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We report the results of detailed observations of the Vela pulsar glitch which occurred on 24 December 1988. The period decrease occurred without warning and took place in much less than 2 minutes. The recovery, which commenced immediately, requires a complex function to model it, including three exponential terms and a damped sinusoid. An offset between data at two observing frequencies commenced at about the glitch epoch and continued for about 40 days; this is consistent with a small increase in dispersion measure or a change in the pulsar magnetic field configuration.
During the course of this colloquium many papers have been presented on observational aspects of pulsar astronomy. In the following discussion I have not attempted to be comprehensive but have selected a number of areas of interest to me.
The basic pulsar properties appear to be consistent over the full range of pulsar periods from 1 ms to 4s, implying that the emission mechanism is the same for all pulsars. There was a general consensus among the observers that the radio emission occurs low down in the pulsar's magnetosphere above the magnetic polar region.
The Southern Hemisphere VLBI Experiment (or SHEVE) was a joint US-Australian-South African venture with both astronomy and geodesy goals. The principle astronomy goal was to make models or maps of the following sources: at 2.3 GHz (with six antennas and 9 usable baselines) – Centaurus A (the nearest active galaxy), Circinus X-1 (a flaring binary), the VELA pulsar, and 26 other active galactic nuclei and quasars; at 8.4 GHz (only one baseline) – Centaurus A and the galactic center.
VLBI observations of the nucleus of Centaurus A were made in April, 1982 at two frequencies with an array of five Australian radio antennas as part of the Southern Hemisphere VLBI Experiment (SHEVE). Observations were undertaken at 2.29 GHz with all five antennas, while only two were operational at 8.42 GHz. The 2.29 GHz data yielded significant information on the structure of the nuclear jet. At 8.42 GHz a compact unresolved core was detected as well.
Six radio telescopes were operated as the first southern hemisphere VLBI array in April and May 1982. Observations were made at 2.3 and 8.4 Ghz. This array produced VLBI images of 28 southern hemisphere radio sources, high accuracy VLBI geodesy between southern hemisphere sites, and subarcsecond radio astrometry of celestial sources south of declination −45 degrees. This paper discusses only the astrophysical aspects of the experiment.
Carbon films were energetically deposited onto copper foil using the physical vapor deposition technique filtered cathodic vacuum arc. Raman spectroscopy and x-ray absorption spectroscopy showed that high quality graphene films of uniform thickness can be deposited onto copper foil at temperatures of 850 °C. The films can be prepared at high deposition rates (∼1 nm/min) and were comparable to graphene films grown at 1050 °C using chemical vapor deposition. This lower growth temperature was made possible by the energetic carbon flux which assisted the arrangement of carbon atoms into graphene layers on the Cu growth surface. Floating substrate potential was found to produce the highest quality graphene and the addition of hydrogen gas during film growth resulted in more defective films.
We have measured a value of 4±5m--2rad for the rotation measure of the radio pulsar PSR0529-66 in the LMC and, after allowing for the dispersion and rotation measures of our Galaxy on the pulsar's line of sight, we deduce that the magnetic field strength in the LMC is in the range 0 to 5μGauss oriented away from the Sun.
The Southern Hemisphere VLBI Experiment (SHEVE) program is aimed at producing high-resolution images of southern radio sources. The radio telescopes of the present SHEVE array are described below and some recent results presented.
Centaurus A (NGC 5128) is the nearest giant radio galaxy. It is a Fanaroff-Riley type 1 (low luminosity) radio source, but the compact radio source in the nucleus is strong enough that VLBI imaging has been possible with both the SHEVE array and the VLBA at several frequencies. These observations have detected a sub-parsec scale counterjet. This shows that jet formation in at least some FR I sources is intrinsically two-sided over very small distances and the radio jets in Centaurus A are probably only moderately relativistic. We also find evidence that the center of activity in Centaurus A is partially obscured by a disk or torus of dense plasma.
Two important factors for understanding the physical nature of compact steep spectrum (CSS) radio sources are determining the correct radio morphological classification of these objects together with their characteristics in wavebands different from the radio (Fanti et al. 1995, A&A, 302, 317). Seven CSS sources (linear dimensions < 30kpc for Ho = 50 kms–1Mpc–1 and α > 0.5, S ≃ v–α) have been found in a complete sample of strong southern radio sources. This group of CSS sources is particularly interesting because some optical and X-ray information is already available as part of a more general study of southern radio sources (Morganti et al. & Siebert et al. these Proceedings). The spectra of all the sources were presented in Tadhunter et al. (1993, MNRAS, 263, 999.) Here we present VLBI observations for three of these sources (0252-71, 1306-09 and 1814-63). The remaining four have already been imaged with VLBI (King et al. these Proceedings).
