Discovery of the most ultra-luminous QSO using Gaia, SkyMapper and WISE
  We report the discovery of the ultra-luminous QSO SMSS~J215728.21-360215.1
with magnitude $z=16.9$ and W4$=7.42$ at redshift 4.75. Given absolute
magnitudes of $M_{145,\rm AB}=-29.3$, $M_{300,\rm AB}=-30.12$ and $\log L_{\rm
bol}/L_{\rm bol,\odot} = 14.84$, it is the QSO with the highest unlensed
UV-optical luminosity currently known in the Universe. It was found by
combining proper-motion data from Gaia DR2 with photometry from SkyMapper DR1
and the Wide-field Infrared Survey Explorer (WISE). In the Gaia database it is
an isolated single source and thus unlikely to be strongly gravitationally
lensed. It is also unlikely to be a beamed source as it is not discovered in
the radio domain by either NVSS or SUMSS. It is classed as a weak-emission-line
QSO and possesses broad absorption line features. A lightcurve from ATLAS
spanning the time from October 2015 to December 2017 shows little sign of
variability.
