Discovery of Sub- To Superluminal Motions in the M87 Jet: An Implication of Acceleration From Sub-Relativistic to Relativistic Speeds

Abstract

The velocity field of the M87 jet from milli-arcsecond (mas) to arcsecond scales is extensively investigated together with new radio images taken by EVN observations. We detected proper motions of components located at between 160 mas from the core and the HST-1 complex for the first time. Newly derived velocity fields exhibits a systematic increase from sub-to-superluminal speed in the upstream of HST-1. If we assume that the observed velocities reflect the bulk flow, we here suggest that the M87 jet may be gradually accelerated through a distance of 10 6 times of the Schwarzschild radius of the supermassive black hole. The acceleration zone is co-spatial with the jet parabolic region, which is interpreted as the collimation zone of the jet (Asada & Nakamura). The acceleration and collimation take place simultaneously, which we suggest a characteristic of magnetohydrodynamic flows. Distribution of the velocity field has a peak at HST-1, which is considered as the site of overcollimation, and shows a deceleration downstream of HST-1 where the jet is conical. Our interpretation of the velocity map in the M87 jet gives a hypothesis in AGNs that the acceleration and collimation zone of relativistic jets extends over the whole scale within the sphere of influence of the supermassive black hole.

The Structure and Propagation of the Misaligned Jet M87

“…A transition from subluminal to superluminal motions at a large distance (>100 mas) has been reported by a yearly EVN monitor program [21]. They suggest that efficient magnetic acceleration occurs from >100 mas of the jet.…”

“…In addition, cm-VLBI offers high-cadence monitoring programs. The M87 jet has intensively been monitored by many VLBI programs over the past years, pinpointing the sites of active gamma-ray flares (e.g., [13][14][15][16][17]) as well as measuring the jet motions (e.g., [18][19][20][21]). Such excellent capabilities of imaging and monitoring of the low-frequency VLBI can be further improved by the recent rapid improvement of data recording rate.…”

Structural Transition in the NGC 6251 Jet: An Interplay With the Supermassive Black Hole and Its Host Galaxy

“…Interestingly, the simultaneity of gradual acceleration and collimation is also detected in the M87 jet with VLBI monitoring observations (NA13; Asada et al 2014). However, such a structural study of a jet over multiple orders of magnitude in axial distance has been conducted only for M87 so far.…”

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

“…Acceleration profile of the jet in M 87. We plot the Lorentz factor measured from WISE analysis of VLBA images at 7 mm (blue), 2 cm (green), from Cheung et al (2007) using VLBA at 1.7 GHz (red), from Giroletti et al (2012) and Asada et al (2014) using EVN at 1.7 GHz (magenta and gray respectively), and from optical images using HST ) (orange). The Lorentz factor and de-projected z axis are computed assuming θ = 18°(z ∼ 3.2 z obs ).…”