735 research outputs found
Slim Disk Model for Narrow-Line Seyfert 1 Galaxies
We argue that both the extreme soft X-ray excess and the large-amplitude
variability of Narrow-Line Seyfert 1 galaxies (NLS1s) can be explained in the
framework of the slim disk model. When the disk luminosity approaches the
Eddington luminosity, the disk becomes a slim disk, exhibiting a multi-color
blackbody spectrum with a maximum temperature, T(bb), of about 0.2 (M/1e5 solar
masses)e(-1/4) keV, and size of the X-ray emitting region, r(bb), of about R(S)
(the Schwarzschild radius). Furthermore, magnetic energy can be amplified up to
a level exceeding radiation energy emitted from the disk, causing substantial
variability in X-rays by consecutive magnetic flares.Comment: Contributed talk presented at the Joint MPE,AIP,ESO workshop on
NLS1s, Bad Honnef, Dec. 1999, to appear in New Astronomy Reviews; also
available at http://wave.xray.mpe.mpg.de/conferences/nls1-worksho
Radiative Spectra from Disk Corona and Inner Hot Flow in Black Hole X-ray Binaries
To understand the origin of hard X-ray emissions from black hole X-ray
binaries during their low/hard states, we calculate the X-ray spectra of
black-hole accretion flow for the following three configurations of hot and
cool media: (a) an inner hot flow and a cool outer disk (inner hot flow model),
(b) a cool disk sandwiched by disk coronae (disk corona model), and (c) the
combination of those two (hybrid model). The basic features we require for
successful models are (i) significant hard X-ray emission whose luminosity
exceeds that of soft X-rays, (ii) high hard X-ray luminosities in the range of
(0.4 - 30) times 10^{37} erg s^{-1}, and (iii) the existence of two power-law
components in the hard X-ray band with the photon indices of Gamma_s ~ 2 >
Gamma_h, where Gamma_s and Gamma_h are the photon indices of the softer (<10
keV) and the harder (>10 keV) power-law components, respectively. Contribution
by non-thermal electrons nor time-dependent evolution are not considered. We
find that Models (a) and (b) can be ruled out, since the spectra are always
dominated by the soft component, and since only one power-law component, at
most, can be reproduced. Only Model (c) can account for sufficiently strong
hard X-ray emissions, as well as the existence of the two power-law components,
for a large ratio of the accretion rate in the corona to that in the thin disk.
The outer disk corona (where the Compton y-parameter is smaller, y < 1)
produces the softer power-law component with photon index of Gamma_s ~ 2,
whereas the inner hot flow (where y gtrsim 1) generates the harder component
with Gamma_h < 2. This model can also account for the observed relationship
between the photon index and the reflection fraction.Comment: 13 pages, 13 figures, accepted for publication in PAS
Evolution of Hydromagnetic Disturbances in Low Ionized Cosmic Plasmas
We consider the propagation of hydromagnetic waves generated by a compact
turbulent source in low ionized plasmas, applying the Lighthill theory. We
assume the plasma to be isothermal, and adopt a uniform, stationary medium
thread by ordered magnetic fields as an initial condition. Then, the distinct
properties of the hydromagnetic waves originating from a source oscillating
with a fixed frequency are studied in the linear regime. As is well known, in
low ionized plasmas, the generated waves dissipate due to ion-neutral damping.
