A phase lag between disc and corona in GRMHD simulations of precessing tilted accretion discs

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 ∼60∘ relative to the BH spin. Misalignment causes the disc to precess at a rate that increases with BH spin and depends on disc morphology. We present general-relativistic magnetohydrodynamic (GRMHD) simulations spanning a full precession period of highly tilted (60∘), moderately thin (h/r=0.1) accretion discs around a rapidly spinning (a≃0.9) BH. While the disc and jets precess in phase, we find that the disc wind/corona, sandwiched between the two, lags behind by ≳10°. 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 can stall when encountering the lagging corona at distances r∼100 BH radii. This interaction may quench large-scale jet formation.