Galactic Radiation at Radio Frequencies. II. The Discrete Sources

Abstract

An account is given of observations on a number of discrete sources of radio- frequency radiation, together with a brief description of the observational techniques used in their detection, and of the methods employed to determine their positions and angular extent.Noise spectra of four of the more intense sources have been measured over a frequency range of 40-160 Mc/s. : in three cases the change of intensity with wave- length is found to be greater than that of the background continuum, and in one case less. Two of the sources have been provisionally identified with astronomically rare objects, the Crab Nebula and N.G.C. 5128.Finally, the short period fluctuations in the intensity of some of the sources, notably that in Cygnus, are discussed. Evidence is presented which, contrary to previous views, suggests that these fluctuations are of terrestrial rather than of extra-terrestrial origin.

The Eta Carinae Nebula and Centaurus A Near 1400mc/s. I. Observations

“…The radio-frequency counterpart of NGC 5128, Centaurus A, is no less peculiar. Early interferometric observations (Stanley and Slee 1950) indicated that it consists of two components: one, a strong source of small angular size coinciding in position with NGC 5128; the other, a broad, faint extended source surrounding NGC 5128. The extended source has been shown in good detail by recent pencil-beam observations at 19·7 Mc/s (Shain 1958) and at 85·5 Mc/s (Sheridan 1958).…”

Galactic Radiation at Radio Frequencies. V. The Sea Interferometer

“…For the sea interference patterns the zero order fringe occurs when the source is first seen on the horizon. The theory of the effect of a finite receiver bandwidth has been given previously in Part II of this series (Stanley and Slee 1950). At the nth fringe it can be shown that, for n<JtilJ,…”

“…This divergence in sea interferometry results in less power in the reflected beam and hence incomplete interference between it and the direct beam. The theory of the effect of the curved Earth has been developed in Part II of this series (Stanley and Slee 1950) and the amplitudes of the interference pattern at minima and maxima are given by P min…”

“…The other, consisting of two aerials spaced many wavelengthS apart along an east-west base line, may be compared to the Michelson interferometer. The relative advantages of the two systems have been previously discussed (Stanley and Slee 1950) but will be briefly reviewed.…”

Setting the radio astronomy flux density scale

“…The earliest measurements by Karl Jansky and Grote Reber (1940) already showed that the Galactic radio emission decreased with increasing wavelength, contrary to the Raleigh Jeans spectrum expected from ordinary thermal radiation. Later observations showed the strongest discrete radio sources and the same non-thermal radio spectrum (Stanley & Slee 1950;Whitfield 1957). However, due to uncertainties in determining the gain of radio telescopes, these early measurements of radio source spectra were limited by the uncertainties in placing the flux density measurements made at different frequencies on the same absolute scale.…”