In this paper, the dependence of the dissipation rate on the frequency of the
oscillating source is investigated. The larger the frequency becomes, the more
substantial is the wave dissipation. Implications of our results on the energy
source in molecular clouds are also discussed. Interestingly, since the outflow
lobes associated with young stellar objects act as compact turbulent sources,
hydromagnetic waves are generated by them. From our order-estimations, about
70% of the energy of the outflow itself propagates as waves or turbulences,
while the remaining 30% dissipates and heats the neutrals via ion-neutral
damping. Then, we confirm that the outflows are significant energy sources in
molecular clouds in the context of the Lighthill theory.Comment: 17 pages LaTeX, 3 PostScript figures, accepted, PASJ (Vol. 51, No. 3,
pp. 337 - 344, 1999
The influence of Galactic wind upon the star formation histories of Local Group galaxies
We examine the possibility that ram pressure exerted by the galactic wind
from the Galaxy could have stripped gas from the Local Group dwarf galaxies,
thereby affecting their star formation histories. Whether gas stripping occurs
or not depends on the relative magnitudes of two counteracting forces acting on
gas in a dwarf galaxy: ram pressure force by the wind and the gravitational
binding force by the dwarf galaxy itself. We suggest that the galactic wind
could have stripped gas in a dwarf galaxy located within the distance of
kpc
(where is the surface radius and is the total binding
energy of the dwarf galaxy, respectively) from the Galaxy within a timescale of
Gyr, thereby preventing star formation there. Our result based on this Galactic
wind model explains the recent observation that dwarfs located close to the
Galaxy experienced star formation only in the early phase of their lifetimes,
whereas distant dwarfs are still undergoing star formation. The present star
formation in the Large Magellanic Cloud can also be explained through our
Galactic wind model.Comment: 7 pages LaTeX, no figures, to appear in MNRA
Where is a Marginally Stable Last Circular Orbit in Super-Critical Accretion Flow?
Impressed by the widespread misunderstanding of the issue, we return to the
old question of the location of the inner edge of accretion disk around black
hole. We recall the fundamental results obtained in the 1970's and 1980's by
Warsaw and Kyoto research groups that proved, in particular, that the inner
edge does not coincide with the location of the innermost stable Keplerian
circular orbit. We give some novel illustrations of this particular point and
of some other fundamental results obtained by Warsaw and Kyoto groups. To
investigate the flow dynamics of the inner edge of accretion disk, we carefully
solve the structure of the transonic flow and plot the effective potential
profile based on the angular-momentum distribution calculated numerically. We
show that the flow does not have a potential minimum for accretion rates, {\dot
M} > 10 L_E/c^2 (with L_E being the Eddington luminosity and
being the speed of light). This property is realized even in relatively
small viscosity parameters
(i.e., \alpha ~ 0.01), because of the effect of pressure gradient. In
conclusion, the argument based on the last circular orbit of a test particle
cannot give a correct inner boundary of the super-critical flow and the inner
edge should be determined in connection with radiation efficiency. The same
argument can apply to optically thin ADAF. The interpretation of the observed
QPO frequencies should be re-considered, since the assumption of Kepler
rotation velocity can grossly over- or underestimate the disk rotation
velocity, depending on the magnitude of viscosity.Comment: 7 pages, 3 figures, accepted for PAS
Binary Black Hole Accretion Flows in Merged Galactic Nuclei
We study the accretion flows from the circumbinary disks onto the
supermassive binary black holes in a subparsec scale of the galactic center,
using a smoothed particles hydrodynamics (SPH) code. Simulation models are
presented in four cases of a circular binary with equal and unequal masses, and
of an eccentric binary with equal and unequal masses. We find that the
circumblack-hole disks are formed around each black holes regardless of
simulation parameters. There are two-step mechanisms to cause an accretion flow
from the circumbinary disk onto supermassive binary black holes: First, the
tidally induced elongation of the circumbinary disk triggers mass inflow
towards two closest points on the circumbinary disk from the black holes. Then,
the gas is increasingly accumulated on these two points owing to the
gravitational attraction of black holes. Second, when the gas can pass across
the maximum loci of the effective binary potential, it starts to overflow via
their two points and freely infalls to each black hole. In circular binaries,
the gas continues to be supplied from the circumbinary disk (i.e. the gap
between the circumbinary disk and the binary black hole is always closed.) In
eccentric binaries, the mass supply undergoes the periodic on/off transitions
during one orbital period because of the variation of periodic potential. The
gap starts to close after the apastron and to open again after the next
periastron passage. Due to this gap closing/opening cycles, the mass-capture
rates are eventually strongly phase dependent. This could provide observable
diagnosis for the presence of supermassive binary black holes in merged
galactic nuclei.Comment: 16 pages, 27 figures, 2 tables, accepted for publication in PASJ.
"High Resolution Version is Available at
"http://www2.yukawa.kyoto-u.ac.jp/~kimitake/bbhs.html" Three observational
references are added. Grammatical errors and typos are correcte
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