Contemporaneous radio and optical observations of S4 0954+65. From top to bottom, the panels show optical brightness in magnitudes, polarization degree (Π) in %, and polarization angle (Ψ) in degrees. The grey shaded area (vertical) marks the duration of the IXPE observation (MJD 60089-60098). In the middle panel, the red and blue shaded areas (horizontal) are centered on the average Π_O and Π_R within the IXPE observation time window, respectively; the thickness of these red and blue horizontally shaded areas corresponds to the standard deviation of Π_O and Π_R within the IXPE observation time window, respectively. In the bottom panel, we indicate the parallel and perpendicular alignment directions of the jet position angle (after shifting Ψ_jet, 43 = −42° ±7° by 180° to account for the inherent 180° ambiguity in polarization angle estimates; see Sect. 3.3) using the green and purple horizontally shaded areas, respectively. The 43 GHz VLBA C and Q1 data points refer to the polarization degree and angle of the two parsec-scale components detected at 43 GHz (see Sect. 3.3).