Galactic Nonthermal Continuum Emission

Abstract

In the course of an investigation of shortwave inteference for the Bell Telephone Laboratories, Karl Jansky in 1930 built a rotating antenna array operable at 14.6-m wavelength (20.5 MHz) and used this instrument to study radio disturbances. He found that the static that was received could usually be categorized according to one of the following descriptions: intermittent and strong, intermittent and weak, or very steady and weak. The first two types of static resulted from thunderstorms which were either local or distant, respectively. However, the third type was conspicuous as it never became intense nor did it ever completely vanish. Moreover, Jansky (1932) noted that the apparent direction to this disturbance rotated through nearly 360° in 24 hours. Such periodic behavior indicated to him that the phenomenon was associated with the Sun, either directly or causally through some process occurring at the subsolar point. These inferences were sharply revised when, after an entire year of observation, it was found that the direction to the disturbance was not correlated with the position of the Sun, but rather referred to a fixed direction in space. Jansky (1933) established this direction to be near right ascension 18^h and declination —10°, i.e., in proximity to the galactic center. Before concluding that a cosmic source of radio radiation existed at the galactic center, other possibilities, such as the presence of a cosmic-ray source in that direction whose particles interacted with the Earth’s atmosphere producing radio photons, had to be evaluated. Also the measured direction of origin was close to that of solar motion with respect to nearby stars.

ArticleNote

The third group [of radio static] is composed of a very steady hiss type static the origin of which is not yet known… [however] the direction of arrival changes gradually throughout the day going almost completely around the compass in 24 hours.

Karl Jansky, 1932.