60 research outputs found
Non-linear resonance in nearly geodesic motion in low-mass X-ray binaries
We have explored the ideas that parametric resonance affects nearly geodesic
motion around a black hole or a neutron star, and that it may be relevant to
the high frequency (twin) quasi-periodic oscillations occurring in some
low-mass X-ray binaries. We have assumed the particles or fluid elements of an
accretion disc to be subject to an isotropic perturbation of a hypothetical but
rather general form. We find that the parametric resonance is indeed excited
close to the radius where epicyclic frequencies of radial and meridional
oscillations are in a 2:3 ratio. The location and frequencies of the highest
amplitude excitation vary with the strength of the perturbation. These results
agree with actual frequency ratios of twin kHz QPOs that have been reported in
some black hole candidates, and they may be consistent also with correlation of
the twin peaks in Sco X-1.Comment: 5 pages; accepted for publication in PAS
Kinematic Density Waves in Accretion Disks
When thin accretion disks around black holes are perturbed, the main
restoring force is gravity. If gas pressure, magnetic stresses, and radiation
pressure are neglected, the disk remains thin as long as orbits do not
intersect. Intersections would result in pressure forces which limit the growth
of perturbations. We find that a discrete set of perturbations is possible for
which orbits remain non-intersecting for arbitrarily long times. These modes
define a discrete set of frequencies. We classify all long-lived perturbations
for arbitrary potentials and show how their mode frequencies are related to
pattern speeds computed from the azimuthal and epicyclic frequencies. We show
that modes are concentrated near radii where the pattern speed has vanishing
radial derivative. We explore these modes around Kerr black holes as a possible
explanation for the high-frequency quasi-periodic oscillations of black hole
binaries such as GRO J1655-40. The long-lived modes are shown to coincide with
diskoseismic waves in the limit of small sound speed. While the waves have long
lifetime, they have the wrong frequencies to explain the pairs of
high-frequency quasi-periodic oscillations observed in black hole binaries.Comment: 28 pages, 6 figures; extended comparison with diskoseismology; added
reference to astro-ph/060368
Effect of turbulent diffusion on iron abundance profiles
We compare the observed peaked iron abundance profiles for a small sample of
groups and clusters with the predictions of a simple model involving the metal
ejection from the brightest galaxy and the subsequent diffusion of metals by
stochastic gas motions. Extending the analysis of Rebusco et al. (2005) we
found that for 5 out of 8 objects in the sample an effective diffusion
coefficient of the order of cm s is needed. For AWM4,
Centaurus and AWM7 the results are different suggesting substantial
intermittence in the process of metal spreading across the cluster. There is no
obvious dependence of the diffusion coefficient on the mass of the system.
We also estimated the characteristic velocities and the spatial scales of the
gas motions needed to balance the cooling losses by the dissipation of the same
gas motions. A comparison of the derived spatial scales and the sizes of
observed radio bubbles inflated in the ICM by a central active galactic nucleus
(AGN) suggests that the AGN/ICM interaction makes an important (if not a
dominant) contribution to the gas motions in the cluster cores.Comment: 12 pages, 4 figures, accepted for publication on MNRA
Metal mixing by buoyant bubbles in galaxy clusters
Using a series of three-dimensional, hydrodynamic simulations on an adaptive
grid, we have performed a systematic study on the effect of bubble-induced
motions on metallicity profiles in clusters of galaxies. In particular, we have
studied the dependence on the bubble size and position, the recurrence times of
the bubbles, the way these bubbles are inflated and the underlying cluster
profile. We find that in hydrostatic cluster models, the resulting metal
distribution is very elongated along the direction of the bubbles. Anisotropies
in the cluster or ambient motions are needed if the metal distribution is to be
spherical. In order to parametrise the metal transport by bubbles, we compute
effective diffusion coefficients. The diffusion coefficients inferred from our
