59 research outputs found
Direct Detection of an Ultraluminous Ultraviolet Source
We present Hubble Space Telescope observations in the far UV of the
ultraluminous X-ray source in NGC 6946 associated with the optical nebula MF
16. Both a point-like source coincident with the X-ray source and the
surrounding nebula are detected in the FUV. The point source has a flux of
5E-16 erg s^-1 cm^-2 Ang^-1 and the nebula has a flux of 1.6E-15 erg s^-1 cm^-2
Ang^-1, quoted at 1533 Ang and assuming an extinction of A_V = 1.54. Thus, MF
16 appears to host the first directly detected ultraluminous UV source (ULUV).
The flux of the point-like source is consistent with a blackbody with T ~
30,000 K, possibly from a massive companion star, but this spectrum does not
create sufficient ionizing radiation to produce the nebular HeII flux and a
second, hotter emission component would be required. A multicolor disk
blackbody spectrum truncated with an outer disk temperature of ~16,000 K
provides an adequate fit to the FUV, B, V, I, and HeII fluxes and can produce
the needed ionizing radiation. Additional observations are required to
determine the physical nature of the source.Comment: 4 pages, accepted for ApJ Letter
Do Ultraluminous X-ray Sources Really Contain Intermediate-mass Black Holes?
An open question remains whether Ultraluminous X-ray Sources (ULXs) really
contain intermediate-mass black holes (IMBHs). We carefully investigated the
XMM-Newton EPIC spectra of the four ULXs that were claimed to be strong
candidates of IMBHs by several authors. We first tried fitting by the standard
spectral model of disk blackbody (DBB) + power-law (PL), finding good fits to
all of the data, in agreement with others. We, however, found that the PL
component dominates the DBB component at 0.3 to 10 keV. Thus, the black
hole parameters derived solely from the minor DBB component are questionable.
Next, we tried to fit the same data by the ``-free disk model'' without the
PL component, assuming the effective temperature profile of where is the disk radius. Interestingly, in spite of one
less free model parameters, we obtained similarly good fits with much higher
innermost disk temperatures, keV. More importantly,
we obtained , just the value predicted by the slim (super-critical)
disk theory, rather than that is expected from the standard disk
model. The estimated black hole masses from the -free disk model are much
smaller; M\ltsim 40 M_\odot. Furthermore, we applied a more sophisticated
slim disk model by Kawaguchi (2003, ApJ, 593,69), and obtained good fits with
roughly consistent black hole masses. We thus conclude that the central engines
of these ULXs are super-critical accretion flows to stellar-mass black holes.Comment: Appear in PASJ, 10 pages, 10 figures (in total) grouped into 3 figure
captions, 3 table
A Note on the Slim Accretion Disk Model
We show that when the gravitational force is correctly calculated in dealing
with the vertical hydrostatic equilibrium of black hole accretion disks, the
relationship that is valid for geometrically thin disks, i.e., constant, where is the sound speed, is the Keplerian
angular velocity, and is the half-thickness of the disk, does not hold for
slim disks. More importantly, by adopting the correct vertical gravitational
force in studies of thermal equilibrium solutions, we find that there exists a
maximally possible accretion rate for each radius in the outer region of
optically thick accretion flows, so that only the inner region of these flows
can possibly take the form of slim disks, and strong outflows from the outer
region are required to reduce the accretion rate in order for slim disks to be
realized.Comment: 14 pages, 5 figures, accepted by Ap
Thermal Equilibria of Magnetically Supported, Black Hole Accretion Disks
We present new thermal equilibrium solutions for optically thin and thick
disks incorporating magnetic fields. The purpose of this paper is to explain
the bright hard state and the bright/slow transition observed in the rising
phases of outbursts in BHCs. On the basis of the results of 3D MHD simulations,
we assume that magnetic fields inside the disk are turbulent and dominated by
the azimuthal component and that the azimuthally averaged Maxwell stress is
proportional to the total pressure. We prescribe the magnetic flux advection
rate to determine the azimuthal magnetic flux at a given radius.
