The Nature of Radio-Intermediate Quasars: What is Radio-Loud and what is Radio-Quiet?

Abstract

We have performed quasi-simultaneous radio flux density measurements at 2.7 and 10 GHz for all PG quasars with radio flux densities between 4-200 mJy. We find that a large fraction of these sources are variable, flat-spectrum quasars. This brings the total fraction of flat-spectrum quasars with a ratio between radio and optical flux of R>10 - a value previously used to define a radio-loud quasar - to 40% 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 which can neither be explained 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 towards average Lorentz factors of gamma_jet=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- (R~25) and flat-spectrum (R~250) quasars.