Multi-epoch monitoring of the faint northern component in 3C 84 during 2016–2021

Abstract We present the results of multi-epoch very long baseline interferometry (VLBI) observations of the sub-parsec-scale northern jet component of 3C 84 (known as the N1 component) at 7 mm from 2016 to 2021. The peak position of the N1 component exhibits positional variations of approximately 0.2 mas at a distance of about 2.5 mas from the central radio core (C1) throughout the observation period. Although a linear fit yields a small proper motion of $\mu _{\rm Dec}=0.054\pm 0.019$ mas yr$^{-1}$, corresponding to an apparent velocity of $\beta _{\rm app}=0.062\pm 0.021$, this displacement is comparable to the observed positional scatter, and the hypothesis that N1 exhibits systematic motion is not statistically supported. In contrast, the peak flux density varies by approximately a factor of two on timescales of several months. We propose two possible explanations for the observed behavior of N1. One is that N1 is the counterpart of the southern hotspot C3, but its motion has been halted by a collision with a compact and dense cloud along its path; in this case, the flux variability may be caused by changes in the free–free absorption opacity of the non-uniform cloud. The other possibility is that N1 is part of the northern jet that is heavily obscured by foreground free–free absorption and is observed through a gap in the absorbing medium; in this case, the variability may reflect intrinsic changes in the jet and its interaction with a non-uniform ambient medium. These results provide new constraints on the physical origin of N1 and on the circumnuclear environment of 3C 84.