We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all Palomar-Green (PG) quasars with radio flux densities between 4 and 200 mJy. We find that a large fraction of these sources are variable, flat-spectrum quasars. This brings to 40% the total fraction of flat-spectrum quasars with a radio-to-optical flux ratio R 10 in the PG quasar sample. We also find that the median R-parameter of these flat-spectrum quasars is lower than those of steep-spectrum radio-loud quasars. This contradicts the predictions of the unified scheme and the idea that all flat-spectrum, core-dominated quasars are relativistically boosted lobe-dominated quasars. We show that this discrepancy is due to a population of flat-spectrum radio-intermediate quasars with 25 R 250 that can be explained neither as relativistically boosted radio-loud quasars nor as normal radio-weak quasars. We point out that a natural explanation for the flat-spectrum radio-intermediate quasars is relativistic boosting in radio-weak quasars. If the flat-spectrum radio-intermediate quasars are considered the boosted counterparts to usual radio-weak quasars, their fraction among radio-weak quasars is roughly 10%, similar to the fraction of boosted radio-loud quasars. This would point toward average Lorentz factors of γjet = 2–4 for radio-loud and radio-weak quasars. The presence of the flat-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-spectrum quasars. Instead, one should use separate R-parameters for the dividing line in steep-spectrum (R 25) and flat-spectrum (R ~ 250) quasars.
展开▼