simple experiments lie at values of around cms at a
radius of 10 kpc. The runs modelled on the Perseus cluster yield diffusion
coefficients that agree very well with those inferred from observations.Comment: 17 pages, 12 figures. Accepted by MNRA
Stability of radiation-pressure dominated disks. I. The dispersion relation for a delayed heating alpha-viscosity prescription
We derive and investigate the dispersion relation for accretion disks with
retarded or advanced heating. We follow the alpha-prescription but allow for a
time offset (\tau) between heating and pressure perturbations, as well as for a
diminished response of heating to pressure variations. We study in detail
solutions of the dispersion relation for disks with radiation-pressure fraction
1 - \beta . For \tau <0 (delayed heating) the number and sign of real solutions
for the growth rate depend on the values of the time lag and the ratio of
heating response to pressure perturbations, \xi . If the delay is larger than a
critical value (e.g., if \Omega \tau <-125 for \alpha =0.1, \beta =0 and \xi
=1) two real solutions exist, which are both negative. These results imply that
retarded heating may stabilize radiation-pressure dominated accretion disks.Comment: 11 pages, 10 figures, to be submitted to A&
The Interpretations For the Low and High Frequency QPO Correlations of X-ray Sources Among White Dwarfs, Neutron Stars and Black Holes
It is found that there exists an empirical linear relation between the high
frequency \nhigh and low frequency \nlow of quasi-periodic oscillations
(QPOs) for black hole candidate (BHC), neutron star (NS) and white dwarf (WD)
in the binary systems, which spans five orders of magnitude in frequency.
For the NS Z (Atoll) sources,
and are identified as the lower kHz QPO frequency
and horizontal branch oscillations (HBOs) \nh (broad noise components); for
the black hole candidates and low-luminosity neutron stars, they are the QPOs
and broad noise components at frequencies between 1 and 10 Hz; for WDs, they
are the ``dwarf nova oscillations'' (DNOs) and QPOs of cataclysmic variables
(CVs). To interpret this relation, our model ascribes to the
Alfv\'en wave oscillation frequency at a preferred radius and to
the same mechanism at another radius. Then, we can obtain \nlow = 0.08
\nhigh and the relation between the upper kHz QPO frequency \nt and HBO to
be \nh \simeq 56 ({\rm Hz}) (\nt/{\rm kHz})^{2}, which are in accordance with
the observed empirical relations. Furthermore, some implications of model are
discussed, including why QPO frequencies of white dwarfs and neutron stars span
five orders of magnitude in frequency. \\Comment: 11 pages, 1 figure, accepted by PAS
QPOs: Einstein's gravity non-linear resonances
There is strong evidence that the observed kHz Quasi Periodic Oscillations
(QPOs) in the X-ray flux of neutron star and black hole sources in LMXRBs are
linked to Einstein's General Relativity. Abramowicz&Klu\'zniak (2001) suggested
a non-linear resonance model to explain the QPOs origin: here we summarize
their idea and the development of a mathematical toy-model which begins to
throw light on the nature of Einstein's gravity non-linear oscillations.Comment: Proceeding of the Einstein's Legacy, Munich 200
The correlations and anticorrelations in QPO data
Double peak kHz QPO frequencies in neutron star sources varies in time by a
factor of hundreds Hz while in microquasar sources the frequencies are fixed
and located at the line \nu_2 = 1.5 \nu_1 in the frequency-frequency plot. The
crucial question in the theory of twin HFQPOs is whether or not those observed
in neutron-star systems are essentially different from those observed in black
holes. In black hole systems the twin HFQPOs are known to be in a 3:2 ratio for
each source. At first sight, this seems not to be the case for neutron stars.
For each individual neutron star, the upper and lower kHz QPO frequencies,
\nu_2 and \nu_1, are linearly correlated, \nu_2=A \nu_1 + B, with the slope A <
1.5, i.e., the frequencies definitely are not in a 1.5 ratio. In this
contribution we show that when considered jointly on a frequency-frequency
plot, the data for the twin kHz QPO frequencies in several (as opposed to one)
neutron stars uniquely pick out a certain preferred frequency ratio that is
equal to 1.5 for the six sources examined so far.Comment: 3 pages, 1 figure, Astronomische Nachrichten, in pres
- âŠ