Probing a Clumpy Circumnuclear Absorber via the Parsec-scale Counterjet in 3C 84

Abstract We present a comprehensive study of the faint counterjet in the radio galaxy 3C 84 using 15 years of 15 and 43 GHz data observed with the Very Long Baseline Array between 2009 and 2024. High-dynamic-range imaging reveals profound morphological and brightness asymmetry between the prominent approaching jet and the highly attenuated receding jet. The primary finding is that the counterjet provides a clean probe of subparsec external free–free absorption (FFA). The northern counterjet components N1 and N2 show strongly inverted 15–43 GHz spectra, whereas the approaching hotspot C3 is persistently optically thin, requiring a frequency-dependent absorbing screen in front of the receding flow. Taking the spectral index of α = −0.82 from C3, we infer that the FFA opacity τ 15 GHz changed from ∼3.0 to ∼1.9 for N1 and from ∼3.7 to ∼2.4 for N2, corresponding to emission measures of (1.7–3.3) × 10 9 pc cm −6 and electron densities of 10 4 –10 5 cm −3 for plausible path lengths. Distinct opacity histories of N1 and N2 indicate a radially and azimuthally structured absorber, consistent with the ionized inner skin of a clumpy torus or torus-associated wind. Proper motions of C3, N1, and N2 imply a characteristic parsec-scale viewing angle of order 20° and a pattern speed of ∼0.45–0.52 c under an assumption of bilateral symmetry. Evidence for 7–11 yr transverse quasi-periodic modulation is present, but remains tentative given the limited temporal baseline and possible core-reference systematics. The counterjet of 3C 84 therefore acts as a time-dependent tomographic probe of the environment of the inner active galactic nuclei and provides a useful template for linking jet propagation to circumnuclear gas on subparsec scales.