Michal Bregman, Tal Alexander
An accreting massive black hole (MBH) in a galactic nucleus is surrounded by
a dense stellar cluster. We analyze and simulate numerically the evolution of a
thin accretion disk due to its internal viscous torques, due to the
frame-dragging torques of a spinning MBH (the Bardeen-Petterson effect) and due
to the orbit-averaged gravitational torques by the stars (Resonant Relaxation).
We show that the evolution of the MBH mass accretion rate, the MBH spin growth
rate, and the covering fraction of the disk relative to the central ionizing
continuum source, are all strongly coupled to the stochastic fluctuations of
the stellar potential via the warps that the stellar torques excite in the
disk. These lead to fluctuations by factors of up to a few in these quantities
over a wide range of timescales, with most of the power on timescales
>~(M_bh/M_d)P(R_d), where M_bh and M_d are the masses of the MBH and disk, and
P is the orbital period at the disk's mass-weighted mean radius R_d. The
response of the disk is stronger the lighter it is and the more centrally
concentrated the stellar cusp. As proof of concept, we simulate the evolution
of the low-mass maser disk in NGC4258, and show that its observed O(10 deg)
warp can be driven by the stellar torques. We also show that the frame-dragging
of a massive AGN disk couples the stochastic stellar torques to the MBH spin
and can excite a jitter of a few degrees in its direction relative to that of
the disk's outer regions.
View original:
http://arxiv.org/abs/1109.5384
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