Vistas in Astronomy最新文献

Object-based compression methods describe images in terms of a set of regions (a partition), and of some information for each region to be used by the receiver to reconstruct the image (its texture). Different techniques can be used to define the partition as well as for coding it and its texture. In this paper we propose a general multi-resolution segmentation algorithm which can deal with many different types of images selecting the appropriated criteria. We also review most outstanding techniques for coding in this context.

We discuss the mechanism of particle diffusive acceleration by shocks developed by non-linear fluid instabilities in collimated flows as the basis of a model to interpret the synchrotron brightness distribution and spectrum of extragalactic jets. Predictions for future VLBI observations are derived.

VLBI between sites with multiple antenna elements, fed from a common local oscillator, holds the prospect of solving for astrometric and geodetic parameters in a new way. The Cluster-Cluster observing mode allows simultaneous observations of multiple sources on the separate “sub-baselines” between sub-elements from one site to the other. The differential analysis benefits from the total cancelation of the temporal structure of the non-geometric contributions (e.g. instrumental terms and the propagation medium). We describe the current status of the analysis of observations in June 1995 using this technique with the WSRT and VLA arrays.

We present observations of the FRI radiogalaxy 3C66B during a joint EVN-MERLIN session in May 1995. The jet has been mapped from the 5 mas scale up to about 3 arcmin. The newly mapped VLBI jet shows two elongated blobs in the direction of the brightest kpc jet (NE). We interpret these two blobs in the light of a new model for VLBI jets, making use of the two-fluid concept.

In this paper we present a technique, based on multiresolution wavelet decomposition, for the merging and data fusion of two images. The images to merge are a high-resolution panchromatic image and a low-resolution multispectral image. The method consists of adding the wavelet coefficients of the high-resolution image to the multispectral (low-resolution) data. We applied the method to merge SPOT and LANDSAT (TM) images. The current data fusion methods may not be satisfying because they can distort the spectral characteristics of the multispectral data. The technique presented here is clearly better than the standard IHS and LHS mergers in preserving both spectral and spatial information. The fusion of reconstructed images and the application of the technique to astronomical images are commented on.

We present here preliminary results of 6 cm VLBI and VLA observations of LS I +61 ° 303 as well as 3.6 cm VLA observations of Cyg X-3. An interesting finding consists of the detection of small amplitude radio flares in the normal VLA interferometer data of LS I +61 ° 303, with an apparent recurrence period of 1.4 h. In addition, the VLA observations of Cyg X-3 exhibit some evidence of possible arcsecond radio jets in this source.

We have used the VLBA to conduct a monitoring campaign of the total intensity and polarization structure of the 43 GHz, v = 1, J = 1-0 SiO masers from three late-type stars. Our data reveal that rings and arcs of emission are the prevalent total intensity structure observed. Coherent structures are also evident in the linearly polarized emission. Dramatic structural variations are common: images taken 6 months apart often bear little relationship to each other. Based on circular polarization measurements we estimate the typical magnetic field strength at the position of the SiO masers to be ∼5–10 G.

The unique object SS433 is briefly reviewed with particular reference to the part played by the stellar wind ejected by the early type companion star. The aims of this observational campaign are described and the theory of the mass loss rate measurements explained. VLBA 1.6 GHz maps are presented as well as preliminary 23 GHz VLA and MERLIN maps. Mass loss rates of around 5 × 10⁻⁶ M⊙ yr⁻¹ are found, consistent with earlier results and the properties of Wolf-Rayet stars.

Many authors have estimated the anisotropies produced by one isolated cosmological non-linear inhomogeneity. This paper is an updated review about these estimates. The main methods used in order to deal with this problem are described. The limitations of these methods are analyzed. Results appear to be particularly interesting in the open non-linear case, in which a general treatment of the anisotropies produced by inhomogeneity distributions is very troublesome. The effects produced by very big structures such as the Great Attractor and the Boötes Void are studied in detail. Some generalities about the origin, detection and features of the Cosmic Microwave Background anisotropies are also presented for the sake of completeness.