Flux monitoring at 327 MHz during SL9-Jupiter collision

Jupiter flux at 327 MHz was monitored using the Ooty radio telescope from July 12th to July 29th during the collision of comet Shoemaker-Levi 9 with Jupiter. Flux was found to increase steadily from July 17th to July 26th by ∼ 2–5 Jy, after which it declined to its pre-event value. The comparison of 327 MHz observations with those at 840 MHz and 2240 MHz indicates that the enhancement was mainly due to the increased synchrotron emission and the contribution of thermal emission was very small at metric-decimetric frequencies. The enhancement in radio emission was found to be more at 840 MHz than at 327 or 2240 MHz. The steepening of the spectrum between 327 and 840 MHz as well as between 2240 and 840 MHz was also noted.     

The Ooty synthesis radio telescope: First results

A 4-km synthesis radio telescope has recently been commissioned at Ootacamund, India for operation at 327 MHz. It consists of the Ooty Radio Telescope (530 m × 30 m) and 7 small antennas which are distributed over an area of about 4 km × 2 km. It has a coverage of about ± 40‡ in declination δ. The beam-width is about 40 arcsec × 90 arcsec at δ = 0‡ and about 40 arcsec × 50 arcsec at δ = 40‡. The sensitivity attained for a 5:1 signal-to-noise ratio is about 15 m Jy after a 10-hour integration. The observational programmes undertaken and some of the results obtained recently are summarized. The radio halo around the edge-on spiral NGC 4631 is found to have a larger scale-height at 327 MHz than is known at higher frequencies. Mapping of interesting radio galaxies at 327 MHz is being carried out; preliminary results for 0511-305 (∼2 Mpc) and 1333-337 (∼750 kpc) are summarized. The very-steep-spectrum radio source in the Abell cluster A85 is found to be resolved; since it has no obvious optical counterpart, it is conceivable that it is a remnant of past activity of a galaxy that has drifted away in about 10⁹ years.     

High-Redshift Radio Galaxies from Deep Fields

Most of the radio galaxies with z > 3 have been found using the red-shift spectral index correlation. We have started a programme with the Giant Metrewave Radio Telescope (GMRT) to exploit this correlation at flux density levels about 100 times deeper than the known high-redshift radio galaxies, with an aim to detect candidate high-redshift radio galaxies. Here we present results from the deep 150 MHz observations of LBDS-Lynx field, which has been imaged at 327, 610 and 1412 MHz with the Westerbork Synthesis Radio Telescope (WSRT) and at 1400 and 4860 MHz with the Very Large Array (VLA). We find about 150 radio sources with spectra steeper than 1. About two-thirds of these are not detected in Sloan Digital Sky Survey (SDSS), hence are strong candidate high-redshift radio galaxies, which need to be further explored with deep infra-red imaging and spectroscopy to estimate the red-shift.     

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.     

Solar radio pulsations at 410 MHz

On 6 September, 1982 very regular, narrow-band radio pulsations of solar origin were observed on the 410 MHz solar radiometer at the Learmonth Solar Observatory. Initial low-amplitude pulsations with a period of about 3 min gave way to large-amplitude pulsations with a period of about 5 min following a 1B solar flare. Position measurements at 327 MHz with the Culgoora Radioheliograph indicated two sources: a strong, extended source located above a unipolar magnetic region near the centre of the disk and a much weaker source near the west limb. Polarisation measurements indicate the burst to be plasma emission.The radio pulsations were unique in their association with both sympathetic radio emission and optical flares at widely different locations. Interpretation of the observations in terms of sausage mode standing oscillations in a coronal flux tube leads to an estimate of the magnetic flux density B = 45 G at the 400 MHz plasma level. Also a 2.8-fold density increase in the loop after the 1B flare is inferred.     

