THE NATURE OF RADIO-INTERMEDIATE QUASARS - WHAT IS RADIO-LOUD AND WHAT IS RADIO-QUIET

We have performed quasi-simultaneous radio flux density measurements a t 2.7 and 10 GHz for all Palomar-Green (PG) quasars with radio flux de nsities between 4 and 200 mJy. We find that a large fraction of these sources are variable, flat-spectrum quasars. This brings to 40% the to tal fraction of flat-spectrum quasars with a radio-to-optical flux rat io R > 10 in the PG quasar sample. We also find that the median R-para meter of these flat-spectrum quasars is lower than those of steep-spec trum radio-loud quasars. This contradicts the predictions of the unifi ed 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-intermediat e quasars with 25 < R < 250 that can be explained neither as relativis tically boosted radio-loud quasars nor as normal radio-weak quasars. W e point out that a natural explanation for the flat-spectrum radio-int ermediate quasars is relativistic boosting in radio-weak quasars. If t he 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 y(je t) = 2-4 for radio-loud and radio-weak quasars. The presence of the fl at-spectrum radio-intermediate quasars changes the definition of ''rad io-loud'' and can bias some conclusions drawn from optically selected quasar samples, where R similar or equal to 1-10 is used as the dividi ng line for both flat- and steep-spectrum quasars. Instead, one should use separate R-parameters for the dividing line in steep-spectrum (R similar or equal to 25) and flat-spectrum (R similar to 250) quasars.