Falcke/Sherwood/Patnaik, The Nature of Radio-Intermediate Quasars
The Nature of Radio-Intermediate Quasars: What is Radio Loud
and what is Radio Quiet?
Heino Falcke (1), William Sherwood (2), Alok Patnaik (2)
(1) Department of Astronomy, University of Maryland, College Park, MD 20742-2421,
(2) Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
Astrophysical Journal, Vol. 471, 106
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.
Paper: Available in full length as PostScript file
and in LaTex (ApJ) Format.
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Questions: Heino Falcke, email@example.com