Radio-optical outlier quasars - a case study with ICRF2 and SDSS
  With Gaia, it will become possible to directly link the radio and optical
reference frames using a large number of common objects. For the most accurate
radio-optical link, it is important to know the level of spatial coincidence
between the quasars' optical positions, and the radio positions determined by
Very Long Baseline Interferometry (VLBI) observations. The "outlier" objects,
for which the positions are significantly offset at the two different
electromagnetic wavebands, may be of astrophysical interest as well. Here we
present a case study to compare the radio positions of ~800 quasars common in
the second realization of the International Celestial Reference Frame (ICRF2)
and in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) catalogue.
Compared to the radio ICRF2, the SDSS provides two orders of magnitude less
accurate astrometric data in the optical. However, its extensive sky coverage
and faint magnitude limit allow us to directly relate the positions of a large
sample of radio sources. This way we provide an independent check of the
overall accuracy of the SDSS positions and confirm that the astrometric
calibration of the latest Data Release 8 (DR8) is poorer than that of the DR7.
We find over 20 sources for which the optical and radio brightness peaks are
apparently not coincident at least at the 3-sigma level of SDSS DR7 positional
accuracy, and briefly discuss the possible causes, including dual active
galactic nuclei.
