Stochastic Wobble of Accretion Discs and Jets from Turbulent Rocket Torques

Abstract

Models of accretion discs and their associated outflows often incorporate assumptions of axisymmetry and symmetry across the disc plane. However, for turbulent discs these symmetries only apply to averaged quantities and do not apply locally. The local asymmetries can induce local imbalances in outflow power across the disc mid-plane, which can in turn induce local tilting torques. Here we calculate the effect of the resulting stochastic torques on disc annuli that are a consequence of standard mean field accretion disc models. The torques induce a random walk of the vector perpendicular to the plane of each averaged annulus. This random walk is characterized by a radially dependent diffusion coefficient which we calculate for small angle tilt. We use the coefficient to calculate a radially dependent time scale for annular tilt and associated jet wobble. The wobble time depends on the square of the wander angle so the age of a given system determines the maximum wobble angle. We apply this to examples of blazars, young stellar objects and binary engines of pre-planetary nebulae and microquasars. It is noteworthy that for an averaging time $t_w\sim 3$ days, we estimate a wobble angle for jets in SS433 of $\theta \sim 0.8$ degrees, not inconsistent with observational data. In general the non-periodic nature of the stochastic wobble could distinguish it from faster periodic jet precession.

12 pages, accepted by MNRAS

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