Gaia Data Release 1: The reference frame and the optical properties of
  ICRF sources
  As part of the data processing for Gaia Data Release~1 (Gaia DR1) a special
astrometric solution was computed, the so-called auxiliary quasar solution.
This gives positions for selected extragalactic objects, including radio
sources in the second realisation of the International Celestial Reference
Frame (ICRF2) that have optical counterparts bright enough to be observed with
Gaia. A subset of these positions was used to align the positional reference
frame of Gaia DR1 with the ICRF2. We describe the properties of the Gaia
auxiliary quasar solution for a subset of sources matched to ICRF2, and compare
their optical and radio positions at the sub-mas level. Their formal standard
errors are better than 0.76~milliarcsec (mas) for 50% of the sources and better
than 3.35~mas for 90%. Optical magnitudes are obtained in Gaia's unfiltered
photometric G band. The comparison with the radio positions of the defining
sources shows no systematic differences larger than a few tenths of a mas. The
fraction of questionable solutions, not readily accounted for by the
statistics, is less than 6%. Normalised differences have extended tails
requiring case-by-case investigations for around 100 sources, but we have not
seen any difference indisputably linked to an optical-radio offset in the
sources.
