22,187 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
Network Community Detection on Metric Space
Community detection in a complex network is an important problem of much
interest in recent years. In general, a community detection algorithm chooses
an objective function and captures the communities of the network by optimizing
the objective function, and then, one uses various heuristics to solve the
optimization problem to extract the interesting communities for the user. In
this article, we demonstrate the procedure to transform a graph into points of
a metric space and develop the methods of community detection with the help of
a metric defined for a pair of points. We have also studied and analyzed the
community structure of the network therein. The results obtained with our
approach are very competitive with most of the well-known algorithms in the
literature, and this is justified over the large collection of datasets. On the
other hand, it can be observed that time taken by our algorithm is quite less
compared to other methods and justifies the theoretical findings
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
A novel method of pure oxy-fuel circulating fluidized bed combustion with zero recirculation flue gas : experimental validation
Applying oxy-fuel combustion requires more advanced combustion control methods to avoid inadmissible high flam temperature. In fluidized beds and pulverized unites, enhanced heat transfer and recirculation flue gas are used. On other hand, higher oxygen concentration has pluses viz. better heat transfer, higher efficiency, compact setup and lower installation and operating costs. In pulverized power unites, pure oxy-fuel combustion is used with 100% O2 in the oxidant. In contrast, the highest experimental O2 % in oxy-fuel circulating fluidized bed (CFB) combustor is 70%. To the best of authors’ knowledge, there is no single CFB power plant operating under pure oxygen condition. In this work, we are aiming to use pure oxygen for oxy-CFB combustion, with new temperature controlling method for CFBs depending on combustion staging by fuel staging rather than using RFG. Fuel staging allows controlling combustion and varying SR. At the first stage, the used oxidant is 100% O2, and fuel is fed to achieve over SR (λ>1), where the excess oxidant absorbs heat and does not take a part in the reaction. The products of the first stage are reach of O2 and subsequently it is used as an oxidant for the second stage. For validation, a series of experiments are conducted using mini-CFB, and an oxidant of 100% O2 concentration is used with three SR ratios λ=1.25, 2.0, and 3.0. The resulted average temperatures along the riser for biomass are 1031°C, 950°C, and 798°C; and for coal 1129 °C, 1051 °C, and 961 °C respectively. The controlling of AFT with pure oxy-fuel combustion eliminates the recycled flue gas (RFG) in oxy-fuel CFB combustion and flue gas recirculation section; this simplifies the power plants’ design, fabrication and its installing-operating costs. Familiarising this concept can accelerate adapting oxy-fuel combustion in CFB power plant for Carbon Capturing and Sequestration (CCS). This contribution can commence and commercialise the third generation of oxy-fuel CFB combustion with zero recycled flue gas. Finally, the concept of controlling AFT by SR (λ) is validated experimentally
Interplay between topology and disorder in a two-dimensional semi-Dirac material
We investigate the role of disorder in a two-dimensional semi-Dirac material
characterized by a linear dispersion in one, and a parabolic dispersion in the
orthogonal, direction. Using the self-consistent Born approximation, we show
that disorder can drive a topological Lifshitz transition from an insulator to
a semi-metal, as it generates a momentum independent off-diagonal contribution
to the self-energy. Breaking time-reversal symmetry enriches the topological
phase diagram with three distinct regimes-- single-node trivial, two-node
trivial and two-node Chern. We find that disorder can drive topological
transitions from both the single- and two-node trivial to the two-node Chern
regime. We further analyze these transitions in an appropriate tight-binding
Hamiltonian of an anisotropic hexagonal lattice, by calculating the real-space
Chern number. Additionally we compute the disorder-averaged entanglement
entropy which signals both the topological Lifshitz and Chern transition as a
function of the anisotropy of the hexagonal lattice. Finally, we discuss
experimental aspects of our results.Comment: 8 pages, 9 figure
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