The majority of quasars are weak in their radio emission, with flux densities comparable to those in the optical, and energies far lower. A small fraction, about 10%, are hundreds to thousands of times stronger in the radio. Conventional wisdom holds that there are two classes of quasars, the radio-quiets and radio-louds, with a deficit of sources having intermediate power. Are there really two separate populations, and if so, is the physics of the radio emission fundamentally different between them? This paper addresses the second question, through a study of radio variability across the full range of radio power, from quiet to loud. The VLA was used during 10 epochs to study three carefully selected samples of 11 radio-quiet quasars, 11 radio-intermediate quasars, and 8 radio-loud quasars. A fourth sample consists of 20 VLA calibrators used for phase correction during the observations, all of which are radio-loud. The basic findings are that the root mean square amplitude of variability is independent of radio luminosity or radio-to-optical flux density ratio and that fractionally large variations can occur on timescales of months or less in both radio-quiet and radio-loud quasars. Combining this with similarities in other indicators, such as radio spectral index and the presence of VLBI-scale components, leads to the suggestion that the physics of radio emission in the inner regions of all quasars is essentially the same, involving a compact, partially opaque core together with a beamed jet. It is possible that differences in large-scale radio structures between radio-loud and radio-quiet quasars could stem from disruption of the jets in low-power sources before they can escape their host galaxies.
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