53 research outputs found
Variability Profiles of Millisecond X-Ray Pulsars: Results of Pseudo-Newtonian 3D MHD Simulations
We model the variability profiles of millisecond period X-ray pulsars. We
performed three-dimensional magnetohydrodynamic simulations of disk accretion
to millisecond period neutron stars with a misaligned magnetic dipole moment,
using the pseudo-Newtonian Paczynski-Wiita potential to model general
relativistic effects. We found that the shapes of the resulting funnel streams
of accreting matter and the hot spots on the surface of the star are quite
similar to those for more slowly rotating stars obtained from earlier
simulations using the Newtonian potential. The funnel streams and hot spots
rotate approximately with the same angular velocity as the star. The spots are
bow-shaped (bar-shaped) for small (large) misalignment angles. We found that
the matter falling on the star has a higher Mach number when we use the
Paczynski-Wiita potential than in the Newtonian case.
Having obtained the surface distribution of the emitted flux, we calculated
the variability curves of the star, taking into account general relativistic,
Doppler and light-travel-time effects. We found that general relativistic
effects decrease the pulse fraction (flatten the light curve), while Doppler
and light-travel-time effects increase it and distort the light curve. We also
found that the light curves from our hot spots are reproduced reasonably well
by spots with a gaussian flux distribution centered at the magnetic poles. We
also calculated the observed image of the star in a few cases, and saw that for
certain orientations, both the antipodal hot spots are simultaneously visible,
as noted by earlier authors.Comment: 9 pages, 10 figures, accepted for publication in ApJ; corrected some
typo
Magnetohydrodynamic Simulations of A Rotating Massive Star Collapsing to A Black Hole
We perform two-dimensional, axisymmetric, magnetohydrodynamic simulations of
the collapse of a rotating star of 40 Msun and in the light of the collapsar
model of gamma-ray burst. Considering two distributions of angular momentum, up
to \sim 10^{17} cm^2/s, and the uniform vertical magnetic field, we investigate
the formation of an accretion disk around a black hole and the jet production
near the hole. After material reaches to the black hole with the high angular
momentum, the disk is formed inside a surface of weak shock. The disk becomes
in a quasi-steady state for stars whose magnetic field is less than 10^{10} G
before the collapse. We find that the jet can be driven by the magnetic fields
even if the central core does not rotate as rapidly as previously assumed and
outer layers of the star has sufficiently high angular momentum. The magnetic
fields are chiefly amplified inside the disk due to the compression and the
wrapping of the field. The fields inside the disk propagate to the polar region
along the inner boundary near the black hole through the Alfv{\'e}n wave, and
eventually drive the jet. The quasi-steady disk is not an advection-dominated
disk but a neutrino cooling-dominated one. Mass accretion rates in the disks
are greater than 0.01 Msun/sec with large fluctuations. The disk is transparent
for neutrinos. The dense part of the disk, which locates near the hole, emits
neutrino efficiently at a constant rate of < 8 \times 10^{51} erg/s. The
neutrino luminosity is much smaller than those from supernovae after the
neutrino burst.Comment: 42 pages, accepted for publication in the Astrophysical Journal. A
paper with higher-resolution figures available at
http://www.ec.knct.ac.jp/~fujimoto/collapsar/mhd-color.pd
Two-dimensional radiation-hydrodynamic model for limit-cycle oscillations of luminous accretion disks
We investigate the time evolution of luminous accretion disks around black
holes, conducting the two-dimensional radiation-hydrodynamic simulations. We
adopt the alpha prescription for the viscosity. The radial-azimuthal component
of viscous stress tensor is assumed to be proportional to the total pressure in
the optically thick region, while the gas pressure in the optically thin
regime. The viscosity parameter, alpha, is taken to be 0.1. We find the
limit-cycle variation in luminosity between high and low states. When we set
the mass input rate from the outer disk boundary to be 100 L_E/c^2, the
luminosity suddenly rises from 0.3L_E to 2L_E, where L_E is the Eddington
luminosity. It decays after retaining high value for about 40 s. Our numerical
results can explain the variation amplitude and duration of the recurrent
outbursts observed in microquasar, GRS 1915+105. We show that the
multi-dimensional effects play an important role in the high-luminosity state.
