29 research outputs found
Entropic-acoustic instability of shocked Bondi accretion I. What does perturbed Bondi accretion sound like ?
In the radial flow of gas into a black hole (i.e. Bondi accretion), the
infall of any entropy or vorticity perturbation produces acoustic waves
propagating outward. The dependence of this acoustic flux on the shape of the
perturbation is investigated in detail. This is the key process in the
mechanism of the entropic-acoustic instability proposed by Foglizzo & Tagger
(2000) to explain the instability of Bondi-Hoyle-Lyttleton accretion. These
acoustic waves create new entropy and vorticity perturbations when they reach
the shock, thus closing the entropic-acoustic cycle. With an adiabatic index
1<gamma<=5/3, the linearized equations describing the perturbations of the
Bondi flow are studied analytically and solved numerically. The fundamental
frequency of this problem is the cut-off frequency of acoustic refraction,
below which ingoing acoustic waves are refracted out. This cut-off is
significantly smaller than the Keplerian frequency at the sonic radius and
depends on the latitudinal number l of the perturbations. When advected
adiabatically inward, entropy and vorticity perturbations trigger acoustic
waves propagating outward, with an efficiency which is highest for non radial
perturbations l=1. The outgoing acoustic flux produced by the advection of
vorticity perturbations is always moderate and peaks at rather low frequency.
By contrast, the acoustic flux produced by an entropy wave is highest close to
the refraction cut-off. It can be very large if gamma is close to 5/3. These
results suggest that the shocked Bondi flow with gamma=5/3 is strongly unstable
with respect to the entropic-acoustic mechanism.Comment: 14 pages, 11 figures, accepted for publication in A&
Spectrum of Optically Thin Advection Dominated Accretion Flow around a Black Hole: Application to Sgr A*
The global structure of optically thin advection dominated accretion flows
which are composed of two-temperature plasma around black holes is calculated.
We adopt the full set of basic equations including the advective energy
transport in the energy equation for the electrons. The spectra emitted by the
optically thin accretion flows are also investigated. The radiation mechanisms
which are taken into accout are bremsstrahlung, synchrotron emission, and
Comptonization. The calculation of the spectra and that of the structure of the
accretion flows are made to be completely consistent by calculating the
radiative cooling rate at each radius. As a result of the advection domination
for the ions, the heat transport from the ions to the electrons becomes
practically zero and the radiative cooling balances with the advective heating
in the energy equation of the electrons. Following up on the successful work of
Narayan et al. (1995), we applied our model to the spectrum of Sgr A*. We find
that the spectrum of Sgr A* is explained by the optically thin advection
dominated accretion flow around a black hole of the mass M_bh=10^6 M_sun. The
parameter dependence of the spectrum and the structure of the accretion flows
is also discussed.Comment: AAS LaTeX file; 26 pages; 12 ps figures; to be published in ApJ. PDF
files are obtainable via following anonymous ftp.
ftp://ftp.kusastro.kyoto-u.ac.jp/pub/manmoto/preprint/spec_sgrA.tar.g
Rms-flux relation of Cyg X-1 with RXTE: dipping and nondipping cases
The rms (root mean square) variability is the parameter for understanding the
emission temporal properties of X-ray binaries (XRBs) and active galactic
nuclei (AGN).
The rms-flux relation with Rossi X-ray Timing Explorer (RXTE) data for the
dips and nondip of black hole Cyg X-1 has been investigated in this paper. Our
results show that there exist the linear rms-flux relations in the frequency
range 0.1-10 Hz for the dipping light curve. Moreover, this linear relation
still remains during the nondip regime, but with the steeper slope than that of
the dipping case in the low energy band. For the high energy band, the slopes
of the dipping and nondipping cases are hardly constant within errors. The
explanations of the results have been made by means of the ``Propagating
Perturbation'' model of Lyubarskii (1997).Comment: 15 pages, 12 figures, Accepted for publication in Astrophysics &
Space Scienc
An Analytical Study on the Multi-critical Behaviour and Related Bifurcation Phenomena for Relativistic Black Hole Accretion
We apply the theory of algebraic polynomials to analytically study the
transonic properties of general relativistic hydrodynamic axisymmetric
accretion onto non-rotating astrophysical black holes. For such accretion
phenomena, the conserved specific energy of the flow, which turns out to be one
of the two first integrals of motion in the system studied, can be expressed as
a 8 degree polynomial of the critical point of the flow configuration.
