The Nature of Radio - Intermediate Quasars : What is

We have performed quasi-simultaneous radio ux density measurements at 2.7 and 10 GHz for all PG quasars with radio ux densities between 4-200 mJy. We nd that a large fraction of these sources are variable, at-spectrum quasars. This brings the total fraction of at-spectrum quasars with a ratio between radio and optical ux of R > 10 | a value previously used to deene a radio-loud quasar | to 40% in the PG quasar sample. We also nd that the median R-parameter of these at-spectrum quasars is lower than those of steep-spectrum radio-loud quasars. This contradicts the predictions of the uniied scheme and the idea that all at-spectrum, core-dominated quasars are relativistically boosted lobe-dominated quasars. We show that this discrepancy is due to a population of at-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 at-spectrum radio-intermediate quasars is relativistic boosting in radio-weak quasars. If the at-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 jet = 2 ? 4 for radio-loud and radio-weak quasars. The presence of the at-spectrum radio-intermediate quasars changes the deenition 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, at-and steep-spectrum quasars. Instead one should use separate R-parameters for the dividing line in steep-(R ' 25) and at-spectrum (R 250) quasars.