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We report the discovery of the ultra-luminous QSO SMSS~J215728.21-360215.1with magnitude $z=16.9$ and W4$=7.42$ at redshift 4.75. Given absolutemagnitudes of $M_{145,\rm AB}=-29.3$, $M_{300,\rm AB}=-30.12$ and $\log L_{\rmbol}/L_{\rm bol,\odot} = 14.84$, it is the QSO with the highest unlensedUV-optical luminosity currently known in the Universe. It was found bycombining proper-motion data from Gaia DR2 with photometry from SkyMapper DR1and the Wide-field Infrared Survey Explorer (WISE). In the Gaia database it isan isolated single source and thus unlikely to be strongly gravitationallylensed. It is also unlikely to be a beamed source as it is not discovered inthe radio domain by either NVSS or SUMSS. It is classed as a weak-emission-lineQSO and possesses broad absorption line features. A lightcurve from ATLASspanning the time from October 2015 to December 2017 shows little sign ofvariability.

Discovery of the most ultra-luminous QSO using Gaia, SkyMapper and WISE