We then construct the corresponding Sturm's chain algorithm to calculate the
number of real roots lying within the astrophysically relevant domain of
. This allows, for the first time in literature, to {\it
analytically} find out the maximum number of physically acceptable solution an
accretion flow with certain geometric configuration, space-time metric, and
equation of state can have, and thus to investigate its multi-critical
properties {\it completely analytically}, for accretion flow in which the
location of the critical points can not be computed without taking recourse to
the numerical scheme. This work can further be generalized to analytically
calculate the maximal number of equilibrium points certain autonomous dynamical
system can have in general. We also demonstrate how the transition from a
mono-critical to multi-critical (or vice versa) flow configuration can be
realized through the saddle-centre bifurcation phenomena using certain
techniques of the catastrophe theory.Comment: 19 pages, 2 eps figures, to appear in "General Relativity and
Gravitation
The Jets and Disc of SS 433 at Super-Eddington Luminosities
We examine the jets and the disc of SS 433 at super-Eddington luminosities
with 600 times Eddington critical accretion rate by time-dependent
two-dimensional radiation hydrodynamical calculations, assuming alpha-model for
the viscosity. One-dimensional supercritical accretion disc models with mass
loss or advection are used as the initial configurations of the disc. As a
result, from the initial advective disc models with alpha =0.001 and 0.1, we
obtain the total luminosities 2.5x10^{40} and 2.0x10^{40} erg/s. The total
mass-outflow rates are 4x10^{-5} and 10^{-4} solar-mass/yr and the rates of the
relativistic axial outflows in a small half opening angle of 1 degree are about
10^{-6} solar-mass/yr: the values generally consistent with the corresponding
observed rates of the wind and the jets, respectively. From the initial models
with mass loss but without advection, we obtain the total mass-outflow and
axial outflow rates smaller than or comparable to the observed rates of the
wind and the jets respectively, depending on alpha. In the advective disc model
with alpha=0.1, the initially radiation-pressure dominant, optically thick disc
evolves to the gas-pressure dominated, optically thin state in the inner region
of the disc, and the inner disc is unstable. Consequently, we find remarkable
modulations of the disc luminosity and the accretion rate through the inner
edge. These modulations manifest themselves as the recurrent hot blobs with
high temperatures and low densities at the disc plane, which develop outward
and upward and produce the QPOs-like variability of the total luminosity with
an amplitude of a factor of 2 and quasi-periods of 10 -- 25 s. This may explain
the massive jet ejection and the QPOs phenomena observed in SS 433.Comment: 11 pages, 15 figures, 1 table, MNRAS in pres
AGN variability time scales and the discrete-event model
We analyse the ultraviolet variability time scales in a sample of 15 Type 1
Active Galactic Nuclei (AGN) observed by IUE. Using a structure function
analysis, we demonstrate the existence in most objects of a maximum variability
time scale of the order of 0.02-1.00 year. We do not find any significant
dependence of these maximum variability time scales on the wavelength, but we
observe a weak correlation with the average luminosity of the objects. We also
observe in several objects the existence of long-term variability, which seems
decoupled from the short-term one. We interpret the existence of a maximum
variability time scale as a possible evidence that the light curves of Type 1
AGN are the result of the superimposition of independent events. In the
framework of the so-called discrete-event model, we study the event energy and
event rate as a function of the object properties. We confront our results to
predictions from existing models based on discrete events. We show that models
based on a fixed event energy, like supernova explosions, can be ruled out. In
their present form, models based on magnetic blobs are also unable to account
for the observed relations. Stellar collision models, while not completely
satisfactory, cannot be excluded.Comment: 13 pages, 10 figures. Accepted for publication in A&
Automatic, Continuous Food Volume Measurement with a Helmholtz Resonator
Rosana G. Moreira, Editor-in-Chief; Texas A&M UniversityThis is a Technical article from International Commission of Agricultural Engineering (CIGR, Commission Internationale du Genie Rural) E-Journal Volume 3 (2001): T. Nishizu, Y. Ikeda, Y. Torikata, S. Manmoto, T. Umehara, and T. Mizukami. Automatic, Continuous Food Volume Measurement with a Helmholtz Resonator. Vol. III, October 2001