Observation of collision of comet Shoemaker-Levy 9 with Jupiter at the Astronomical and Geophysical Observatory in Modra

A report about observations of collision of the comet Shoemaker-Levy 9 with Jupiter is presented. The observations were made both in optical and radio wavelengths. Some consequences of impacts of individual fragments of the comet were detected visually in the 20-cm refractor. 21 days prolonged series of records on a video tape was taken in the focus of the 60-cm reflector, too. 13 serious events from July 16, 1994 to July 23, 1994 on 42.7 MHz radio wavelengths were registered. Its coherence with the pair Shoemaker-Levy 9 and Jupiter in the interaction is the subject of further investigation. More comprehensive results will be published after collecting information about similar observations on radio waves all over the world.     

Comet plasma densities deduced from refraction of occulted radio sources

Eighty MHz observations of the occultation of the radio source Culgoora-1 0300 + 16 by the plasma tail of Comet Kohoutek (1973f) were made in February/March 1974 with the Culgoora radioheliograph. No detectable source broadening or change in flux density was observed, but the results showed a 2' arc anomaly in the observed position. This is greater than can be attributed to ionospheric refraction or experimental error. We suggest that it arose from refraction in the plasma tail of the comet. Similar observations of the occulation of the radio source Culgoora-1 2313-14 by the plasma tail of Comet West (1975n) were made at Culgoora in February 1976. These results were inconclusive but did suggest that the cometary plasma may have had some influence on the observed source position. The results are used to derive, from simple models, the distribution of electron density in comet tails. Peak electron densities of approximately 2 to 5 × 10⁴ cm^?3 and density gradients of ～0.05 cm^?3 km^?1 are indicated.     

Search for effects of comet S-L 9 fragment impacts on low radio frequency emission from Jupiter

Decametric radio observations of Jupiter were made before, during, and after the impacts of the fragments of the comet S-L 9 with the planet, from the University of Florida Radio Observatory, the Maipu Radio Astronomy Observatory of the University of Chile, and the Owens Valley Radio Observatory of the California Institute of Technology. The decametric radiation was monitored at frequencies from 16.7 to 32 MHz. The minimum detectable flux densities were on the order of 30 kJy, except for that of the large 26.3 MHz array in Florida, which was about 1 kJy. There was no significant enhancement or suppression of the decametric L-burst or S-burst emission with respect to normal activity patterns that might be attributed to the fragment entries. However, a burst of left-hand elliptically polarized radiation having a considerably longer duration than an L-burst was observed almost simultaneously with the impact of the large fragment Q2, and another with right-hand elliptical polarization was observed simultaneously with Q1. We consider the possibility that these two bursts were emitted just above the local electron cyclotron frequencies from the southern and northern ends, respectively, of magnetic flux tubes that had been excited in some way by the proximity of fragments Q2 and Q1.In addition to the monitoring of the decametric radiation, a search was conducted for possible comet-enhanced Jovian synchrotron radiation at 45 MHz using a large dipole antenna array at the observatory in Chile. This frequency is above the cutoff of the decametric radiation, but is considerably below the lowest frequency at which the synchrotron emission has previously been detected. The minimum detectable flux density with the 45 MHz antenna was about 5 Jy. No synchrotron emission at all was found before, during, or after the entry of the comet fragments.     

A Hidden Radio Halo in the Galaxy Cluster A 1682?

High sensitivity observations of radio halos in galaxy clusters at frequencies ν ≤ 330 MHz are still relatively rare, and very little is known compared to the classical 1.4 GHz images. The few radio halos imaged down to 150–240 MHz show a considerable spread in size, morphology and spectral properties. All clusters belonging to the GMRT Radio Halo Survey with detected or candidate cluster-scale diffuse emission have been imaged at 325 MHz with the GMRT. Few of them were also observed with the GMRT at 240 MHz and 150 MHz. For A 1682, imaging is particularly challenging due to the presence of strong and extended radio galaxies at the center. Our data analysis suggests that thew radio galaxies are superposed to very low surface brightness radio emission extended on the cluster scale, which we present here.     

