Kinematics of the jet in M 87 on scales of 100–1000 Schwarzschild radii

Abstract

Context. Very long baseline interferometry (VLBI) imaging of radio emission from extragalactic jets provides a unique probe of physical mechanisms governing the launching, acceleration, and collimation of relativistic outflows. Aims. VLBI imaging of the jet in the nearby active galaxy M 87 enables morphological and kinematic studies to be done on linear scales down to ∼ 100 Schwarzschild radii (R s ). Methods. The two-dimensional structure and kinematics of the jet in M 87 (NGC 4486) have been studied by applying the Waveletbased Image Segmentation and Evaluation (WISE) method to 11 images obtained from multi-epoch Very Long Baseline Array (VLBA) observations made in January-August 2007 at 43 GHz (λ = 7 mm).Results. The WISE analysis recovers a detailed two-dimensional velocity field in the jet in M 87 at sub-parsec scales. The observed evolution of the flow velocity with distance from the jet base can be explained in the framework of MHD jet acceleration and Poynting flux conversion. A linear acceleration regime is observed up to z obs ∼ 2 mas. The acceleration is reduced at larger scales, which is consistent with saturation of Poynting flux conversion. Stacked cross correlation analysis of the images reveals a pronounced stratification of the flow. The flow consists of a slow, mildly relativistic layer (moving at β ∼ 0.5 c), associated either with instability pattern speed or an outer wind, and a fast, accelerating stream line (with β ∼ 0.92, corresponding to a bulk Lorentz factor γ ∼ 2.5). A systematic difference of the apparent speeds in the northern and southern limbs of the jet is detected, providing evidence for jet rotation. The angular velocity of the magnetic field line associated with this rotation suggests that the jet in M87 is launched in the inner part of the disk, at a distance r 0 ∼ 5 R s from the central engine.Conclusions. The combined results of the analysis imply that MHD acceleration and conversion of Poynting flux to kinetic energy play the dominant roles in collimation and acceleration of the flow in M 87.

The Structure and Propagation of the Misaligned Jet M87

“…The presence of superluminal motions near the jet base is in contrast to that picture, implying that an efficient magnetic-to-kinetic conversion takes place rather near the jet base. A quite similar acceleration profile has also been reported by Mertens et al based on VLBA 43 GHz data [31].…”

“…On the other hand, such a scattering of the observed speeds can also be caused if the sampling interval is sparse. To date, the most detailed program was performed by Walker et al in 2007Walker et al in -2008 with VLBA at 43 GHz [19,31], where they monitored M87 every 3 weeks (January-August 2007) or ∼5 days (January-April 2008). They found fast (superluminal) motions that might be missed in the other monitoring programs.…”

A Jet Source of Event Horizon Telescope Correlated Flux in M87

“…This is motivated theoretically as a boundary condition in the equatorial plane and VLBI observations that indicate apparent velocities, v app = 0.1−0.4c, < 2 mas from the core. Furthermore, v app increases from ∼ 0.15c to 1.5 −2.0c in the first 10 mas indicating strong magnetic forces, requiring the jet base to be magnetically dominated (Mertens et al 2016;Hada et al 2016Hada et al , 2017a. In this Poynting flux dominated regime, the conservation of angular momentum condition provides a constraint on the azimuthal magnetic field,…”

“…130 mas is large compared to the 0.040 mas EHTC. However, inspection of 5 GHz, Hada et al (2014), 15 GHz, Lister et al (2013), 43 GHz, (Mertens et al (2016); Hada et al (2014) and 86 GHz, Hada et al (2016); Kim et al (2016), VLBI images indicate that there is no strong optically thick component between 0.06 mas and 100 mas that could produce a significant contribution to the opticl/UV flux density -the unresolved 86 GHz core is the only possible source of the HST detected flux. We can get a tight bound on the high frequency synchrotron tail by assuming that most of this emission is associated with the smaller EHTC.…”

Probing the Internal Structure of Magnetized, Relativistic Jets with Numerical Simulations

“…From an observational point of view, delving into these models demands the resolution of the structure across the jet with the highest angular resolutions, looking for the imprints of helical magnetic fields [7], flow stratification [8,9], and shocks. Relying on a magneto-fluid dynamical description, numerical simulations can help to characterize the internal structure of jets (transversal structure, magnetic field structure, internal shocks).…”