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In 1983 ultra high energy γ-rays were detected from Cygnus X-3. This was of particular interest since it is the first identifiable source of cosmic rays. Since then, there has been an explosion of interest in the field with more than twenty air shower arrays planned or being used to search for γ-rays at energies above 1014 eV. Observational techniques for both very high energy and ultra high energy γ-ray astronomy are described and existing and proposed experiments reviewed with particular reference to their geographical locations. The results obtained so far are summarised together with some of the models proposed to explain them.
Abstract: The Australia Telescope is an advanced radio astronomy facility planned to satisfy the major research requirements of Australian scientists. The construction project, begun in 1983, is now well along the way to completion as an Australian Bicentennial activity. The civil works at the main Culgoora site are complete. The construction of the seven 22-m antennas has begun: the first will be handed over in November 1986, the last a year later. Most other areas of the project – feed construction, receiver construction, computer development etc. are on schedule.
On 14 March 1986 the European Space Agency’s Giotto spacecraft passed within 600 km of the nucleus of Halley’s comet. During the encounter a radio science experiment was conducted with the aim of determining the total amount of cometary material which impacted the probe. In this paper the theory and implementation of the experiment are discussed and a summary of experimental results is given.
The 1665 and 1667 MHz OH intensity towards Halley’s comet has been monitored during the period October 1985 to April 1986. The flux density variation during the course of the apparition roughly follows the predictions of Schloerb and Gerard (1985), although we find a systematically lower flux than they predicted. The relative intensities of these lines are approximately in the ratio expected for thermodynamic equilibrium.
Meteor rates have been measured with a large HF Radar at a number of frequencies. At the top end of the HF band our results match those of Greenhow (1963). However at lower frequencies we find high echo rates which indicate that past observations measured only a few percent of the total meteor flux incident on the Earth’s atmosphere. This explains the ‘missing mass’ discrepancy observed when radar results are compared with satellite or visual data. Accounting for’this missing mass results in a four-fold increase in the calculated total meteoroid mass influx to the surface of the Earth from 4000 to 16,000 tonnes per year. Our results also imply that the majority of echoes originate from altitudes above 100 km.
A complete orbital light-curve of V2051 Oph in the IR H band is presented, together with a second eclipse in the J band. Simultaneous Rc band data were obtained. Eclipse depths in Rc, J and H are 1.8 mag, 1.0 mag and 0.8 mag respectively. No evidence for ellipsoidal variations due to the secondary was seen and constraints on the secondary are discussed.
The star AB Dor (HD 36705) was first identified as an interesting object because of its strong Call H and K emission features (Bidelman and MacConnell 1973; Houk and Cowley 1975). It has a spectral class of G8 and appears to be a single star, since no radial velocity variations have thus far been detected, despite numerous attempts (e.g. Collier 1982; Innis et al. 1985a). Probably the most unusual and important feature about the star is its rapid rotational velocity, with a Vsini of 80 km s-1 (Collier 1982), which is more than 20 times that of a normal star of similar spectral class. AB Dor also shows a substantial photometric wave, commonly interpreted as indicating the presence of starspots. This wave has a typical amplitude of 0.05 to 0.15 magnitudes in V and a period of 0.514 days (e.g. Innis et al. 1985b). Combining this with the Vsini value gives a lower limit of 0.76 R⊙ for the stellar radius, while assuming the radius of a normal G8 dwarf yields an axial inclination of 60° ± 10°.
The single G8V active chromosphere star HD36705 (AB Dor) was observed at 8.4 GHz with the Parkes 64 m telescope during three observing sessions involving a total of 21 days in the interval 1985 December to 1986 February. Subsequent photometric observations were made of the star with the 0.25 m and 0.45 m telescopes of the Monash Observatory in 1986 March-April. Two strong radio flares, each lasting three days, were detected; they yielded peak radio powers of P8.4≈4×109 W Hz-1, comparable with the microwave power emitted by the RS CVn binaries. Significant circular polarization of 13% left-hand was measured on only one of the six active days. The 8.4 GHz flux density showed smooth variation over an interval of several hours, consistent with the flare source being partly occulted by the stellar disk as the star rotated. When all the radio data was phase-binned using the known rotation period of 0.514 day we found two radio maxima corresponding to radio sources at stellar longitudes ~180° apart. The subsequent photometric data showed intensity variations that were consistent with the starspots at the same approximate longitudes. We thus interpret our radio curve as showing the presence of comparatively small (<0.5 D*) radio sources in the corona above the star spots. The upper limit to source diameter gives a peak brightness temperature ≥2×l010 K, which can be achieved by gyro-synchrotron emission only if the source is optically thick and the electrons, with average energy ~ 2 MeV, have a hard energy spectrum. The observed radiation can be due only to very high harmonics of the gyro-frequency, leading to an estimate for the magnetic field strength of ~30G.
Properties of the microwave emission from HR1099 are examined in an attempt to determine whether the emission arises as gyro-synchrotron radiation from mildly relativistic electrons trapped in magnetic fields above starspots on the active K subgiant component. It is shown that radio curves do not exhibit a systematic variation in phase with the rotation rate, as one might expect for emission from a source situated above a long-lived starspot. However, there is some evidence that the radio flaring occurs at two preferred longitude zones. Whether these zones agree with starspot locations remains to be determined by light curve modelling. What we can say with confidence is that the measured spectral index of the microwave emission does not fit a simple gyro-synchrotron source model, such as that proposed to explain the observed reversal with frequency of the sense of circular polarization.
