Transverse Oscillations of the M87 Jet Revealed by KaVA Observations

Recent VLBI monitoring has found transverse motions of the M87 jet. However, due to the limited cadence of previous observations, details of the transverse motion have not been fully revealed yet. We have regularly monitored the M87 jet at KVN and VERA Array (KaVA) 22 GHz from December 2013 to June 2016. The average time interval of the observation is ~ 0.1 year, which is suitable for tracking short-term structural changes. From these observations, the M87 jet is well represented by double ridge lines in the region 2 - 12 mas from the core. We found that the ridge lines exhibit transverse oscillations in all observed regions with an average period of $0.94\pm0.12$ years. When the sinusoidal fit is performed, we found that the amplitude of this oscillation is an order of $\sim0.1$ mas, and the oscillations in the northern and southern limbs are almost in phase. Considering the amplitude, it does not originate from Earth's parallax. We propose possible scenarios of the transverse oscillation, such as the propagation of jet instabilities or magneto-hydrodynamic (MHD) waves or perturbed mass injection around magnetically dominated accretion flows.