We find magnetically supported, thermally stable solutions for both optically
thin and thick disks, in which the heating enhanced by the strong magnetic
field balances the radiative cooling. The temperature in a low- disk is
lower than that in an ADAF/RIAF but higher than that in a standard disk. We
also study the radial dependence of the thermal equilibrium solutions.
The optically thin, low- branch extends to , in which the temperature anti-correlates with the mass accretion
rate. Thus optically thin low- disks can explain the bright hard state.
Optically thick, low- disks have the radial dependence of the effective
temperature . Such disks will be observed as
staying in a high/soft state. Furthermore, limit cycle oscillations between an
optically thick low- disk and a slim disk will occur because the
optically thick low- branch intersects with the radiation pressure
dominated standard disk branch. These limit cycle oscillations will show a
smaller luminosity variation than that between a standard disk and a slim disk.Comment: 23 pages, 9 figures, accepted for publication in Ap
Why Is Supercritical Disk Accretion Feasible?
Although the occurrence of steady supercritical disk accretion onto a black
hole has been speculated about since the 1970s, it has not been accurately
verified so far. For the first time, we previously demonstrated it through
two-dimensional, long-term radiation-hydrodynamic simulations. To clarify why
this accretion is possible, we quantitatively investigate the dynamics of a
simulated supercritical accretion flow with a mass accretion rate of ~10^2
L_E/c^2 (with L_E and c being, respectively, the Eddington luminosity and the
speed of light). We confirm two important mechanisms underlying supercritical
disk accretion flow, as previously claimed, one of which is the radiation
anisotropy arising from the anisotropic density distribution of very optically
thick material. We qualitatively show that despite a very large radiation
energy density, E_0>10^2L_E/(4 pi r^2 c) (with r being the distance from the
black hole), the radiative flux F_0 cE_0/tau could be small due to a large
optical depth, typically tau 10^3, in the disk. Another mechanism is photon
trapping, quantified by vE_0, where v is the flow velocity. With a large |v|
and E_0, this term significantly reduces the radiative flux and even makes it
negative (inward) at r<70r_S, where r_S is the Schwarzschild radius. Due to the
combination of these effects, the radiative force in the direction along the
disk plane is largely attenuated so that the gravitational force barely exceeds
the sum of the radiative force and the centrifugal force. As a result, matter
can slowly fall onto the central black hole mainly along the disk plane with
velocity much less than the free-fall velocity, even though the disk luminosity
exceeds the Eddington luminosity. Along the disk rotation axis, in contrast,
the strong radiative force drives strong gas outflows.Comment: 8 pages, 7 figures, accepted for publication in Ap
An analytic relation for the thickness of accretion flows
We take the vertical distribution of the radial and azimuthal velocity into
account in spherical coordinates, and find that the analytic relation
c_{s0}/(v_K \Theta) = [(\gamma -1)/(2\gamma)]^{1/2} is valid for both
geometrically thin and thick accretion flows, where c_{s0} is the sound speed
on the equatorial plane, v_K is the Keplerian velocity, \Theta is the
half-opening angle of the flow, and \gamma is the adiabatic index.Comment: 4 pages, 2 figures, accepted by Science in China Series
The hard X-ray spectral evolution in X-ray binaries and its application to constrain the black hole mass of ultraluminous X-ray sources
We investigate the relationship between the hard X-ray photon index
and the Eddington ratio () in six X-ray
binaries (XRBs) with well constrained black hole masses and distances. We find
that different XRBs follow different anti-correlations between and
when is less than a critical value, while and
generally follow the same positive correlation when is larger than the
critical value. The anti-correlation and the positive correlation may suggest
that they are in different accretion modes (e.g., radiatively inefficient
accretion flow (RIAF) and standard disk). We fit both correlations with the
linear least-square method for individual sources, from which the crosspoint of
two fitted lines is obtained. Although the anti-correlation varies from source
to source, the crosspoints of all sources roughly converge to the same point
with small scatter(), which may
correspond to the transition point between RIAF and standard accretion disk.