In this state, the outflow is driven by the strong radiation force, and some
part of radiation energy dissipated inside the disk is swallowed by the black
hole due to the photon-trapping effects. This trapped luminosity is comparable
to the disk luminosity. We also calculate two more cases: one with a much
larger accretion rate than the critical value for the instability and the other
with the viscous stress tensor being proportional to the gas pressure only even
when the radiation pressure is dominant. We find no quasi-periodic light
variations in these cases. This confirms that the limit-cycle behavior found in
the simulations is caused by the disk instability.Comment: 6 pages, 4 figures, accepted for publication in ApJ (ApJ 01 April
2006, v640, 2 issue
Particle Acceleration in Advection-Dominated Accretion Disks with Shocks: Green's Function Energy Distribution
The distribution function describing the acceleration of relativistic
particles in an advection-dominated accretion disk is analyzed using a
transport formalism that includes first-order Fermi acceleration, advection,
spatial diffusion, and the escape of particles through the upper and lower
surfaces of the disk. When a centrifugally-supported shock is present in the
disk, the concentrated particle acceleration occurring in the vicinity of the
shock channels a significant fraction of the binding energy of the accreting
gas into a population of relativistic particles. These high-energy particles
diffuse vertically through the disk and escape, carrying away both energy and
entropy and allowing the remaining gas to accrete. The dynamical structure of
the disk/shock system is computed self-consistently using a model previously
developed by the authors that successfully accounts for the production of the
observed relativistic outflows (jets) in M87 and \SgrA. This ensures that the
rate at which energy is carried away from the disk by the escaping relativistic
particles is equal to the drop in the radial energy flux at the shock location,
as required for energy conservation. We investigate the influence of advection,
diffusion, and acceleration on the particle distribution by computing the
nonthermal Green's function, which displays a relatively flat power-law tail at
high energies. We also obtain the energy distribution for the particles
escaping from the disk, and we conclude by discussing the spectrum of the
observable secondary radiation produced by the escaping particles.Comment: Published in Ap
Corotational Damping of Diskoseismic C-modes in Black Hole Accretion Discs
Diskoseismic c-modes in accretion discs have been invoked to explain
low-frequency variabilities observed in black-hole X-ray binaries. These modes
are trapped in the inner-most region of the disc and have frequencies much
lower than the rotation frequency at the disc inner radius. We show that
because the trapped waves can tunnel through the evanescent barrier to the
corotational wave zone, the c-modes are damped due to wave absorption at the
corotation resonance. We calculate the corotational damping rates of various
c-modes using the WKB approximation. The damping rate varies widely depending
on the mode frequency, the black hole spin parameter and the disc sound speed,
and is generally much less than 10% of the mode frequency. A sufficiently
strong excitation mechanism is needed to overcome this corotational damping and
make the mode observable.Comment: 10 pages, 5 figures, MNRAS in pres
A simple model of radiative emission in M87
We present a simple physical model of the central source emission in the M87
galaxy. It is well known that the observed X-ray luminosity from this galactic
nucleus is much lower than the predicted one, if a standard radiative
efficiency is assumed. Up to now the main model invoked to explain such a
luminosity is the ADAF (Advection-Dominated-Accretion-Flow) model. Our approach
supposes only a simple axis-symmetric adiabatic accretion with a low angular
momentum together with the bremsstrahlung emission process in the accreting
gas. With no other special hypothesis on the dynamics of the system, this model
agrees well enough with the luminosity value measured by Chandra.Comment: 11 pages, 6 figures, accepted for publication in The Astrophysical
Journa
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
A General Relativistic Magnetohydrodynamics Simulation of Jet Formation
We have performed a fully three-dimensional general relativistic
magnetohydrodynamic (GRMHD) simulation of jet formation from a thin accretion
disk around a Schwarzschild black hole with a free-falling corona. The initial
simulation results show that a bipolar jet (velocity ) is created as
shown by previous two-dimensional axisymmetric simulations with mirror symmetry
at the equator. The 3-D simulation ran over one hundred light-crossing time
units ( where ) which is
considerably longer than the previous simulations. We show that the jet is
initially formed as predicted due in part to magnetic pressure from the
twisting the initially uniform magnetic field and from gas pressure associated
with shock formation in the region around . At later times,
the accretion disk becomes thick and the jet fades resulting in a wind that is
ejected from the surface of the thickened (torus-like) disk. It should be noted
that no streaming matter from a donor is included at the outer boundary in the
simulation (an isolated black hole not binary black hole). The wind flows
outwards with a wider angle than the initial jet. The widening of the jet is
consistent with the outward moving torsional Alfv\'{e}n waves (TAWs). This
evolution of disk-jet coupling suggests that the jet fades with a thickened
accretion disk due to the lack of streaming material from an accompanying star.Comment: 27 pages, 8 figures, revised and accepted to ApJ (figures with better
resolution: http://gammaray.nsstc.nasa.gov/~nishikawa/schb1.pdf
Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?
