Vistas in Astronomy最新文献

The possibilities for radio wave diagnostics of interstellar flares in RS CVn and Algol type binary systems using both VLBI and single dish measurements are discussed. The model for interstellar flares based on the Ohm's generalised law and the circuit analog for a current-carrying filament between two stars of the binary system is proposed. It has been shown that intense “core” radio radiation can exist between two components. Filament oscillations can be a driver for the re-current flares and/or for the quasi-periodic variations of radio emission. The oscillation period depends on the kinetic temperature of a filament. We suggest that the coherent plasma radiation plays an important role in the flare radio emission. Interstellar flare radio emission is a good diagnostic tool for the plasma parameters in a flare.

In the past decades, clustering has been widely used in areas such as pattern recognition, data analysis, and image processing. Recently, clustering has been recognized as a useful method for knowledge discovery in spatial databases. To efficiently detect clusters from large spatial databases with a limited amount of available memory, special database techniques have been developed. In this article, we present a survey of these methods from a database perspective.

Radio supernovae are rare events among supernovae, usually taking place in distant galaxies. In recent years, great progress has been made in the study of radio supernovae with high angular resolution, due to advances in the VLBI technique and to the lucky occurrence of the powerful supernova SN 1993J in M81 only 3.6 Mpc distant. Its spatial structure has been unveiled and its expansion rate determined at 3.6 cm. Here we present new preliminary VLBI results on the expansion of this supernova at 6 and 13 cm. We resolve the shell at both wavelengths. The size estimates at 6 cm indicate a lower expansion rate than previously estimated. This may be the first evidence of deceleration.

Parsec-scale observations of quasars have shown that the magnetic fields along the jets do not always lie parallel to the jet direction but there are some cases of misalignment. Preliminary results from dual frequency polarization VLBI are presented for quasar 4C71.07 (0836+710). Our results show that Faraday rotation is not the origin of the misalignment. The magnetic field is found to be misaligned to the structural jet direction by a maximum of 28 ± 4°. The consequences for the magnetic field geometry and the interaction with the narrow line region are discussed.

Ground-based VLBI is a front-running technique in studying the Universe with the sharpest angular resolution. However, many astrophysical problems require even higher angular resolution and the only way to achieve this, for a given wavelength, is to create an interferometer with a baseline larger than the diameter of the Earth by placing at least one telescope in space. In this contribution the scientific rationale and technical aspects of the present and future generations of Space VLBI missions are briefly discussed.

We show how the total intensity data acquired with the European VLBI telescopes are affected by a systematic error, the physical origin of which lies in the instrumental polarization of the telescopes. The effect on the data can be modeled and therefore removed by data processing.

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.

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.

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.

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.