Early Parkes Observations of Planets and Cosmic Radio Sources

We discuss early Parkes observations of the radio emission from the planets Mercury, Venus, Mars, Saturn, and Uranus. The sensitive Parkes 11 cm system was used to detect a surprisingly high observed nighttime temperature on Mercury, the first, but unrecognized, hint that the Mercury actually rotates with respect to the Sun, as well as detecting the faint radio emission from Uranus. We also discuss the anomalous spectrum of PKS 1934-63, the first recognized GPS source.Comment: Published electronically in Proceedings "Science with Parkes @ 50 Years Young", 2012, Ed. Robert Braun. Conference link is, http://www.atnf.csiro.au/research/conferences/Parkes50th/ProcPapers/kellermann.pd

The Diverse Properties of GPS Sources

We discuss the morphology and kinematics of five gigahertz-peaked spectrum (GPS) sources that have been observed with the VLBA. We find a wide range of observed properties including core-jet structure, superluminal motion, variability, extended structure, and polarization, all of which appear to deviate from commonly-accepted GPS paradigms (e.g., O'Dea 1998). We suggest that the observed low frequency cutoff in GPS sources may be primarily due to free-free absorption rather than synchrotron self-absorption.Comment: Proceedings of the 6th European VLBI Network Symposium, Ros E., Porcas R.W., Lobanov, A.P., & Zensus, J.A. (eds), MPIfR, Bonn, Germany. (4 pages, 5 figures, needs evn2002.cls style file

The Discovery of Quasars and its Aftermath

Although the extragalactic nature of quasars was discussed as early as 1960, it was rejected largely because of preconceived ideas about what appeared to be an unrealistically high radio and optical luminosity. Following the 1962 occultations of the strong radio source 3C 273 at Parkes, and the subsequent identification with an apparent stellar object, Maarten Schmidt recognized that the relatively simple hydrogen line Balmer series spectrum implied a redshift of 0.16 Successive radio and optical measurements quickly led to the identification of other quasars with increasingly large redshifts and the general, although for some decades not universal, acceptance of quasars as being by far the most distant and the most luminous objects in the Universe. Curiously, 3C 273, which is one of the strongest extragalactic sources in the sky, was first cataloged in 1959 and the magnitude 13 optical counterpart was observed at least as early as 1887. Since 1960, much fainter optical counterparts were being routinely identified using accurate radio interferometer positions, measured primarily at the Caltech Owens Valley Radio Observatory. However, 3C 273 eluded identification until the series of lunar occultation observations led by Cyril Hazard, although inexplicably there was an earlier mis-identification with a faint galaxy located about an arc minute away from the true position. Ironically, due to calculation error, the occultation position used by Schmidt to determine the redshift of 3C 273 was in error by 14 arcseconds, and a good occultation position was not derived until after Schmidt had obtained his 200 inch spectrum.Comment: Paper published in the Journal of Astronomical History and Heritage 17(3) 267-282 (2014) which is a revision and extension of a previous version published in the Bulletin of the Astronomical Society of India 41, 1-17 (2013

Space VLBI Observations Show Tb>1012KT_b > 10^{12} K in the Quasar NRAO 530

We present here space-based VLBI observations with VSOP and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 GHz and 5 GHz. The brightness temperature of the core at 1.6 GHz is $5 \times 10^{11}$ K. The size is near the minimum observable value in the direction of NRAO~530 due to interstellar scattering. The 5 GHz data show a single component with a brightness temperature of $\sim 3 \times 10^{12}$ K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit (Readhead 1994). This is strong evidence for relativistic motion in a jet requiring model-dependent Doppler boosting factors in the range 6 to 60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model with a Doppler boosting factor of 15.Comment: 12 pages, 2 figures. Accepted for publication in ApJ Letter