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Very Long Baseline Interferometry (VLBI) allows for high-resolution andhigh-sensitivity observations of relativistic jets, that can revealperiodicities of several years in their structure. We perform an analysis oflong-term VLBI data of the quasar S5 1928+738 in terms of a geometric model ofa helical structure projected onto the plane of the sky. We monitor thedirection of the jet axis through its inclination and position angles. Wedecompose the variation of the inclination of the inner 2 milliarcseconds ofthe jet of S5 1928+738 into a periodic term with amplitude of ~0.89 deg and alinear decreasing trend with rate of ~0.05 deg/yr. We also decompose thevariation of the position angle into a periodic term with amplitude of ~3.39deg and a linear increasing trend with rate of ~0.24 deg/yr. We interpret theperiodic components as arising from the orbital motion of a binary black holeinspiraling at the jet base and derive corrected values of the mass ratio andseparation from the accumulated 18 years of VLBI data. Then we identify thelinear trends in the variations as due to the slow reorientation of the spin ofthe jet emitter black hole induced by the spin-orbit precession and wedetermine the precession period T_SO=4852+/-646 yr of the more massive blackhole, acting as the jet emitter. Our study provides indications, for the firsttime from VLBI jet kinematics, for the spinning nature of the jet-emittingblack hole.

A spinning supermassive black hole binary model consistent with VLBI observations of the S5 1928+738 jet