A Phase Lag between Disk and Corona in GRMHD Simulations of Precessing Tilted Accretion Disks

TL;DR

This study uses GRMHD simulations to reveal a phase lag between the corona and the disc in tilted accretion disks around spinning black holes, impacting jet formation and X-ray emission timing.

Contribution

First GRMHD simulations of precessing tilted accretion disks show a corona lag behind the disc, affecting emission and jet dynamics in black hole systems.

Findings

Corona lags behind the disc by over 10 degrees.

Hard X-ray emission lags softer emission, explaining observed X-ray timing.

Weak jets stall when encountering the lagging corona.

Abstract

In the course of its evolution, a black hole (BH) accretes gas from a wide range of directions. Given a random accretion event, the typical angular momentum of an accretion disc would be tilted by $\sim$60$^\circ$ relative to the BH spin. Misalignment causes the disc to precess at a rate that increases with BH spin and tilt angle. We present the first general-relativistic magnetohydrodynamic (GRMHD) simulations spanning a full precession period of highly tilted (60$^\circ$), moderately thin ($h/r=0.1$) accretion discs around a rapidly spinning ($a\simeq0.9$) BH. While the disc and jets precess in phase, we find that the corona, sandwiched between the two, lags behind by $\gtrsim 10^{\circ}$. For spectral models of BH accretion, the implication is that hard non-thermal (corona) emission lags behind the softer (disc) emission, thus potentially explaining some properties of the hard energy lags seen in Type-C low frequency quasi-periodic oscillations in X-Ray binaries. While strong jets are unaffected by this disc-corona lag, weak jets stall when encountering the lagging corona at distances $r \sim 100$ black hole radii. This interaction may quench large-scale jet formation.