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In a previous work, we have identified the spin of the dominant black hole of a binary from its jet properties. Analysing Very Long Baseline Array (VLBA) observations of the quasar S5 1928+738, taken at 15-GHz during 43 epochs between 1995.96 and 2013.06, we showed that the inclination angle variation of the inner (<2 mas) jet symmetry axis naturally decomposes into a periodic and a monotonic contribution. The former emerges due to the Keplerian orbital evolution, while the latter is interpreted as the signature of the spin-orbit precession of the jet emitting black hole. In this paper, we revisit the analysis of the quasar S5 1928+738 by including new 15-GHz VLBA observations extending over 29 additional epochs, between 2013.34 and 2020.89. The extended data set confirms our previous findings which are further supported by the flux density variation of the jet. By applying an enhanced jet precession model that can handle arbitrary spin orientations κ with respect to the orbital angular momentum of a binary supermassive black hole system, we estimate the binary mass ratio as ν = 0.21 ± 0.04 for κ = 0 (i.e. when the spin direction is perpendicular to the orbital plane) and as ν = 0.32 ± 0.07 for κ = π/2 (i.e. when the spin lies in the orbital plane). We estimate more precisely the spin precession velocity, halving its uncertainty from $(-0.05\pm 0.02)$ to $(-0.04\pm 0.01)^{\circ }\, \mathrm{yr}^{-1}$.

We report on the radio brightening of the blazar TXS 0506+056 (at z = 0.3365), and we support its identification by the IceCube Neutrino Observatory as a source of the high-energy (HE) neutrino IC-170922A. Data from the Monitoring Of Jets in AGN with VLBA Experiments (MOJAVE)/Very Long Baseline Array (VLBA) survey indicate that its radio brightness has abruptly increased since 2016 January. When decomposing the total radio flux density curve (in the period 2008 January to 2018 July), provided by the Owens Valley Radio Observatory, into eight Gaussian flares, the peak time of the largest flare overlaps with the HE neutrino detection, while the total flux density has exhibited a threefold increase since 2016 January. We reveal the radio structure of TXS 0506+056 by analysing very long baseline interferometry (VLBI) data from the MOJAVE/VLBA survey. The jet components maintain quasi-stationary core separations. The structure of the ridge line is indicative of a jet curve in the region 0.5–2 mas (2.5–9.9 pc projected) from the VLBI core. The brightness temperature of the core and the pc-scale radio morphology support a helical jet structure at small inclination angle (<8${^{\circ}_{.}}$2). The jet pointing towards the Earth is a key property facilitating multimessenger observations (HE neutrinos, γ-rays and radio flares). The radio brightening preceding the detection of a HE neutrino is similar to the one reported for the blazar PKS 0723–008 and IceCube event ID5.