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ON DISCRETE SOURCES OF COSMIC RADIO EMISSION
Investigating the radio emission coming from the constellation Cygnus, Hey and others[^1] found that it fluctuates in a manner similar to the radio emission of sunspots. In their opinion, this indicated the presence in Cygnus of discrete “radio-bright” sources, analogous in the character of the origin of their radiation to sunspots.
In a short note published in one of the issues of Nature[^2] in 1948, Bolton and Stanley reported that they had succeeded in measuring the angular size of the discrete source in the constellation Cygnus, which in the meter range proved to be less than \(8'\). The method for measuring the angular size of the source used by the authors is typical for this kind of measurement and consists in the fact that the radiation is received by an antenna device with a multi-lobe pattern. At the output of the receiving device there is then observed an alternation of maxima and minima corresponding to the displacement of the source in the zone of the lobes of the receiving antenna pattern. The angular size of the source is found from the ratio of the maximum readings of the output instrument to the minimum ones. To obtain a multi-lobe pattern, the authors used an antenna located on a cliff above the sea, so that the angular size of the lobe was of the order of several minutes; this determined the resolving power of their apparatus.
The method described is, obviously, the radiophysical analogue of the optical method for determining the angular diameters of stars proposed by Fizeau and fundamentally improved by Michelson.
Taking the size of the source they found to be equal to \(8'\), Bolton and Stanley found its effective temperature to be \(4\cdot10^8\) K, where by effective temperature they understand, as is customary, the temperature
blackbody of the same angular size, producing the same intensity as that observed experimentally. In a later note\(^3\) Bolton gives the results of a search for new discrete sources (the region around \(1/4\) of the celestial sphere was surveyed) at a frequency of \(100\) MHz, the sensitivity of Bolton’s installation being \(10^{-24}\ \mathrm{W}/\mathrm{m}^{2}\mathrm{Hz}\), and the experimental technique the same as in\(^2\). Data on the six newly discovered discrete sources and on the previously discovered source in the constellation Cygnus are given in the table. Sources 8.48 and 5.47 are located near Telescopium A and Centaurus A, respectively, which prevents their coordinates from being determined with sufficient accuracy. Most of these sources lie in regions of low stellar density and are not active in the optical radio-emission range. However, Bolton notes, perhaps “radio-bright” regions of the celestial sphere are regions of high density of discrete sources.
Sources of cosmic radio emission known as of February 1, 1948
| Temporary name | Position: right ascension | Position: declination | Intensity at 100 MHz, in \(\mathrm{W}/\mathrm{m}^{2}\mathrm{Hz}\) | Angular width | Type |
|---|---|---|---|---|---|
| Cygnus A (1.46) |
\(19^{h}59^{m}\) | \(+41^{\circ}47'\) | \(6\cdot10^{-23}\) | \(<8'\) | Variable |
| Telescopium A (2.47) |
\(5^{h}\ 13\ m\) | \(+28^{\circ}\) | \(10^{-23}\) | \(<30'\) | Apparently variable, with a large period |
| Coma Berenices A (4.47) |
\(12^{h}04^{m}\) | \(+20^{\circ}30'\) | \(1.5\cdot10^{-23}\) | \(<15'\) | Constant |
| Hercules A (7.48) |
\(16^{h}21^{m}\) | \(+15^{\circ}\) | \(2\cdot10^{-23}\) | \(<1^{\circ}\) | — |
| (8.48) | — | — | \(2\cdot10^{-24}\) | — | Constant |
| (5.47) | — | — | \(3\cdot10^{-24}\) | \(<1^{\circ}\) | Constant |
| Centaurus A (6.47) |
— | — | \(10^{-23}\) | \(<15'\) | Apparently variable, like Cygnus A |
The modern experiment does not answer this question. Moreover, as in the case of the source in the constellation Cygnus, the new sources are difficult to identify with any outstanding stellar objects. The discovery of discrete sources compels a reconsideration of the question of the origin of cosmic radio emission and the attribution of part of the observed emission intensity to individual “anomalously radio-bright” stars or nebulae.
Sometimes the discovery of new discrete sources has been a “by-product” of an investigation. While observing, at a frequency of 80 Mc/s, the polarization of the radiation coming from the constellation Cygnus, Ryle and Smith^4 discovered two new discrete sources of radio emission, one of which lies in Cassiopeia (\(23^{h}17^{m}50^{s};\ 58^\circ\)), and the other in the constellation Ursa Major (\(12^{h}20^{m};\ 58^\circ\)). The radiation of the source in Cassiopeia is less subject to fluctuations than the radiation of the source in Cygnus. At the same time, the new source is more intense than the source in Cygnus. The angular diameter of the newly discovered sources, which at a frequency of 80 Mc/s proved to be less than \(6'\), was measured with the aid of two antennas separated by 0.5 km.
As for the polarization of the radiation coming from the discrete source in the constellation Cygnus, Ryle and Smith found this radiation to be naturally polarized, which, in their opinion, indicates the absence in the source of a sufficiently strong magnetic field.
The discovery of discrete sources of radio emission stimulated attempts to identify them with visible celestial objects. Bolton et al.^5, after carefully measuring the positions of three discrete sources lying in the constellations Taurus, Virgo, and Centaurus, came to the conclusion that these sources belong to nebulae. The angular size of the galactic nebula identified with the source in Taurus is of the order of \(5'\), which at a frequency of 100 Mc/s leads to a value of the effective temperature of the nebula of \(2\cdot10^{6}\) K. The question of whether the other two nebulae belong to our Galaxy or not remains open.
G. Getmantsev
CITED LITERATURE
- J. S. Hey, S. J. Parsons and J. W. Phillips, Nature 158, 234 (1946).
- J. G. Bolton, G. J. Stanley, Nature 161, 312 (1948).
- J. G. Bolton, Nature 162, 141 (1948).
- M. Ryle and F. G. Smith, Nature 162, 462 (1948).
- J. G. Bolton, G. J. Stanley, O. B. Slee, Nature 164, 101 (1949).