614 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
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
Black Hole Spin-Orbit Misalignment in Galactic X-ray Binaries
In black hole X-ray binaries, a misalignment between the spin axis of the
black hole and the orbital angular momentum can occur during the supernova
explosion that forms the compact object. In this letter we present population
synthesis models of Galactic black hole X-ray binaries, and study the
probability density function of the misalignment angle, and its dependence on
our model parameters. In our modeling, we also take into account the evolution
of misalignment angle due to accretion of material onto the black hole during
the X-ray binary phase. The major factor that sets the misalignment angle for
X-ray binaries is the natal kick that the black hole may receive at its
formation. However, large kicks tend to disrupt binaries, while small kicks
allow the formation of XRBs and naturally select systems with small
misalignment angles. Our calculations predict that the majority (>67%) of
Galactic field BH XRBs have rather small (>10 degrees) misalignment angles,
while some systems may reach misalignment angles as high as ~90 degrees and
even higher. This results is robust among all population synthesis models. The
assumption of small small misalignment angles is extensively used to
observationally estimate black hole spin magnitudes, and for the first time we
are able to confirm this assumption using detailed population synthesis
calculations.Comment: 30 pages, 2 figures, submitted to ApJ
Excitation of Trapped Waves in Simulations of Tilted Black Hole Accretion Disks with Magnetorotational Turbulence
We analyze the time dependence of fluid variables in general relativistic,
magnetohydrodynamic simulations of accretion flows onto a black hole with
dimensionless spin parameter a/M=0.9. We consider both the case where the
angular momentum of the accretion material is aligned with the black hole spin
axis (an untilted flow) and where it is misaligned by 15 degrees (a tilted
flow). In comparison to the untilted simulation, the tilted simulation exhibits
a clear excess of inertial variability, that is, variability at frequencies
below the local radial epicyclic frequency. We further study the radial
structure of this inertial-like power by focusing on a radially extended band
at 118 (M/10Msol)^-1 Hz found in each of the three analyzed fluid variables.
The three dimensional density structure at this frequency suggests that the
power is a composite oscillation whose dominant components are an over dense
clump corotating with the background flow, a low order inertial wave, and a low
order inertial-acoustic wave. Our results provide preliminary confirmation of
earlier suggestions that disk tilt can be an important excitation mechanism for
inertial waves.Comment: 8 Pages, 6 Figures, accepted for publication in Ap
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
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