Engineering topics
Publications and source records attributed to Jones, D. L..
We present VLBI and Australia Telescope Compact Array images, and derive source frame radio-core brightness temperatures for three prominent, flat-spectrum extragalactic radio sources, notable because they have not been detected as gamma-ray sources with the EGRET instrument.
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The radio sky at frequencies of several MHz and below is essentially unexpected with high angular resolution due to refraction and opacity in the Earth's ionosphere.
We observed the nuclear region of NGC 4261 (3C270) with the VLBA to determine the morphology of the central radio source on parsec scales.
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Hipparcos proper motion and parallax data are combined for nearby stars with ground-based radial velocity measurements in order to identify stars which may have passed, or will pass, close enough to the sun to perturb the Oort cloud. Close stellar encounters could deflect large numbers of comets into the inner solar system, with possible serious consequences for impact hazards on the earth. Only one star, Gliese 710 is found with a predicted closest approach of less than 0.5 pc, although several stars come within 1 pc during a 8.5 M year interval. In most cases, the uncertainty in closest approach distance is dominated by uncertainties in the barycenter motion of binary systems. A program to obtain new radial velocities for stars in the sample with no previously published values is underway.
We have combined HIPPARCOS proper motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars which may have passed close enough to the sun to perturb the Oort cloud.
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We report submilliarsecond-precise astrometric measurement for the late-type star AB Doradus via a combination of VLBI (very long baseline interferometry) and Hipparos satellite data. Our astrometric analysis results in the precise determination of the kinematics of this star, that reveals an orbital motion readily explained as caused by the gravitational interaction with a low-mass companion.
The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring. We have obtained 22 GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components. Previous VLBI observations at lower frequencies indicate that the brightness temperatures of these components are significantly lower than 10(exp 10) K (Jauncey, et al. 1991), less than is typical for compact synchrotron radio sources and less than is implied by the short timescales of flux density variations. A possible explanation is that interstellar scattering is broadening the apparent angular size of the source and thereby reducing the observed brightness temperature. Our VLBA data support this hypothesis. At 22 GHz the measured brightness temperature is at least 10(exp 11) K, and the deconvolved 2 size of the core in the southwest compact component is proportional to upsilon(sup -2) between 1.7 and 22 GHz. VLBI observations at still higher frequencies should be unaffected by interstellar scattering.
Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211, whilst obscured by our galaxy at optical wavelengths, has recently provided a lensing galaxy redshift of 0.89 through the detection of a new absorption feature, most likely due to neutral hydrogen in a second redshift system at z=0.19.
Measurements of the properties of gravitational lenses have the power to tell us what sort of universe we live in. The brightest known radio Einstein ring/gravitational lens PKS 1830-211, whilst obscured by our Galaxy at optical wavelengths, has recently provided a lensing galaxy redshift of 0.89 through the detection of molecular absorption in the millimetre waveband.
We present high-resolution very long baseline interferometry images of three southern radio sources that the Energetic Gamma-Ray Experiment Telescope (EGRET), on board the Compton Gamma Ray Observatory, has identified as greater than 100 MeV gamma-ray sources.
Dual frequency VLBA observations of the nucleus of NGC 4261 (3C270) reveal highly symmetric radio structures at both 1.6 and 8.4 GHz. Analysis of these images shows that the central 10 pc of this source is not significantly affected by free-free absorption, even though the nucleus of the galaxy is known to contain a nearly edge-on disk of gas and dust. The lack of detectable absorption implies that the density of ionized gas in the central 10 pc is less than (is similar to) 10(sup 3) cm (sup -3), assuming a temperature of (is similar to) 10(sup 4)K.
An array of satellites is proposed to make astronomic observations in the low frequency range of a few tens of MHz down to roughly 100 kHz, a range that cannot be observed through the ionosphere. The array would be in a solar orbit to avoid radio interference from Earth and to simplify trajectory tracking and control.
Exploring the frequency range from a few tens of MHZ down to a few tens of kHz commonly requires multiple antennas operating as an aperture synthesis interferometer to obtain angular resolution. This design must overcome the corrupting effects of interplanetary scintillation, interfering signals and nearly isotropic antennas that see strong sources such as the Sun and Jupiter at all times.
The remarkably strong radio gravitational lens PKS 1830-211 consists of a one arcsecond diameter Einstein ring with two bright compact (milliarcsecond) components located on opposite sides of the ring. We have obtained 22GHz VLBA data on this source to determine the intrinsic angular sizes of the compact components.