- Astronomy Group, Department of Physics, Queen's University at Kingston, Ontario, Canada.
MANY recent studies18 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 identified3,6,9 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.
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