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.

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.

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.

On 27 July 1994 the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory detected an outburst of high energy X-rays from a previously unknown source in the constellation Scorpius. This source was designated GRO J1655-40. Approximately 12 days after the start of the X-ray outburst, a strong outburst of radio emission occurred. Here we present very long baseline interferometry (VLBI) observations of GRO J1655-40 made with an array of telescopes in Australia, South Africa, and the western United States. These observations show that the radio source which appeared two weeks after the initial X-ray outburst consisted of two prominent components which separated with an apparent speed of 1.5 ± 0.4c. When the various possibilities for the geometry of the radio source are taken into account the apparent speed implies an intrinsic speed between 0.5c and 0.9c. Our results and those of other investigators imply a strong link between the accretion of material onto a highly compact object and the ejection of relativistic components of radio emission.

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.

Image fusion techniques are increasingly applied to astronomical images in order to improve sensitivity or to compare images. We discuss in this paper the ability of multiscale transforms for many applications: geometrical and photometrical registrations, co-addition and compression. A strategy for multiscale image fusion is derived.

We analyse VLBI data of a sample of radio quasars in order to study the milliarcsecond scale structural properties of quasars in the widest available redshift range of 0.2 < z < 4.5. The statistical study is based on a combination of the 5 GHz Caltech-Jodrell VLBI survey and our data for a set of extremely high redshift sources. We show that the apparently less prominent milliarcsecond scale structures found in z > 3 quasars do not contradict the assumption of intrinsic similarity of radio sources at different redshifts and may be explained by a difference in spectral properties of cores and extended components (jets).

The discovery and use of the Saros, a lunar cycle of 18 years and 10 or 11 days, is reviewed from its earliest origins two millennia ago to the present day, when it is known with precision and enables the accurate prediction of both time and type of solar and lunar eclipses. The theoretical basis for the Saros is discussed, along with other historically known lunar cycles. The geometry of the Sun-Moon-Earth system is found to repeat itself after one Saros, not only at eclipses but also at any phase of the cycle, indicating that the Moon moves in a nearly periodic orbit. The search for periodic orbits using the Saros has led to the discovery of a set of eight periodic orbits of period equal to one Saros whose time evolutions closely resemble that of the real Moon. Finally, the potential of the Saros in studying the dynamics and stability of the Earth-Moon system is examined and the existence of other Saros-like cycles of longer periods in the present, past and future of the Earth-Moon-Sun system is explored.

By the 5th Century B.C. three great cultures were in contact in the Eastern Mediterranean area. The newest of these, the Greek, began by borrowing and improving on the discoveries of Babylonians and Egyptians. The knowledge of the gnomon is attested as one such borrowing. An overview is attempted of the state of timekeeping, with particular emphasis on the measure of shadows, in these two older cultures, revealing the uncertainties as to just what they had to offer the Greeks, and how they had come to their conclusions.