1. Astronomy Group, Department of Physics, Queen's University at Kingston, Ontario, Canada.

Abstract

MANY recent studies^1–8 of the continuum spectra of discrete radio sources have provided examples of sources with "opaque" microwave spectra, in which the flux density has a maximum at a frequency greater than 3 GHz. At least two classes of object have such spectra: compact extragalactic sources, frequently identified^3,6,9 with quasi-stellar objects or nuclei of galaxies, and galactic HII regions with high electron densities¹. The extragalactic objects are thought to be sources of synchrotron radiation within which the relativistic plasma is opaque at microwave frequencies; some exhibit radio variability which can be explained by an expanding optically thick synchrotron source¹⁰. The galactic sources are thought to be optically thick emitters of bremsstrahlung radiation; a nonthermal mechanism¹¹ has also been proposed for these objects. The source VRO 42.22.01 (BL Lac) may be an example of a third class characterized by rapid microwave variability. It is uncertain whether this object is a star with a faint diffuse envelope² or an extragalactic source for which the line of sight is coincidentally close to the galactic plane.