Vistas in Astronomy最新文献

We present results from simultaneous EVN and MERLIN observations of the FR 1 radio galaxy 3C 264 at 5 GHz. The combination of these two arrays allows us to obtain radio images with resolutions ranging from 4.6 to 118 mas. Moreover, we compare similar resolution images from MERLIN and the HST, compute the spectral index along the jet between optical and radio wavelengths, and confirm the synchrotron nature of the optical jet in 3C 264.

The observational evidence for jets and phenomena arising from rotation powered radio pulsars is reviewed, including many recent and exciting discoveries at X-ray wavelengths. The well studied jets of the Crab pulsar are summarised, including recent results from the HST. The evolutionary links between the known binary radio pulsars and jets sources in X-ray binaries are discussed.

We have used three antennas of the EVN (Effelsberg, Medicina and Jodrell Bank) to observe in W3(OH) and in five other star-forming regions, simultaneously, and for the first time, the two main lines of the ${}^2\text{φ}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, $\text{J}\text{=}\text{5}\text{2}$ excited state of OH with right and left circularly polarized feeds. The data were correlated with the MkIII MPIfR correlator in Bonn and processed with the AIPS package at the Observatoire de Bordeaux. In W3(OH) we made cleaned maps of all individual channels for each line and polarization. These maps made with a 5×6.5 mas beam reveal complex kinematics and spatial structure with both extended emission and unresolved features. This fact and polarization properties demon-strate the masing nature of the emission. Maser features are identified by searching for emission over adjacent channels, and adjacent positions (within about one synthesized beam) in both polarizations after we had mapped and selected one channel as a phase reference. We have been able to identify OH Zeeman pairs and to estimate the magnetic field strength across W3(OH); the field varies from about 1 to 10 mG. At the time of this conference, three other star-forming regions (M17, ON1 and W51) show fringes while two other regions are still incompletely processed.

We present a new image compression method, for very high quality lossy compression. This method caters for image data in regimes of (i) detector imperfections, which motivates a robust approach based on the median transform, and (ii) noise, which is explicitly sought and separated out, since noise is inherently non-compressible. An in-depth assessment is carried out on real data, relative to the standard JPEG compression method. Comparable visual quality is based on 260:1 compression with the new method, and 40:1 compression with JPEG. The assessment procedure, based on the astronomical images used, is an objective approach for determining very high quality visual reconstructions.

In this paper an axiomatic characterisation of feature subset selection is presented. Two axioms are presented: sufficiency axiom—preservation of learning information, and necessity axiom—minimising encoding length. The sufficiency axiom concerns the existing dataset and is derived based on the following understanding: any selected feature subset should be able to describe the training dataset without losing information, i.e. it is consistent with the training dataset. The necessity axiom concerns the predictability and is derived from Occam's razor, which states that the simplest among different alternatives is preferred for prediction. The two axioms are then restated in terms of relevance in a concise form: maximising both the r(X; Y) and r(Y; X) relevance. Based on the relevance characterisation, four feature subset selection algorithms are presented and analysed: one is exhaustive and the remaining three are heuristic. Experimentation is also presented and the results are encouraging. Comparison is also made with some well-known feature subset selection algorithms, in particular, with the built-in feature selection mechanism in C4.5.

High-precision photometric analysis of images of crowded stellar fields needs sophisticated algorithms. The photometric precision is, however, a strong function of the completeness of source detection. We discuss several aspects of this problem, both with relation to single- and multi-channel applications. In single images, we separate detection into two phases, source image enhancement and actual detection. Comparative tests show a point spread function pixel fitting technique to give the best results. For the fraction of undetectable stars still affecting image statistics, we have developed a technique to extract information about their effect on the faint-end luminosity function. We give two examples of multi-channel image fusion applications: (1) a combination of low- and high-resolution images and (2) removal of undersampling effects by sub-pixel image displacements. Preliminary results show considerable potential for these techniques.

Databases in the real world are often dynamic, incomplete, noisy and very large and most of the problems which appear in the Data Mining field are caused by these characteristics. However, it is often unnecessary to obtain results (rules, clusters, etc.) with a high degree of precision since the input data is itself imprecise. For these reasons, we propose a new approach to cope with these situations. We refer to the “Soft Computing” methodology which combines the ability of Fuzzy Logic to represent and manage imprecise data and knowledge together with the accepted capacities for learning and heuristic computation of Neural Networks and Genetic Algorithms. The aim of the paper is to present some of the possiblities offered by the use of Soft Computing.

We present new results from a radio mapping campaign on the X-ray binary (XRB), Cygnus X-3 using both MERLIN and the VLBA. This has produced two significant results. Firstly, observations using MERLIN at 5 GHz during a quiescent phase in June 1995 showed the presence of N and S radio lobes associated with a bright core. The position angle and separation of these lobes are consistent with the idea of a large scale jet in this source. Secondly, observations with the VLBA at 15 GHz during a period of small scale flaring have shown the source to be expanding at apparently superluminal velocities. This expansion was then followed by a period of apparent contraction with similar superluminal speeds.

With a time sampling of up to one observation per month, the analysis of geodetic VLBI data can reveal a detailed picture of the kinematics in numerous compact radio sources. The database comprises simultaneous observations obtained at two frequencies (8.4 and 2.3 GHz) which have been collected over a period of about 15 years. For our analysis of short-time structural variability in the jets of AGN we selected several sources, mainly on the basis of their flux density variability in the radio and, in case of correlated outbursts and intra-day variability, also in the optical regime. These sources are generally found to be active in the γ-ray regime as well. This fact enables us to study possible correlations between the structural properties seen at radio wavelengths and the flux-density behaviour in the γ-ray regime. The results for the γ-active quasar PKS 0528 + 134 are presented.

A short review of data fusion in remote sensing with emphasis on statistically based data fusion methods is given. The introductory part defines data fusion and image registration, and provides a historical background and some general references. Multivariate data are the necessary basis for any data fusion algorithm. The possible levels of data fusion and the frequent occurrence of various types of multivariate data in remote sensing are discussed. Finally, the paper presents a number of statistically based data fusion methods and discusses data fusion in the Bayesian framework.