ERRATIC JET WOBBLING IN THE BL LACERTAE OBJECT OJ287 REVEALED BY SIXTEEN YEARS OF 7 mm VLBA OBSERVATIONS

Abstract

We present the results from an ultra-high-resolution 7 mm Very Long Baseline Array (VLBA) study of the relativistic jet in the BL Lacertae object OJ287 from 1995 to 2011 containing 136 total intensity images. Analysis of the image sequence reveals a sharp jet-position-angle swing by > 100 • during [2004,2006], as viewed in the plane of the sky, that we interpret as the crossing of the jet from one side of the line of sight to the other during a softer and longer term swing of the inner jet. Modulating such long term swing, our images also show for the first time a prominent erratic wobbling behavior of the innermost ∼ 0.4 mas of the jet with fluctuations in position angle of up to ∼ 40 • over time scales ∼ 2 yr. This is accompanied by highly superluminal motions along non-radial trajectories, which reflect the remarkable non-ballistic nature of the jet plasma on these scales. The erratic nature and short time scales of the observed behavior rules out scenarios such as binary black hole systems, accretion disk precession, and interaction with the ambient medium as possible origins of the phenomenon on the scales probed by our observations, although such processes may cause longer-term modulation of the jet direction. We propose that variable asymmetric injection of the jet flow; perhaps related to turbulence in the accretion disk; coupled with hydrodynamic instabilities, leads to the non-ballistic dynamics that cause the observed non-periodic changes in the direction of the inner jet.

OJ 287 as a Rotating Helix

“…A detailed discussion of the magnetic field is beyond the scope of this paper, but we can report that the EVPA in the inner jet region in the post-jump era was along the jet, meaning that the toroidal components of the magnetic field were dominant. This also was seen at 43 GHz; see (Agudo et al [5] Figure 2).…”

“…It became visible in 2004 at 43 GHz, when its angular separation from the core became large enough. The opacity in the core was not responsible for the appearance of this new component, as shown by the flux density curve for the core in (Agudo et al [5] Figure 3). That flux first rose as the blend of core plus new component appeared, then dropped sharply as the blend was separated into two components each with its own flux.…”

“…This work was motivated in part by the report of a jump in Position Angle (PA) of the jet by Agudo et al [5] at 43 GHz in 2004, and we sought a similar effect at 15 GHz. Another motivation was our results on BL Lac [6,7], especially the identification of moving components and patterns as MHD waves.…”

“…However, we shall see that in OJ 287 the ridgelines are ambiguous at some epochs, implying a more complex jet structure. OJ 287 has also been studied in detail at many epochs at 43 and 86 GHz by Agudo et al [5] and Hodgson et al [1], respectively. We have less angular resolution at 15 GHz but are more sensitive to the jet structure, and our emphasis is on the jet and not on the inner regions that are accessible to the higher frequencies.…”

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

“…Jet rotation has so far not been directly detected in AGN jets, with the reported rotational speeds clearly related to the rotation of patterns inside the flow (e.g. Lobanov et al 2003;Agudo et al 2007Agudo et al , 2012Cohen et al 2015) resulting from plasma instability in the flow. If our measured rotation is associated with conserved angular momentum in the jet and if the jet is launched from the accretion disk via the BP mechanism, its direction should be the same as the rotational direction of the disk itself.…”