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We have performed quasi-simultaneous radio flux density measurements at 2.7and 10 GHz for all PG quasars with radio flux densities between 4-200 mJy. Wefind that a large fraction of these sources are variable, flat-spectrumquasars. This brings the total fraction of flat-spectrum quasars with a ratiobetween radio and optical flux of R>10 - a value previously used to define aradio-loud quasar - to 40% in the PG quasar sample. We also find that themedian R-parameter of these flat-spectrum quasars is lower than those ofsteep-spectrum radio-loud quasars. This contradicts the predictions of theunified scheme and the idea that all flat-spectrum, core-dominated quasars arerelativistically boosted lobe-dominated quasars. We show that this discrepancyis due to a population of flat-spectrum radio-intermediate quasars with25<R<250 which can neither be explained as relativistically boosted radio-loudquasars nor as normal radio-weak quasars. We point out that a naturalexplanation for the flat-spectrum radio-intermediate quasars is relativisticboosting in radio-weak quasars. If the flat-spectrum radio-intermediate quasarsare considered the boosted counterparts to usual radio-weak quasars, theirfraction among radio-weak quasars is roughly 10%, similar to the fraction ofboosted radio-loud quasars. This would point towards average Lorentz factors ofgamma_jet=2-4 for radio-loud and radio-weak quasars. The presence of theflat-spectrum radio-intermediate quasars changes the definition of 'radio-loud'and can bias some conclusions drawn from optically selected quasar samples,where R ~ 1-10 is used as the dividing line for both, flat- and steep-spectrumquasars. Instead one should use separate R-parameters for the dividing line insteep- (R~25) and flat-spectrum (R~250) quasars.

The Nature of Radio‐Intermediate Quasars: What Is Radio‐loud and What Is Radio‐quiet?