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The radio-loud quasar SDSS J013127.34-032100.1at a redshift z=5.18 is one ofthe most distant radio-loud objects. The radio to optical flux ratio (i.e. theradio-loudness) of the source is large, making it a promising blazar candidate.Its overall spectral energy distribution, completed by the X-ray flux andspectral slope derived through Target of Opportunity Swift/XRT observations, isinterpreted by a non-thermal jet plus an accretion disc and molecular torusmodel. We estimate that its black hole mass is (1.1+-0.2)1e10 Msun. for anaccretion efficiency eta=0.08, scaling roughly linearly with eta. Althoughthere is a factor ~2 of systematic uncertainty, this black hole mass is thelargest found at these redshifts in a radio loud object. We derive a viewingangle between 3 and 5 degrees. This implies that there must be other (hundreds)sources with the same black hole mass of SDSS J013127.34-032100.1, but whosejets are pointing away from Earth. We discuss the problems posed by theexistence of such large black hole masses at such redshifts, especially injetted quasars. In fact, if they are associated to rapidly spinning blackholes, the accretion efficiency is high, implying a slower pace of black holegrowth with respect to radio-quiet quasars.

SDSS J013127.34-032100.1: a candidate blazar with a 11 billion solar mass black hole at $z$=5.18