Centaurus A is the closest active extragalactic radio source, at a distance of approximately 3.5 Mpc, and is identified with the peculiar elliptical galaxy NGC 5128. As such it is a very important target for observations of the small-scale (sub-parsec) and large-scale (kpc) structures in extragalactic jets. Here we present Mk-II VLBI observations made at 8.4 GHz over a 4.3 year period from early 1991 until mid-1995, as well as a 4.8 GHz observation that was co-eval with one of the 8.4 GHz observations. All of the observations were made with the SHEVE array except for the last observation which was made with the VLBA. The dual-frequency observations identify the core of the radio source, while the multi-epoch observations show the complex structural evolution at a resolution of 0.1 pc. Subluminal motion of ≈ 0.15c is evident. Structural changes are observed on time scales shorter than four months.
Public agencies at all levels of government and other organizations that manage archaeological resources often face the problem of many undertakings that collectively impact large numbers of individually significant archaeological resources. Such situations arise when an agency is managing a large area, such as a national forest, land management district, park unit, wildlife refuge, or military installation. These situations also may arise in regard to large-scale development projects, such as energy developments, highways, reservoirs, transmission lines, and other major infrastructure projects that cover substantial areas. Over time, the accumulation of impacts from small-scale projects to individual archaeological resources may degrade landscape or regional-scale cultural phenomena. Typically, these impacts are mitigated at the site level without regard to how the impacts to individual resources affect the broader population of resources. Actions to mitigate impacts rarely are designed to do more than avoid resources or ensure some level of data recovery at single sites. Such mitigation activities are incapable of addressing research question at a landscape or regional scale.
The Llanherne telescope is a meridian transit instrument with an instantaneous bandwidth of two octaves in the range 35 to 150 MHz, and was constructed primarily for studying the low frequency properties of pulsars. The antenna is a 78 × 156 metre filled aperture phased array comprising 4096 wire dipole elements arranged in a 64 × 64 matrix. Uniform illumination of the elements produces a single pencil beam response which is scanned electronically along the meridian from Declination -90° to +30°. The beamwidth at 100 MHz is 1 degree in right ascension and two degrees sec(Dec.) in declination and varies proportionally with wavelength, giving a transit time of 4 sec(Dec) × 100/f(MHz) minutes between half power points.
Observations at a number of frequencies indicate that for at least two pulsars the average pulse shape has a slow but quite definite frequency dependence. Figure 1 shows average pulse shapes for CP 1919, CP 0950 and CP 1133. With the exception of those at 408 MHz these results were obtained at Parkes. The 408 MHz pulse shapes were obtained at Jodrell Bank by Lyne and Rickett. Circumstances of the observations are listed in Table I. Linearly polarized feeds were used at all frequencies.
Pulsar rotation measures have been used to investigate the structure of the local Galactic magnetic field. The Galactic field is found to be concentrated in the spiral arms and hence to be a spiral field. From the pulsars within 2kpc of the Sun, the field in the local spiral feature was modelled with Gaussian profiles in altitude and azimuth. In this model the field has a peak strength of 4.3 ± 0.2 μGauss directed towards Galactic longitude l = 73°±6°.
PKS 1830–211 is the strongest known radio gravitational lens by almost an order of magnitude and has the potential to provide a measurement of H0, provided the lensing system can be parameterized. Attempts to identify optical counterparts, to measure redshifts, have so far proved unsuccessful and this has lead to radio and millimetre spectral line observations. We present our discovery of an absorption system at z = 0.19. A brief description is also made of our ATCA observations to measure the lensing time delay for this source.
Daily timing observations of the Vela pulsar have been made over the last 14 years. During this time there has been seven large period jumps, or glitches, four of these have been observed as they occurred. We examine the time scale of these glitches and several other events which appear to be mini-glitches.
PKS 1934–638 is an archetypal GPS source, peaking at 1.4 GHz and exhibits almost no flux density variability. VLBI images at frequencies of .843, 2.3, 4.8, & 8.4 were made with the southern hemisphere VLBI array and they reveal that the source is a 42 mas compact double. There is no detectable change in separation over the last 20 years, yielding an upper limit of ~ 0.03c ± 0.2c on any expansion velocity. The spectral shapes of the two components are remarkably similar, despite indications of finer structure on longer baselines. Magnetic field calculations indicate fields of a few mGauss and the results are consistent with equipartition.