The Structure and Dynamics of the Sub-parsec Scale Jet in M87 Based on 50 VLBA Observations Over 17 Years at 43 GHz (1802.06166v1)

Abstract: The central radio source in M87 provides the best opportunity to study jet formation because it has a large angular size for the gravitational radius of the black hole and has a bright jet that is well resolved by VLBI observations. We present intensive monitoring observations from 2007 and 2008, plus roughly annual observations that span 17 years, all made with the the Very Long Baseline Array at 43 GHz with a resolution of about 30 by 60 Rs. Our high-dynamic-range images clearly show the wide-opening-angle structure and the counter-jet. The jet and counter-jet are nearly symmetric in the inner 1.5 milli-arcseconds (mas; 0.12 pc in projection) with both being edge brightened. Both show deviations from parabolic shape in the form of an initial rapid expansion and subsequent contraction followed by further rapid expansion and, beyond the visible counter-jet, subsequent collimation. Proper motions and counter-jet/jet intensity ratios both indicate acceleration from apparent speeds of $\lesssim 0.5c$ to $\gtrsim 2c$ in the inner about 2 mas and suggest a helical flow. The jet displays a sideways shift with an approximately 8 to 10 year quasi-periodicity. The shift propagates outwards non-ballistically and significantly more slowly than the flow speed revealed by the fastest moving components. Polarization data show a systematic structure with magnetic field vectors that suggest a toroidal field close to the core.

• The paper presents a detailed analysis of 50 VLBA observations over 17 years, uncovering multiphase expansion and collimation in M87's jet.
• It demonstrates that jet components accelerate from subluminal to ~2c with evidence of clockwise helical motions and a sheath-spine structure.
• The study identifies long-term transverse motions suggesting an 8-10 year periodic variation, likely linked to instability wave propagation.

Overview of the Sub-Parsec Scale Jet in M87

The paper of M87's sub-parsec scale jet presents a comprehensive analysis based on a dataset of images accumulated using the Very Long Baseline Array (VLBA) at 43 GHz over 17 years, with a focus on 50 observations. This paper meticulously details the jet's dynamical structure, expansion characteristics, and motion analysis. Key topics include the jet's expansion and collimation, proper motion implications, jet/counter-jet brightness ratio, internal structure, and long-term evolutionary motion patterns in the jet.

Jet Structure and Expansion

The jet in M87 is characterized by a wide-opening angle and pronounced edge-brightening, facilitating a detailed paper of its collimation and expansion regimes. The data indicates three distinct expansion-collimation stages: an initial rapid expansion close to the core followed by contraction, and two subsequent expansions at further distances. This multiphase structural complexity contrasts with the previous simplistic parabolic models, indicating significant interaction with the surrounding medium and providing insights into the jet's launching region dynamics.

Dynamics of Jet Flow

The paper reports evident acceleration in jet components from subluminal to mildly superluminal speeds, with maximum observed speeds of approximately 2c at distances beyond 2 mas from the core. This acceleration confirms prior analyses and is pivotal for understanding jet-launching mechanisms. Comparison of component speeds and jet/counter-jet intensity ratios yield insights into jet material speeds, revealing a clockwise helical motion around the jet’s axis. This finding is significant for understanding the beaming effects impacting jet emission observations.

Internal Jet Structure

Transverse slice profiles of the jet and counter-jet imply the existence of a high-emissivity sheath surrounding a darker central spine, with an estimated sheath thickness of about one-fourth the jet radius. The inferred dynamics of this structure suggest the possibility of a high-speed, Doppler-deboosted spine that may reach relativistic velocities, offering potential explanations for observed superluminal motion beyond the core.

Long-Term Jet Variability

The analysis of long-term observations reveals transverse jet movements, possibly linked to slow periodic variations in the source's direction, with a suggested period of 8 to 10 years. These changes propagate along the jet at speeds lower than fast material components but consistent with instability wave propagation models such as the Kelvin-Helmholtz instability, indicating a complex interaction between the jet and its cocooning medium.

Implications and Future Directions

The insights from the extensive dataset suggest a sheath-spine jet model with significant dynamical evolution near the black hole. The spectral and structural asymmetries observed across jet and counter-jet components inform on the internal and external dynamics critical for theoretical modeling of jet propagation and emission. Future research directions involve further high-resolution observations, potential Faraday rotation studies, and advanced modeling efforts to simulate M87 dynamics accurately. Understanding these intricate details will enhance our comprehension of relativistic jet phenomena and their role in shaping galactic environments.