Motivated by the observational evidence for the similarity of the X-ray
spectral evolution of ultraluminous X-ray sources (ULXs) to that of XRBs, we
then constrain the black hole masses for seven ULXs assuming that their X-ray
spectral evolution is similar to that of XRBs. We find that the BH masses of
these seven luminous ULXs are around 10^{4}\msun, which are typical
intermediate-mass BHs (IMBHs). Our results are generally consistent with the BH
masses constrained from the timing properties (e.g., break frequency) or the
model fitting with a multi-color disk.Comment: accepted for publication in ApJ, 18 pages, 2 figures, Comments is
welcomed
Spectral Evolution of NGC 1313 X-2: Evidence Against The Cool Disk Model
The presence of a cool multicolor disk component with an inner disk
temperature kT=0.1~0.3 keV at a luminosity L>10^40 erg/s has been interpreted
as evidence that the ultraluminous X-ray source NGC 1313 X-2 harbors an
intermediate-mass black hole (IMBH). The temperature of a disk component should
vary with luminosity as . However, upon investigating the
spectral evolution with multiple XMM-Newton observations, we found that the
cool disk component failed to follow this relation with a confidence level of
0.999964. Indeed, the luminosity decreases as the temperature increases, and
the luminosities at high temperatures are more than an order of magnitude less
than expected from the extrapolation of luminosities at low
temperatures. This places a strong constraint against the validity of modeling
the X-ray spectra of NGC 1313 X-2 as emission from the accretion disk of an
IMBH. The decrease in luminosity with increasing temperature of the soft
component follows the trend suggested by a model in which the soft emission
arises from an outflow from a stellar-mass black hole with super-Eddington
accretion viewed along the symmetry axis. Alternatively, the spectra can be
adequately fitted by a p-free disk model with kT=~2 keV and p=~0.5. The
spectral evolution is consistent with the relation and appears
to be a high luminosity extension of the L-kT relation of Galactic black holes.
This, again, would suggest that the emission is from a super-Eddington
accreting stellar mass black hole.Comment: 5 pages, 4 figures, ApJ Letter in press; minor changes in v2 to match
the published versio
Global Structure of Three Distinct Accretion Flows and Outflows around Black Holes through Two-Dimensional Radiation-Magnetohydrodynamic Simulations
We present the detailed global structure of black hole accretion flows and
outflows through newly performed two-dimensional radiation-magnetohydrodynamic
simulations. By starting from a torus threaded with weak toroidal magnetic
fields and by controlling the central density of the initial torus, rho_0, we
can reproduce three distinct modes of accretion flow. In model A with the
highest central density, an optically and geometrically thick supercritical
accretion disk is created. The radiation force greatly exceeds the gravity
above the disk surface, thereby driving a strong outflow (or jet). Because of
the mild beaming, the apparent (isotropic) photon luminosity is ~22L_E (where
L_E is the Eddington luminosity) in the face-on view. Even higher apparent
luminosity is feasible if we increase the flow density. In model B with a
moderate density, radiative cooling of the accretion flow is so efficient that
a standard-type, cold, and geometrically thin disk is formed at radii greater
than ~7R_S (where R_S is the Schwarzschild radius), while the flow is
radiatively inefficient otherwise. The magnetic-pressure-driven disk wind
appears in this model. In model C the density is too low for the flow to be
radiatively efficient. The flow thus becomes radiatively inefficient accretion
flow, which is geometrically thick and optically thin. The magnetic-pressure
force, in cooperation with the gas-pressure force, drives outflows from the
disk surface, and the flow releases its energy via jets rather than via
radiation. Observational implications are briefly discussed.Comment: 19 pages, 13 figures, accepted for publication in Ap
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