Pinpointing the jet apex in 3C 84
  Jets which are powered by an AGN are a crucial element in the study of their
central black holes (BH) and their immediate surroundings. The formation of
such jets is the subject of intense research, mainly based on the dichotomy
presented by the two main jet launching scenarios $-$ the one from Blandford &
Payne (1982), and the one from Blandford & Znajek (1977). In this work we study
the prominent and nearby radio galaxy 3C 84 (NGC 1275) with 15, 43, and 86 GHz
quasi-simultaneous VLBI observations. From these we determine the jet apex to
be located $83\pm7\,\mu$as ($0.028-0.11$pc) upstream of the 86 GHz VLBI core,
applying a two dimensional cross-correlation analysis. A byproduct of this
analysis are spectral index maps, in which we identify a robust spectral index
gradient in the north-south direction, for the first time at such high
resolution, for the 43-86 GHz pair. The magnetic field strength at distances
from the VLBI core comparable to measurements from the literature ($\sim10$
Schwarzschild radii) for other prominent AGN, like NGC 1052 and M 87, is
computed to be $70-600$G. Implications for the magnetic field topology are also
discussed.
