Multi-frequency VLBI Observations of the M 84 Inner Jet/Counterjet

Observational studies of inner-most regions of the edge-on jets in nearby active galactic nuclei (AGN) are crucial to understand their kinematics and morphology. For the inner jet of the nearby low luminosity AGN in M 84, we present new high-sensitivity observations with very long baseline interferometry since 2019, as well as archival Very Long Baseline Array observations in 2014. We find that the compact core in M 84 has an inverted-to-flat spectrum from 1.5 to 88 GHz. Based on the turnover frequency of $4.2\pm 0.2$ GHz in the spectrum, we estimated a magnetic field strength of 1-10mG and an electron number density of $\sim 10^5 cm^{-3}$ in the core region. Three inner jet components within $\sim 3$ mas from the core are identified and traced in the images at 22 GHz, whose apparent speeds are 0.11 c, 0.27 c, and 0.32 c, respectively. We calculate the viewing angle of $\sim58$ degree for the inner jet based on the proper motion and the flux ratio of jet-to-counterjet. A propagating sinusoidal model with a wavelength of $\sim 3.4$ mas is used to fit the helical morphology of the jet extended to 20 mas ($\sim 2.2\times 10^4$ Schwarzschild Radii).
Comment: 9 pages, 5 figures, accepted by the ApJ