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In a previous work, we have identified the spin of the dominant black hole ofa binary from its jet properties. Analysing Very Long Baseline Array (VLBA)observations of the quasar S5~1928+738, taken at $15$-GHz during $43$ epochsbetween $1995.96$ and $2013.06$, we showed that the inclination angle variationof the inner ($<2$~mas) jet symmetry axis naturally decomposes into a periodicand a monotonic contribution. The former emerges due to the Keplerian orbitalevolution, while the latter is interpreted as the signature of the spin-orbitprecession of the jet emitting black hole. In this paper, we revisit theanalysis of the quasar S5~1928+738 by including new $15$-GHz VLBA observationsextending over $29$ additional epochs, between $2013.34$ and $2020.89$. Theextended data set confirms our previous findings which are further supported bythe flux density variation of the jet. By applying an enhanced jet precessionmodel that can handle arbitrary spin orientations $\kappa$ with respect to theorbital angular momentum of a binary supermassive black hole system, weestimate the binary mass ratio as $\nu=0.21\pm0.04$ for $\kappa=0$ (i.e. whenthe spin direction is perpendicular to the orbital plane) and as$\nu=0.32\pm0.07$ for $\kappa=\pi/2$ (i.e. when the spin lies in the orbitalplane). We estimate more precisely the spin precession velocity, halving itsuncertainty from $(-0.05\pm0.02)^\circ\,\mathrm{yr}^{-1}$ to$(-0.04\pm0.01)^\circ\,\mathrm{yr}^{-1}$.

Signatures of a spinning supermassive black hole binary on the mas-scale jet of the quasar S5 1928+738 based on 25 years of VLBI data