AGN torus emission for a homogeneous sample of bright flat-spectrum radio quasars^⋆

¹ SISSA, via Bonomea 265, 34136 Trieste, Italy
e-mail: gianluca.castignani@oca.eu
² INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

Received: 29 March 2014
Accepted: 9 October 2014

Abstract

We have selected a complete sample of 80 flat-spectrum radio quasars (FSRQs) from the Wilkinson Microwave Anisotropy Probe (WMAP) seven-year catalog within the Sloan Digital Sky Survey (SDSS) area, all with measured redshift, and have inspected their spectral energy distributions (SEDs) looking for evidence of an active galactic nucleus 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 12 of these FSRQs belong to the subsample of 55 sources showing the optical-ultraviolet bump interpreted as thermal emission from a standard accretion disk. Torus luminosities have been estimated for the eight objects whose torus was identified by the fitting algorithm. For the other 47 FSRQs in the subsample, including the 4 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 disk. This implies that the inferred ratios of torus to disk 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_disk ~ 1 while studies of radio quiet quasars yield average ratios ⟨ L_torus/L_disk ⟩ ≃ 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 as those of radio quiet quasars. At variance with another investigation of optically selected BL Lacs, we find that the Wide-field Infrared Survey Explorer (WISE) infrared colors do not allow us to draw any conclusions 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.