2021A&A...650L..18P

Nearby radio galaxies that contain jets are extensively studied with very long baseline interferometry (VLBI), addressing jet launching and the physical mechanisms at play around massive black holes. 3C 84 is unique in this regard because the combination of its proximity and large super massive black hole mass provides a high spatial resolution to resolve the complex structure at the jet base. For 3C 84, an angular scale of 50 µas corresponds to 200-250 Schwarzschild radii (R_s). Recent RadioAstron VLBI imaging at 22 GHz has revealed an east-west elongated feature at the northern end of the VLBI jet, which challenges past interpretations. Here we propose instead that the jet apex is not located within the 22 GHz VLBI core region but more upstream in the jet. We base our arguments on a 2D cross-correlation analysis of quasi-simultaneously obtained VLBI images at 15, 43, and 86 GHz, which measures the opacity shift of the VLBI core in 3C 84. With the assumption of the power-law index (k_r) of the core shift being set to 1, we find the jet apex to be located 83±7µas north (upstream) of the 86 GHz VLBI core. Depending on the assumptions for k_r and the particle number density power-law index, n, we find a mixed toroidal-poloidal magnetic field configuration, consistent with a region that is offset from the central engine by about 400-1500 R_s. The measured core shift is then used to estimate the magnetic field strength, which amounts to B=1.80-4.0 G near the 86 GHz VLBI core. We discuss some physical implications of these findings.