A review of radio observations of comets

The current status of cometary radio observations is reviewed. Radio continuum observations made at different wavelengths can be used to model the properties of cometary particles. Continuum observations have been successful for two comets but the interpretation of the data is subject to some disagreement. Radar observations are important for determining the size, angular momentum, direction of motion, and surface properties of the cometary nucleus. One comet, p/Encke, has been successfully observed by radar. The reasons why radio observations can fail are discussed. These include the undue influence of the highly volatile “comet frost” which often coats new comets, small errors in radio ephemerides, the inopportune scheduling of observing periods at less than optimum cometary heliocentric distances and velocities, and poor spectroscopic properties of the molecular transitions chosen for observations. In order to clarify the sometimes confusing observations which have been reported, cometary radio spectroscopy is reviewed in chronological order, comet by comet, starting from the earliest reported searches for polyatomic molecules in the early 1970s through progress in understanding cometary OH and into current searches for glycine, the simplest amino acid. The results of current OH ultraviolet pumping models are briefly discussed and several formalisms for computing molecular production rates arepresented. Radio observational programs which can aid in discriminating between current theories of terrestrial biological evolution are introduced. Both specific and general conclusions are drawn from the available material on cometary radio spectroscopy.     

A pre-launch low frequency VLBI survey for RadioAstron

Preliminary results from two low frequency Very Long Baseline Interferometry (VLBI) observations (EA009 and INTAS1) at 327 MHz are described. Sixty one sources were observed. Twenty five sources are recommended for further detailed investigation. Only two sources (3C345 and 3C309.1) could be mapped. These experiments are part of a long term pre-launch RadioAstron survey at 327 MHz that has two objectives: (i) to establish a suitable list of candidates for low frequency Space VLBI (SVLBI) observations and (ii) to study the limitations on SVLBI due to scattering effects in the interstellar and interplanetary medium.     

彗木碰撞期间3.6和12cm射电观测

本文介绍了1994年7月SL－9彗星撞击木星期间3．6cm和12cm射电连续谱观测的结果．在3．6cm波长上，撞击事件无明显的视效应，在12cm波长上，木星的射电平均视流量增加约20％．     

3.6/12 cm radio observation of Jupiter during the impact by Comet SL-9

We present 3.6/12 cm dual-frequency radio continuum observations of Jupiter with the Shanghai Observatory 25 m radio telescope during the impact by comet SL-9. There was no appreciable variation in the daily average flux density at 3.6 cm wavelength, but a relative increase of about 20% at 12 cm was observed during the impact.     

Magnetospheric source of UHF radio bursts at mid-latitudes during the magnetic storm on October 22, 1999

We discuss the results of our simultaneous observations of interplanetary and geomagnetic field fluctuations as well as solar wind parameters and meter radio emission in near-Earth space at mid-latitudes (near Kharkov) based on ground measurements before and during a unique magnetic storm on October 22, 1999. The electron flux dynamics in interplanetary space, geostationary orbit, and the magnetosphere is analyzed to find the interconnection with UHF radio background bursts at a frequency of 151 MHz. We conclude that the acceleration processes in the inner magnetospheric layers affect the generation processes of high-frequency radio bursts and that this phenomenon should be studied further using the SINP (MSU) instruments onboard the CORONAS-F satellite.     

Cluster-scale magnetic fields

The search for non thermal radio emission from clusters of galaxies is a powerful tool to investigate the existence of magnetic fields on such large scale. Unfortunately, such observations are scarce thus far, mainly because of the very faint large scale radio emission expected in clusters of galaxies. In the present contribution we will first review the status of the radio observations of clusters of galaxies, carried out with the aim of detecting large scale radio emission.We will then focus on the large scale radio emission detected at 327 MHz and 610 MHz in the Coma cluster of galaxies. The features of the detected radio emission suggest that a magnetic field with an intensity of the order of ~ 10^–7 Gauss must be present on a scale of about 2 Mpc (forH _o = 100km s ^–1 Mpc ^–1). The morphology of the radio emission is similar to that of the most recent X-ray images derived with ROSAT, and follows the distribution of the galaxies in the cluster. All these pieces of information will be taken into account in the discussion on the possible origin of this large scale magnetic field.     

Common origin for UV and radio fluctuations?

Brightness temperature fluctuations induced by a shock wave transit through the solar transition region are computed at several decimetric wavelengths. A simplified method previously used to reproduce the observed UV line intensity fluctuations is shown to give oscillation amplitudes which are consistent with experimental results. The detectability of shock-induced radio fluctuations is briefly discussed, to check the possibility of a common origin for the observed UV and radio phenomena.     

