8,256 research outputs found
A new approach to multi-frequency synthesis in radio interferometry
We present a new approach to multi-frequency synthesis in radio astronomy.
Using Bayesian inference techniques, the new technique estimates the sky
brightness and the spectral index simultaneously. In principle, the bandwidth
of a wide-band observation can be fully exploited for sensitivity and
resolution, currently only limited by higher order effects like spectral
curvature. Employing this new approach, we further present a multi-frequency
extension to the imaging algorithm RESOLVE. In simulations, this new algorithm
outperforms current multi-frequency imaging techniques like MS-MF-CLEAN.Comment: 13 pages, 5 fugures, submitted to Astronomy and Astrophysic
Artifacts in incomplete data tomography - with applications to photoacoustic tomography and sonar
We develop a paradigm using microlocal analysis that allows one to
characterize the visible and added singularities in a broad range of incomplete
data tomography problems. We give precise characterizations for photo- and
thermoacoustic tomography and Sonar, and provide artifact reduction strategies.
In particular, our theorems show that it is better to arrange Sonar detectors
so that the boundary of the set of detectors does not have corners and is
smooth. To illustrate our results, we provide reconstructions from synthetic
spherical mean data as well as from experimental photoacoustic data
Towards an Intelligent Tutor for Mathematical Proofs
Computer-supported learning is an increasingly important form of study since
it allows for independent learning and individualized instruction. In this
paper, we discuss a novel approach to developing an intelligent tutoring system
for teaching textbook-style mathematical proofs. We characterize the
particularities of the domain and discuss common ITS design models. Our
approach is motivated by phenomena found in a corpus of tutorial dialogs that
were collected in a Wizard-of-Oz experiment. We show how an intelligent tutor
for textbook-style mathematical proofs can be built on top of an adapted
assertion-level proof assistant by reusing representations and proof search
strategies originally developed for automated and interactive theorem proving.
The resulting prototype was successfully evaluated on a corpus of tutorial
dialogs and yields good results.Comment: In Proceedings THedu'11, arXiv:1202.453
Solar total and spectral irradiance reconstruction over the last 9000 years
Changes in solar irradiance and in its spectral distribution are among the
main natural drivers of the climate on Earth. However, irradiance measurements
are only available for less than four decades, while assessment of solar
influence on Earth requires much longer records. The aim of this work is to
provide the most up-to-date physics-based reconstruction of the solar total and
spectral irradiance (TSI/SSI) over the last nine millennia. The concentrations
of the cosmogenic isotopes 14C and 10Be in natural archives have been converted
to decadally averaged sunspot numbers through a chain of physics-based models.
TSI and SSI are reconstructed with an updated SATIRE model. Reconstructions are
carried out for each isotope record separately, as well as for their composite.
We present the first ever SSI reconstruction over the last 9000 years from the
individual 14C and 10Be records as well as from their newest composite. The
reconstruction employs physics-based models to describe the involved processes
at each step of the procedure. Irradiance reconstructions based on two
different cosmogenic isotope records, those of 14C and 10Be, agree well with
each other in their long-term trends despite their different geochemical paths
in the atmosphere of Earth. Over the last 9000 years, the reconstructed secular
variability in TSI is of the order of 0.11%, or 1.5 W/m2. After the Maunder
minimum, the reconstruction from the cosmogenic isotopes is consistent with
that from the direct sunspot number observation. Furthermore, over the
nineteenth century, the agreement of irradiance reconstructions using isotope
records with the reconstruction from the sunspot number by Chatzistergos et al.
(2017) is better than that with the reconstruction from the WDC-SILSO series
(Clette et al. 2014), with a lower chi-square-value
The Uses of Argument in Mathematics
Stephen Toulmin once observed that `it has never been customary for
philosophers to pay much attention to the rhetoric of mathematical debate'.
Might the application of Toulmin's layout of arguments to mathematics remedy
this oversight?
Toulmin's critics fault the layout as requiring so much abstraction as to
permit incompatible reconstructions. Mathematical proofs may indeed be
represented by fundamentally distinct layouts. However, cases of genuine
conflict characteristically reflect an underlying disagreement about the nature
of the proof in question.Comment: 10 pages, 5 figures. To be presented at the Ontario Society for the
Study of Argumentation Conference, McMaster University, May 2005 and LOGICA
2005, Hejnice, Czech Republic, June 200
What determines large scale galaxy clustering: halo mass or local density?
Using dark matter simulations we show how halo bias is determined by local
density and not by halo mass. This is not totally surprising, as according to
the peak-background split model, local density is the property that constraints
bias at large scales. Massive haloes have a high clustering because they reside
in high density regions. Small haloes can be found in a wide range of
environments which determine their clustering amplitudes differently. This
contradicts the assumption of standard Halo Occupation Distribution (HOD)
models that the bias and occupation of haloes is determined solely by their
mass. We show that the bias of central galaxies from semi-analytic models of
galaxy formation as a function of luminosity and colour is not correctly
predicted by the standard HOD model. Using local density instead of halo mass
the HOD model correctly predicts galaxy bias. These results indicate the need
to include information about local density and not only mass in order to
correctly apply HOD analysis in these galaxy samples. This new model can be
readily applied to observations and has the advantage that the galaxy density
can be directly observed, in contrast with the dark matter halo mass.Comment: 11 pages, 9 figure
Lithium enrichment on the single active K1-giant DI Piscium -- Possible joint origin of differential rotation and Li enrichment
We investigate the surface spot activity of the rapidly rotating,
lithium-rich active single K-giant DI Psc to measure the surface differential
rotation and understand the mechanisms behind the Li-enrichment. Doppler
imaging was applied to recover the surface temperature distribution of DI Psc
in two subsequent rotational cycles using the individual mapping lines Ca I
6439, Fe I 6430, Fe I 6421 and Li I 6708. Surface differential rotation was
derived by cross-correlation of the subsequent maps. Difference maps are
produced to study the uniformity of Li-enrichment on the surface. These maps
are compared with the rotational modulation of the Li I 6708 line equivalent
width. Doppler images obtained for the Ca and Fe mapping lines agree well and
reveal strong polar spottedness, as well as cool features at lower latitudes.
Cross-correlating the consecutive maps yields antisolar differential rotation
with shear coefficient -0.083 +- 0.021. The difference of the average and the
Li maps indicates that the lithium abundance is non-activity related. There is
also a significant rotational modulation of the Li equivalent width.Comment: 8 pages, 7 figures, accepted in A&
- …