Pinpointing the jet apex of 3C 84
  Nearby radio galaxies that contain jets are extensively studied with 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 SMBH mass provides a high spatial resolution to resolve the
complex structure at the jet base. For 3C 84 an angular scale of 50 ${\mu}$as
corresponds to 200 - 250 Schwarzschild radii ($R_s$). Recent RadioAstron VLBI
imaging at 22 GHz revealed an east-west elongated feature at the northern end
of the VLBI jet, which challenges interpretations. Here we propose instead that
the jet apex is not located within the 22 GHz VLBI core region but more
upstream of 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 $83 \pm 7$ ${\mu}$as 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 which 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.