The topology and polarization of subbeams associated with the 'drifting' subpulse emission of pulsar B0943+10 – V. A new look at the low-frequency burst-mode emission

This paper reports new observations of pulsar B0943+10 carried out at the Pushchino Radio Astronomy Observatory (PRAO) at the low radio frequencies of 42, 62 and 112 MHz. B0943+10 is well known for its exquisitely regular burst-mode (B-mode) drifting subpulses as well as its weaker and chaotic quiescent mode. Earlier Arecibo investigations at 327 MHz have identified remarkable, continuous changes in its B-mode subpulse drift rate and integrated-profile shape with durations of several hours. These PRAO observations reveal that the changes in profile shape during the B-mode lifetime are strongly frequency dependent – namely the measured changes in the component amplitude ratio are more dramatic at 327 and 112 MHz as compared with those at 62 and 42 MHz. The differences, however, are most marked during the first several tens of minutes after B-mode onset; after an hour or so the profile shape changes tend to be more similar at all four frequencies. We also have found that the linear polarization of the integrated profile increases continuously throughout the lifetime of the B mode, going from hardly 10 per cent just after onset to some 40–50 per cent after several hours. Pulsar B0943+10's B mode thus provides a unique new opportunity to investigate continuous systematic changes in the plasma flow within the polar flux tube. While refraction in the pulsar's magnetosphere may well play some role, we find that the various frequency-dependent effects, both between and within the two modes, can largely be understood geometrically. If the modes and B-mode decay reflect systematic variations in the carousel-'spark' radius and emission height then a specific set of profile and linear polarization changes would be expected.     

The structure of the Cygnus loop at 34.5 MHz

We have observed the large supernova remnant Cygnus Loop at 34.5 MHz with the low frequency radio telescope at Gauribi-danur, India. A radio map of the region with a resolution of 26 arcmin × 40 arcmin (α × δ) is presented. The integrated flux density of the Cygnus Loop at this frequency is 1245 ± 195 Jy. The radio fluxes of different parts of the nebula at this frequency were also measured and used to construct their spectra. It is found that the spectrum of the region associated with the optical nebulosity NGC 6992/5 is not flat at low frequencies, and also exhibits a break at a frequency around 400 MHz. The spectrum of the region associated with NGC 6960 also shows a break but around 1000 MHz, while the spectrum of the region associated with NGC 6974 is straight in the entire frequency range 25 to 5000 MHz. The implication of these results on the basis of existing theories of the origin of radio emission from supernova remnants is discussed.     

Quiet Sun and slowly varying component at meter and decameter wavelengths

Comparison of maps of the Sun obtained over the period June 29 to July 8, 1982 at 169 MHz with the Nançay Radioheliograph and at 73.8, 50, and 30.9 MHz with the Clark Lake Radioheliograph shows that the slowly varying component at meter and decameter wavelengths is not always thermal emission. During the period under study weak noise storm continua were the most frequent sources of slowly varying component at 169 and 73.8 MHz. Most filaments show no radio counterpart on the disk. A streamer has been detected on the disk from 169 to 30.9 MHz with an optimum observability at 50 MHz. The brightest source of the slowly varying component from 73.8 to 30.9 MHz for most of the period was located above an extended coronal hole in a region where a depression was observed at 169 MHz. In favorable cases, electron densities can be derived from the positions of noise storms and radio streamers; these are in agreement with previous K-corona eclipse observations.     

IPS observations at stelab for the fruitful coming decade

We have been carrying out solar wind measurements using the interplanetary scintillation (IPS) method. Our IPS observation system is operated at a frequency of 327MHz and consists of four stations located at Toyokawa, Fuji, Sugadaira and Kiso. The present system, however, has insufficient sensitivity to measure enough IPS sources for observing the solar wind with adequate spatial and temporal resolution. Therefore we have been excuting the upgrade project since 1994 in order to observe a larger number of compact radio sources. The Fuji system has been improved successfully and has achieved sensitivity by a factor over five compared with the previous system. The upgrade project is now in progress for the Toyokawa and Sugadaira station.