Known Southern flare stars and RSCVn-like variables are being observed with the Molonglo Observatory Synthesis Telescope in an attempt to detect quiescent (non-flaring) emission. Two flare stars out of 7 and one RSCVn out of 8 have been detected. Quiescent emission has not been observed previously from these sources at such a low frequency. All sources so far detected have mean flux densities below 10 mJy and in at least two of them the emission varies with a time scale of about one day.
Time dependent solutions of the nonlinear modal equations for cellular convection in a fluid layer heated below have demonstrated the existence of a nonlinear bifurcation which leads to a stable regime with reduced heat flux and vertical velocities. This new state is brought about by the growth, to a significant level, of the vertical component of vorticity after an initial quasi-steady state has been established. The growth rate mechanism has been investigated analytically and compared with the numerical results. These vorticity modified solutions exhibit favourable features Which could be established in the solar convection zone.
The five major members of the Sculptor Group of galaxies and NGC 45 have been observed with the Molonglo observatory synthesis telescope. NGC 247 and NGC 300 were not detected and upper limits to their 843 MHz flux densities are given. Radio emission from NGC 7793 is discussed with particular attention to its morphology, radial dependence compared with that of the blue light and possible coincidences between HII regions and 843 MHz peak flux densities.
As part of a program to investigate southern extragalactic radio sources, the Molonglo Reference Catalogue source 1452-517 has been observed with, the MOST and FST. It was found to have a compact core and broad lobes giving an angular size of around 20 arcmin. Despite the low galactic latitude (b = 6°) a probable identification has been made with a magnitude 18 galaxy indicating a physical size of over 1 Mpc.
Observations of the galactic background radio emission at a number of frequencies between 2.7 MHz and 1.07 MHz were made in association with the plasma depletion experiments of Spacelab-2 to test the concept of making low frequency radio observations through an artificial ionospheric window. Following the Shuttle OMS burn at a time when foF2 was 1.99 MHz, a decrease in the maximum ionospheric electron density of approximately 30% occurred. The first observations of the radio emission at high galactic latitudes with good angular resolution (25 degrees) were obtained during this event.
Propagation of 150 μs radio pulses (f= 1.91 MHz) along a magnetospheric field-aligned ionization duct have been observed, under conditions of low ionospheric critical frequency. The echoes, presumably reflected from the ionosphere at the magnetic conjugate point, were received at the transmitting site near Hobart. Positively identified echoes were observed on six separate occasions during periods of moderate solar activity, with delay times of approximately 260 ms, representing a value L = 2.95.
The Buckland Park air shower array is being developed particularly for use as an ultra-high-energy gamma ray astronomy telescope. The properties of this instrument are described with an emphasis on improvements being made to its angular resolution. Some early data are presented to illustrate the way in which the data obtained will be used.
Three mirrors of the White Cliffs Solar Power Station are currently being used for very high energy γ-ray Astronomy while the University of Adelaide very high energy γ-ray telescope is being designed. Use is made of fast-timing to obtain γ-ray arrival directions to an accuracy approaching 1 °. The experimental arrangement and operation of the telescope is described and our current observing program is outlined.
The Parkes radio telescope has been fitted with a new control system. Part of this system is a computer for transforming position requests into commands to the telescope drive. The high computing power coupled with a new control for the master control unit has allowed modes of telescope control not previously available. These new capabilities include scanning the telescope in any astronomical coordinate system, the definition and use of beams offset in azimuth and elevation from the telescope’s optical axis and the definition of arbitrary coordinate frames which may move with respect to standard reference frames. The new control system fully supports the new standard epoch of J2000.0 and uses the newly adopted constants for Earth axis precession and nutation.
Operation of the six 13.7 m antennas of the Fleurs synthesis telescope as a sub-array has provided a new and surprisingly versatile astronomical tool. With enhanced reliability and fully automated operation, unattended observing over several days is possible. Interleaved ‘multiple-snapshot’ observations of many fields per day can be made.
The array has shown itself to be particularly suitable for the measurement of precision (a few arcsecond) positions for the optical identification of a large number of radio sources, a survey of compact sources and the monitoring of the activity of several radio stars over periods of weeks. At present a program of recalibration is under way to improve the positional accuracy and dynamic range of the instrument.
The recent upgrading of the Tidbinbilla two-element interferometer to simultaneous S-band (2.3 GHz) and X-band (8.4 GHz) operation has provided a powerful new astronomical facility for weak radio source measurement in the Southern Hemisphere. The new X-band system has a minimum fringe spacing of 38 arcsec, and about the same positional measurement capability (approximately 2 arcsec) and sensitivity (1 s rms noise of 10 mJy) as the previous S-band system. However, the far lower confusion limit will allow detection and accurate positional measurements for sources as weak as a few millijanskys. This capability will be invaluable for observations of radio stars, X-ray sources and other weak, compact radio sources.