2019ApJ...886...85A

We study the properties of the innermost jet of the flat spectrum radio quasar 1633+382 (4C 38.41) based on very long baseline interferometry (VLBI) data from the radio monitoring observations of the Boston University VLBI program at 43 GHz. Analysis of the components suggests a semi-parabolic jet geometry with jet radius R following the relation R ∝ r^0.7 with distance r, with indications of a jet geometry break toward a conical geometry. Brightness temperature falls with distance following T_B ∝ r^–2.1. Combining this information, magnetic field and electron densities are found to fall along the jet as B ∝ r^–1.5 and n ∝ r^–1.1, respectively, suggesting that the magnetic configuration in the jet may be dominated by the poloidal component. Our analysis of the jet structure suggests that the innermost jet regions do not follow a ballistic trajectory and, instead, match a sinusoidal morphology, which could be due to jet precession from a helical pattern or Kelvin-Helmholtz instabilities.