Description
Many accreting black holes and neutron stars exhibit rapid variability
in their X-ray light curves, termed quasi-periodic oscillations (QPOs);
The commonly observed type is the low-frequency (< 10 Hz), type-C QPOs,
thought to originate from the Lense-Thirring precession of a hot,
geometrically thick accretion flow misaligned with the black hole spin
axis. Numerous simulations in the past have demonstrated that such
misaligned accretion flows precess at frequencies matching the
observations. However, none of these simulations involves more realistic
geometries like a truncated disc (a geometrically thin disc surrounding
an inner hot accretion flow), leaving an open question: What is the
effect of the outer, thin disc on the precession of the inner, hot
accretion flow? To address this, we perform GRMHD simulations of a
truncated disc with the inner, hot flow misaligned with the spin axis of
the black hole. The key finding of our simulations is that the presence
of an outer-thin disc decreases the precession rate of the inner torus
by nearly 95 per cent due to the exchange of the angular momentum
between the inner and outer discs, thus relieving some of the remaining
tensions between the Lense-Thirring precession model and observations.
The misalignment also excites variability in the inner, hot flow at
higher frequencies, which is otherwise absent in the aligned discs.
