Very Long Baseline Interferometry ( VLBI ) allows for high-resolution and high-sensitivity observations of relativistic jets , that can reveal periodicities of several years in their structure .
We perform an analysis of long-term VLBI data of the quasar S5 1928+738 in terms of a geometric model of a helical structure projected onto the plane of the sky .
We monitor the direction of the jet axis through its inclination and position angles .
We decompose the variation of the inclination of the inner 2 milliarcseconds of the jet of S5 1928+738 into a periodic term with amplitude of \sim 0.89 ^ { \circ } and a linear decreasing trend with rate of \sim 0.05 ^ { \circ } \text { yr } ^ { -1 } .
We also decompose the variation of the position angle into a periodic term with amplitude of \sim 3.39 ^ { \circ } and a linear increasing trend with rate of \sim 0.24 ^ { \circ } \text { yr } ^ { -1 } .
We interpret the periodic components as arising from the orbital motion of a binary black hole inspiraling at the jet base and derive corrected values of the mass ratio and separation from the accumulated 18 years of VLBI data .
Then we identify the linear trends in the variations as due to the slow reorientation of the spin of the jet emitter black hole induced by the spin-orbit precession and we determine the precession period T _ { \mathrm { SO } } = 4852 \pm 646 yr of the more massive black hole , acting as the jet emitter .
Our study provides indications , for the first time from VLBI jet kinematics , for the spinning nature of the jet-emitting black hole .