Do jets precess... or even move at all?

TL;DR

The paper argues that black hole jets are unlikely to precess due to disc effects on short timescales, and suggests alternative physics like disc warping or tearing for observed rapid precession.

Contribution

It challenges the common assumption that jet precession results from disc effects, proposing that additional disc physics is necessary to explain rapid precession.

Findings

Disc generally has too little angular momentum to cause jet precession.

AGN jet directions are stable on short timescales, changing only over millions of years.

Rapid jet precession indicates non-standard disc physics like warping or tearing.

Abstract

Observations of accreting black holes often provoke suggestions that their jets precess. The precession is usually supposed to result from a combination of the Lense-Thirring effect and accretion disc viscosity. We show that this is unlikely for any type of black hole system, as the disc generally has too little angular momentum compared with a spinning hole to cause any significant movement of the jet direction across the sky on short timescales. Uncorrelated accretion events, as in the chaotic accretion picture of active galactic nuclei, change AGN jet directions only on timescales \gtrsim 10^7 yr. In this picture AGN jet directions are stable on shorter timescales, but uncorrelated with any structure of the host galaxy, as observed. We argue that observations of black-hole jets precessing on timescales short compared to the accretion time would be a strong indication that the accretion disc, and not the standard Blandford-Znajek mechanism, is responsible for driving the jet. This would be particularly convincing in a tidal disruption event. We suggest that additional disc physics is needed to explain any jet precession on timescales short compared with the accretion time. Possibilities include the radiation warping instability, or disc tearing.