The hard state of X-ray binaries (XRBs) is characterized by a power law
spectrum in the X-ray band, and a flat/inverted radio/IR spectrum associated
with occasionally imaged compact jets. It has generally been thought that the
hard X-rays result from Compton upscattering of thermal accretion disk photons
by a hot, coronal plasma whose properties are inferred via spectral fitting.
Interestingly, these properties-especially those from certain magnetized corona
models-are very similar to the derived plasma conditions at the jet footpoints.
Here we explore the question of whether the `corona' and `jet base' are in fact
related, starting by testing the strongest premise that they are synonymous. In
such models, the radio through the soft X-rays are dominated by synchrotron
emission, while the hard X-rays are dominated by inverse Compton at the jet
base - with both disk and synchrotron photons acting as seed photons. The
conditions at the jet base fix the conditions along the rest of the jet, thus
creating a direct link between the X-ray and radio emission. We also add to
this model a simple iron line and convolve the spectrum with neutral
reflection. After forward-folding the predicted spectra through the detector
response functions, we compare the results to simultaneous radio/X-ray data
obtained from the hard states of the Galactic XRBs GX339-4 and Cygnus X-1.
Results from simple Compton corona model fits are also presented for
comparison. We demonstrate that the jet model fits are statistically as good as
the single-component corona model X-ray fits, yet are also able to address the
simultaneous radio data.Comment: Accepted to the Astrophysical Journal. 14 pages, emulateapj.st
Two temperature viscous accretion flows around rotating black holes: Description of under-fed systems to ultra-luminous X-ray sources
We discuss two temperature accretion disk flows around rotating black holes.
As we know that to explain observed hard X-rays the choice of Keplerian angular
momentum profile is not unique, we consider the sub-Keplerian regime of the
disk. Without any strict knowledge of the magnetic field structure, we assume
the cooling mechanism is dominated by bremsstrahlung process. We show that in a
range of Shakura-Sunyaev viscosity parameter 0.2\gsim\alpha\gsim0.0005, flow
behavior varies widely, particularly by means of the size of disk, efficiency
of cooling and corresponding temperatures of ions and electrons. We also show
that the disk around a rotating black hole is hotter compared to that around a
Schwarzschild black hole, rendering a larger difference between ion and
electron temperatures in the former case. With all the theoretical solutions in
hand, finally we reproduce the observed luminosities () of two extreme cases
-- the under-fed AGNs and quasars (e.g. Sgr ) with L\gsim 10^{33}
erg/sec to ultra-luminous X-ray sources with erg/sec, at
different combinations of mass accretion rate, ratio of specific heats,
Shakura-Sunyaev viscosity parameter and Kerr parameter, and conclude that Sgr
may be an intermediate spinning black hole.Comment: 21 pages including 5 figures; few typos corrected; to appear in New
Astronom
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