  The radio-loud quasar SDSS J013127.34-032100.1at a redshift z=5.18 is one of
the most distant radio-loud objects. The radio to optical flux ratio (i.e. the
radio-loudness) of the source is large, making it a promising blazar candidate.
Its overall spectral energy distribution, completed by the X-ray flux and
spectral slope derived through Target of Opportunity Swift/XRT observations, is
interpreted by a non-thermal jet plus an accretion disc and molecular torus
model. We estimate that its black hole mass is (1.1+-0.2)1e10 Msun. for an
accretion efficiency eta=0.08, scaling roughly linearly with eta. Although
there is a factor ~2 of systematic uncertainty, this black hole mass is the
largest found at these redshifts in a radio loud object. We derive a viewing
angle 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 whose
jets are pointing away from Earth. We discuss the problems posed by the
existence of such large black hole masses at such redshifts, especially in
jetted quasars. In fact, if they are associated to rapidly spinning black
holes, the accretion efficiency is high, implying a slower pace of black hole
growth with respect to radio-quiet quasars.
