulated lenses taken from a
cosmological hydrodynamical simulation, our modeling pipeline recovers both H_0
and within estimated uncertainties. Our result is contrary to some
recent claims that lensing time delays imply either a low H_0 or galaxy
profiles much steeper than r^{-2}. We diagnose these claims as resulting from
an invalid modeling approximation: that small deviations from a power-law
profile have a small effect on lensing time-delays. In fact, as we show using
using both perturbation theory and numerical computation from a
galaxy-formation simulation, a first-order perturbation of an isothermal lens
can produce a zeroth-order change in the time delays.Comment: Replaced with final version accepted for publication in ApJ; very
minor changes to text; high resolution figures may be obtained at
justinread.ne
Superconducting and ferromagnetic phases induced by lattice distortions in SrFe2As2
Single crystals of SrFe2As2 grown using a self-flux solution method were
characterized via x-ray, transport and magnetization studies, revealing a
superconducting phase below T_c = 21 K characterized by a full electrical
resistivity transition and partial diamagnetic screening. The reversible
destruction and reinstatement of this phase by heat treatment and mechanical
deformation studies, along with single-crystal X-ray diffraction measurements,
indicate that internal crystallographic strain originating from c-axis-oriented
planar defects plays a central role in promoting the appearance of
superconductivity under ambient pressure conditions in ~90% of as-grown
crystals. The appearance of a ferromagnetic moment with magnitude proportional
to the tunable superconducting volume fraction suggests that these phenomena
are both stabilized by lattice distortion.Comment: 4 pages, 4 figure
Look before you Hop: Conversational Question Answering over Knowledge Graphs Using Judicious Context Expansion
Fact-centric information needs are rarely one-shot; users typically ask follow-up questions to explore a topic. In such a conversational setting, the user's inputs are often incomplete, with entities or predicates left out, and ungrammatical phrases. This poses a huge challenge to question answering (QA) systems that typically rely on cues in full-fledged interrogative sentences. As a solution, we develop CONVEX: an unsupervised method that can answer incomplete questions over a knowledge graph (KG) by maintaining conversation context using entities and predicates seen so far and automatically inferring missing or ambiguous pieces for follow-up questions. The core of our method is a graph exploration algorithm that judiciously expands a frontier to find candidate answers for the current question. To evaluate CONVEX, we release ConvQuestions, a crowdsourced benchmark with 11,200 distinct conversations from five different domains. We show that CONVEX: (i) adds conversational support to any stand-alone QA system, and (ii) outperforms state-of-the-art baselines and question completion strategies
Study of Phase Stability in NiPt Systems
We have studied the problem of phase stability in NiPt alloy system. We have
used the augmented space recursion based on the TB-LMTO as the method for
studying the electronic structure of the alloys. In particular, we have used
the relativistic generalization of our earlier technique. We note that, in
order to predict the proper ground state structures and energetics, in addition
to relativistic effects, we have to take into account charge transfer effects
with precision.Comment: 22 pages, 7 figures. Accepted for publication in JPC
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