Jet tilt instability from stream–disc interactions in MAD discs

ABSTRACT Magnetically arrested accretion discs (MADs) around a rapidly rotating black hole (BH) have been proposed as a model for jetted tidal disruption events (TDEs). However, the stream and disc interact strongly at times, and this will lead to different dynamics than expected in the standard MAD model. Here, we employ global GRMHD simulations of an MAD disc interacting with an injected stream with a penetrating pericenter $R_p\sim 10 r_g$ and a range of density contrasts $f_\rho \equiv \rho _d/\rho _s$, or how dense the disc is relative to the stream. We demonstrate for the first time that an MAD or semi-MAD state can be sustained and jets powered by the BH spin can be produced even when the stream is much denser than the disc, i.e. in the first month(s) of a jetted TDE. We also demonstrate that the strength of the self-intersection shock decreases as $f_\rho$, and time, increases. The jet or funnel can become significantly tilted (by $10{\!-\!}30^\circ$) due to the self-intersection outflow when $f_\rho \le 0.1$. In models with a powerful jet and $f_\rho \le 0.01$, the tilted jet interacts with and ultimately tilts the disc by as much as 23 deg from the incoming stream and this tilted state is stable for the duration of the simulation. As $f_\rho$ increases, the tilt of the jet and disc is expected to realign with the BH spin once $f_\rho \ge 0.1$. The jet tilt could rapidly realign due to outer disc collapse or the self-intersection radius increasing. Our results provide an alternative explanation for the observed X-ray jet shut-off in days-weeks in jetted TDEs.