We have selected a complete sample of 80 flat-spectrum radio quasars ( FSRQs ) from the WMAP 7-yr catalog within the SDSS area , all with measured redshift , and have inspected their SEDs looking for evidence of an AGN torus emission .
A SED fitting algorithm has found such evidence for seven objects and an uncertain indication for one more .
A further analysis has picked out four additional FSRQs whose observed SEDs may be consistent with the presence of a torus .
All these 8 + 4 FSRQs belong to the sub-sample of 55 sources showing the optical–ultraviolet bump interpreted as thermal emission from a standard accretion disc .
Torus luminosities have been estimated for the eight objects whose torus was identified by the fitting algorithm .
For the other 47 FSRQs in the sub-sample , including the four with indications of torus emission not spotted by the algorithm , we have derived upper limits to the torus luminosity .
Our analysis shows that the torus can show up clearly only under quite special conditions : low luminosity and preferentially low peak frequency of the beamed synchrotron emission from the jet ; high torus luminosity , close to that of the accretion disc .
This implies that the inferred ratios of torus to disc luminosity are biased high .
The median value , considering upper limits as detections ( survival analysis techniques are found to be inapplicable because of the excessive fraction of upper limits ) , is L _ { torus } / L _ { disc } \sim 1 while studies of radio quiet quasars yield average ratios \langle L _ { torus } / L _ { disc } \rangle \simeq 1 / 3 – 1 / 2 .
On the other hand , although our poor statistics does not allow us to draw firm conclusions , our results are compatible with the FSRQ tori having the same properties of those of radio quiet quasars .
At variance with Plotkin et al .
( 28 ) , who investigated a sample of optically selected BL Lacs , we find that the Wide-field Infrared Survey Explorer ( WISE ) infrared colors do not allow us to draw any conclusion on the presence or absence of tori associated with WMAP selected blazars .
With the latter selection blazars of all types ( FSRQs with and without evidence of torus , BL Lacs , blazars of unknown type ) occupy the same region of the WISE color - color plane , and their region overlaps that of SDSS quasars with point-like morphology .