Low-frequency solar radiophysics with LOFAR and FASR

Low-frequency radio observations offer unique diagnostics of the solar corona and solar wind. After a prolongued hiatus, there is renewed interest in this important frequency regime. Two new ground-based instruments will provide critical new low-frequency observations: the low-frequency array (LOFAR) and the frequency agile solar radiotelescope (FASR). This brief topical review summarizes low-frequency radio phenomena that will be accessible to detailed study by LOFAR and FASR in the coming decade. Energy release, drivers of space weather, and studies of the solar wind are emphasized. Both instruments are expected to play important roles in both basic research problems and national and international space weather capabilities. While FASR is a solar-dedicated instrument, LOFAR is not. Solar observing requirements for LOFAR are briefly discussed.     

Phaseless VLBI mapping of compact extragalactic radio sources

The problem of phaseless aperture synthesis is of current interest in phase-unstable VLBI with a small number of elements when either the use of closure phases is not possible (a two-element interferometer) or their quality and number are not enough for acceptable image reconstruction by standard adaptive calibration methods. Therefore, we discuss the problem of unique image reconstruction only from the spectrum magnitude of a source. We suggest an efficient method for phaseless VLBI mapping of compact extragalactic radio sources. This method is based on the reconstruction of the spectrum magnitude for a source on the entire UV plane from the measured visibility magnitude on a limited set of points and the reconstruction of the sought-for image of the source by Fienup's method from the spectrum magnitude reconstructed at the first stage. We present the results of our mapping of the extragalactic radio source 2200+420 using astrometric and geodetic observations on a global VLBI array. Particular attention is given to studying the capabilities of a two-element interferometer in connection with the putting into operation of a Russian-made radio interferometer based on Quasar RT-32 radio telescopes.     

On the Dynamic Disconnection of Rising Omega Loops

The tilt-angle variation of solar bipolar magnetic regions (BMRs) in space and time is a probable indicator of large-scale properties of magnetic field distribution, the dynamo and other different processes affecting the rising of magnetic flux tubes. We analyze the tilt-angle distribution and evolution of clearly oriented BMRs using sunspot position and area measurements from the Greenwich Photo-Heliographic Results (GPHR) supplemented by high-quality full-disk sunspot drawings made in the Haynald Observatory in Hungary. Our work is made on the basis of the 14th sunspot cycle and we compare our results with present theories of magnetic flux evolution in the convective zone. Our results support the assumption that the scatter of tilt angles of BMRs around Joy`s law is determined by the convective turbulence. Furthermore, we find that regular BMRs grouped by age generally do not show the phenomenon of toroidal relaxation towards the east–west direction. We interpret this by the disconnection of -loops from the bottom of the convection zone.     

Dynamics of Two Planets in the 2/1 Mean-Motion Resonance

The dynamics of two planets near a first-order mean-motion resonance is modeled in the domain of the general three-body planar problem. The system studied is the pair Uranus-Neptune (2/1 resonance). The phase space of the resonance and near-resonance regions is studied by means of surfaces of section and spectral analysis techniques. After a thorough investigation of the topology of the phase space, we find that several regimes of motion are possible for the Uranus-Neptune system, and the regions of transition between the regimes of motion are the seats of chaotic motion. This revised version was published online in July 2006 with corrections to the Cover Date.     

B0712+472: a new radio four-image gravitational lens

A new four-image gravitational lens system, B0712+472, has been discovered during the Cosmic Lens All-Sky Survey. This system consists of four flat-spectrum radio images that are also seen on a Hubble Space Telescope ( HST ) image, together with the lensing galaxy. We present MERLIN, VLA and VLBA maps and WHT spectra of the system as well as the HST images. The light distribution of the lensing galaxy is highly elongated and so too is the mass distribution deduced from modelling. We suggest a redshift of ∼1.33 for the lensed object; the lens redshift will require further investigation. The discovery of this new system further increases the ratio of four-image to two-image lens systems currently known, exacerbating problems of required ellipticity of matter distributions in lensing galaxies.     

Gamma-ray bursts: post-burst evolution of fireballs

The post-burst evolution of fireballs that produce γ-ray bursts (GRBs) is studied, assuming the expansion of fireballs to be adiabatic and relativistic. Numerical results as well as an approximate analytic solution for the evolution are presented. Owing to the adoption of a new relation between t R and γ, our results differ markedly from previous studies. Synchrotron radiation from the shocked interstellar medium is carefully calculated, using a conventional set of equations. The observed X-ray flux of GRB afterglows can be reproduced easily. Although the optical afterglows seem much more complicated, our results can still present a rather satisfactory agreement with observations. We also find that the expansion will no longer be highly relativistic about 4 d after the main GRB. We thus suggest that the marginally relativistic phase of the expansion should be investigated so as to check the afterglows observed a week or more later.     

Radio properties of the Shapley Concentration — II. J1324–3138: a remnant of a radio galaxy in the Abell cluster A3556?

In this paper we present a detailed study of the radio galaxy J1324–3138, located at a projected distance of 2 arcmin from the centre of the Abell cluster of galaxies A3556, belonging to the core of the Shapley Concentration, at an average redshift of z  = 0.05. We have observed J1324–3138 over a wide range of frequencies: at 327 MHz (VLA), 843 MHz (MOST), and at 1376, 2382, 4790 and 8640 MHz (ATCA).   Our analysis suggests that J1324–3138 is a remnant of a tailed radio galaxy, in which the nuclear engine has switched off and the radio source is now at a late stage of its evolution, confined by the intracluster gas. The radio galaxy is not in pressure equilibrium with the external medium, as is often found for extended radio sources in clusters of galaxies. We favour the hypothesis that the lack of observed polarized radio emission in the source is a result of Faraday rotation by a foreground screen, i.e. the source is seen through a dense cluster gas, characterized by a random magnetic field.   An implication of the head–tail nature of the source is that J1324–3138 is moving away from the core of A3556 and that, possibly, a major merging event between the core of A3556 and the subgroup hosting J1324–3138 has already taken place.