Transverse Oscillations and Wave Propagation in the Magnetically Dominated M87 Jet

TL;DR

This study analyzes transverse oscillations in the M87 jet, revealing wave propagation with superluminal speeds and suggesting they are likely MHD waves or jet instabilities, based on high-cadence VLBI observations.

Contribution

It provides the first detailed analysis of transverse wave propagation in the M87 jet, identifying superluminal wave speeds and modeling oscillations as MHD waves or instabilities.

Findings

Oscillations propagate downstream with a wavelength of 9-10 mas.

Observed wave speeds are approximately 2.7-2.9 times the speed of light.

Oscillations may be due to MHD waves or jet instabilities.

Abstract

We present an in-depth analysis of transverse oscillations in the M87 jet, as identified in our previous study (Ro et al. 2023a), which reported oscillatory patterns with a characteristic period of $\sim$1 year in the edge-brightened jet structure extending up to 12\,mas from the core. This work is based on high-cadence KaVA 22\,GHz observations conducted from December 2013 to June 2016. By analyzing the transverse velocity profiles and the spatial evolution of the oscillations, we find that the oscillations propagate downstream along the jet, with a wavelength of $\sim9-10$\,mas. A single-mode sinusoidal wave model applied to the ridge lines successfully reproduces the observed transverse oscillations and yields superluminal wave speeds of $\sim2.7-2.9\,c$, consistent with the bulk jet velocity in this region. These findings suggest that the transverse oscillations may be interpreted either as transverse MHD waves -- possibly excited by jet precession, nutation, or quasi-periodic magnetic flux eruptions near the central engine -- or as manifestations of jet instabilities, such as current-driven instabilities (CDIs). Further investigation is required to distinguish between these scenarios and to clarify the dominant physical mechanism.