88 research outputs found
Effective Inner Radius of Tilted Black Hole Accretion Disks
One of the primary means of determining the spin of an astrophysical black
hole is by actually measuring the inner radius of a surrounding accretion disk
and using that to infer the spin. By comparing a number of different estimates
of the inner radius from simulations of tilted accretion disks with differing
black-hole spins, we show that such a procedure can give quite wrong answers.
Over the range 0 <= a/M <= 0.9, we find that, for moderately thick disks (H/r ~
0.2) with modest tilt (15 degrees), the inner radius is nearly independent of
spin. This result is likely dependent on tilt, such that for larger tilts, it
may even be that the inner radius would increase with increasing spin. In the
opposite limit, we confirm through numerical simulations of untilted disks
that, in the limit of zero tilt, the inner radius recovers approximately the
expected dependence on spin.Comment: 5 pages, 4 figures, accepted to ApJ Letter
High-Frequency and Type-C QPOs from Oscillating, Precessing Hot, Thick Flow
Motivated by recent studies showing an apparent correlation between the
high-frequency quasi-periodic oscillations (QPOs) and the low-frequency, type-C
QPO in low-mass, black hole X-ray binaries (LMXBs), we explore a model that
explains all three QPOs in terms of an oscillating, precessing hot flow in the
truncated-disk geometry. Our model favors attributing the two high-frequency
QPOs, often occurring in a near 3:2 frequency ratio, to the breathing and
vertical epicyclic frequency modes of the hot, thick flow, although we can not
rule out the Keplerian and m=-1 radial epicyclic modes. In either case, the
type-C QPO is attributed to precession. The correlation of the QPOs comes from
the fact that all three frequencies are associated with the same geometrical
structure. While the exact QPO frequencies are sensitive to the black hole mass
and spin, their evolution over the course of an outburst is mainly tied to the
truncation radius between the geometrically thin, optically thick disk and the
inner, hot flow. We show that, in the case of the LMXB GRO J1655-40, this model
can explain the one simultaneous observation of all three QPOs and that an
extrapolation of the model appears to match lower frequency observations where
only two of the three components are seen. Thus, this model may be able to
unify multiple QPO observations using the properties of a single, simple,
geometrical model.Comment: 7 pages, 4 figures, accepted to MNRA
Application of the Cubed-Sphere Grid to Tilted Black-Hole Accretion Disks
In recent work we presented the first results of global general relativistic
magnetohydrodynamic (GRMHD) simulations of tilted (or misaligned) accretion
disks around rotating black holes. The simulated tilted disks showed dramatic
differences from comparable untilted disks, such as asymmetrical accretion onto
the hole through opposing "plunging streams" and global precession of the disk
powered by a torque provided by the black hole. However, those simulations used
a traditional spherical-polar grid that was purposefully underresolved along
the pole, which prevented us from assessing the behavior of any jets that may
have been associated with the tilted disks. To address this shortcoming we have
added a block-structured "cubed-sphere" grid option to the Cosmos++ GRMHD code,
which will allow us to simultaneously resolve the disk and polar regions. Here
we present our implementation of this grid and the results of a small suite of
validation tests intended to demonstrate that the new grid performs as
expected. The most important test in this work is a comparison of identical
tilted disks, one evolved using our spherical-polar grid and the other with the
cubed-sphere grid. We also demonstrate an interesting dependence of the
early-time evolution of our disks on their orientation with respect to the grid
alignment. This dependence arises from the differing treatment of current
sheets within the disks, especially whether they are aligned with symmetry
planes of the grid or not.Comment: 15 pages, 11 figures, submitted to Ap
Hydrodynamic Simulations of Tilted Thick-Disk Accretion onto a Kerr Black Hole
We present results from fully general relativistic three-dimensional
numerical studies of thick-disk accretion onto a rapidly-rotating (Kerr) black
hole with a spin axis that is tilted (not aligned) with the angular momentum
vector of the disk. We initialize the problem with the solution for an aligned,
constant angular momentum, accreting thick disk, which is then allowed to
respond to the Lense-Thirring precession of the tilted black hole. The
precession causes the disk to warp, beginning at the inner edge and moving out
on roughly the Lense-Thirring precession timescale. The propagation of the warp
stops at a radius in the disk at which other dynamical timescales, primarily
the azimuthal sound-crossing time, become shorter than the precession time. At
this point, the warp effectively freezes into the disk and the evolution
becomes quasi-static, except in cases where the sound-crossing time in the bulk
of the disk is shorter than the local precession timescale. We see evidence
that such disks undergo near solid-body precession after the initial warping
has frozen in. Simultaneous to the warping of the disk, there is also a
tendency for the midplane to align with the symmetry plane of the black hole
due to the preferential accretion of the most tilted disk gas. This alignment
is not as pronounced, however, as it would be if more efficient angular
momentum transport (e.g. from viscosity or magneto-rotational instability) were
considered.Comment: 51 pages, 34 figures (color figures reduced for this archive), this
version accepted to Ap
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