166 research outputs found
Turbulent Stresses in Local Simulations of Radiation-Dominated Accretion Disks, and the Possibility of the LIghtman-Eardley Instability
We present the results of a series of radiation-MHD simulations of a local
patch of an accretion disk, with fixed vertical gravity profile but with
different surface mass densities and a broad range of radiation to gas pressure
ratios. Each simulation achieves a thermal equilibrium that lasts for many
cooling times. After averaging over times long compared to a cooling time, we
find that the vertically integrated stress is approximately proportional to the
vertically-averaged total thermal (gas plus radiation) pressure. We map
out--for the first time on the basis of explicit physics--the thermal
equilibrium relation between stress and surface density: the stress decreases
(increases) with increasing surface mass density when the simulation is
radiation (gas) pressure dominated. The dependence of stress on surface mass
density in the radiation pressure dominated regime suggests the possibility of
a Lightman-Eardley inflow instability, but global simulations or shearing box
simulations with much wider radial boxes will be necessary to confirm this and
determine its nonlinear behavior.Comment: accepted for publication in The Astrophysical Journa
Does the Slim-Disk Model Correctly Consider Photon-Trapping Effects?
We investigate the photon-trapping effects in the super-critical black hole
accretion flows by solving radiation transfer as well as the energy equations
of radiation and gas. It is found that the slim-disk model generally
overestimates the luminosity of the disk at around the Eddington luminosity
(L_E) and is not accurate in describing the effective temperature profile,
since it neglects time delay between energy generation at deeper inside the
disk and energy release at the surface. Especially, the photon-trapping effects
are appreciable even below L ~ L_E, while they appear above ~ 3L_E according to
the slim disk. Through the photon-trapping effects, the luminosity is reduced
and the effective temperature profile becomes flatter than r^{-3/4} as in the
standard disk. In the case that the viscous heating is effective only around
the equatorial plane, the luminosity is kept around the Eddington luminosity
even at very large mass accretion rate, Mdot>>L_E/c^2. The effective
temperature profile is almost flat, and the maximum temperature decreases in
accordance with rise in the mass accretion rate. Thus, the most luminous radius
shifts to the outer region when Mdot/(L_E/c^2) >> 10^2. In the case that the
energy is dissipated equally at any heights, the resultant luminosity is
somewhat larger than in the former case, but the energy-conversion efficiency
still decreases with increase of the mass accretion rate, as well. The most
luminous radius stays around the inner edge of the disk in the latter case.
Hence, the effective temperature profile is sensitive to the vertical
distribution of energy production rates, so is the spectral shape. Future
observations of high L/L_E objects will be able to test our model.Comment: 10 pages, 7 figures, accepted for publication in Ap
Super-critical Accretion Flows around Black Holes: Two-dimensional, Radiation-pressure-dominated Disks with Photon-trapping
The quasi-steady structure of super-critical accretion flows around a black
hole is studied based on the two-dimensional radiation-hydrodynamical (2D-RHD)
simulations. The super-critical flow is composed of two parts: the disk region
and the outflow regions above and below the disk. Within the disk region the
circular motion as well as the patchy density structure are observed, which is
caused by Kelvin-Helmholtz instability and probably by convection. The
mass-accretion rate decreases inward, roughly in proportion to the radius, and
the remaining part of the disk material leaves the disk to form outflow because
of strong radiation pressure force. We confirm that photon trapping plays an
important role within the disk. Thus, matter can fall onto the black hole at a
rate exceeding the Eddington rate. The emission is highly anisotropic and
moderately collimated so that the apparent luminosity can exceed the Eddington
luminosity by a factor of a few in the face-on view. The mass-accretion rate
onto the black hole increases with increase of the absorption opacity
(metalicity) of the accreting matter. This implies that the black hole tends to
grow up faster in the metal rich regions as in starburst galaxies or
star-forming regions.Comment: 16 pages, 12 figures, accepted for publication in ApJ (Volume 628,
July 20, 2005 issue
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
Formation of Obscuring Walls by Radiation Force from Circumnuclear Starbursts and Implications for Starburst-Active Galactic Nucleus Connection
We explore the formation of dusty gas walls induced by a circumnuclear
starburst around an AGN. We concentrate our attention on the role of the
radiation force by a starburst as well as an AGN, where the effects of optical
depth of dusty gas are taken into consideration. In two-dimensional
axisymmetric space, we analyze the configuration and the stability of
geometrically thin walls which are in balance between radiation pressure and
gravity. As a result, it is shown that the radiation force by the circumnuclear
starburst works to stabilize optically thick walls surrounding the nucleus. In
the case of a brighter starburst with a fainter AGN, there form double walls,
an inner one of which is located between the nucleus and the circumnuclear
starburst, and an outer one of which enshrouds both the starburst regions and
the nucleus. The total extinction (Av) of both walls turns out to be ~10 for a
brighter starburst. As a consequence, double walls could heavily obscure the
nucleus to make the AGN type 2. The outer wall may provide an explanation for
the recent indications for large-scale obscuring materials in Seyfert 2's.
Also, it is predicted that the AGN type is shifts from type 2 to type 1 in
several times 10^7 yr according to the stellar evolution in the starburst. In
contrast, if the AGN itself is much brighter than the starburst as a quasar is,
then neither wall forms regardless of the starburst activity and the nucleus is
likely to be identified as type 1. To conclude, the radiatively-supported gas
walls could be responsible for the putative correlation between AGN type and
the starbursts, whereby Seyfert 2 galaxies are more frequently associated with
circumnuclear starbursts than type 1, whereas quasars are mostly observed as
type 1 regardless of star-forming activity in the host galaxies.Comment: 10 pages, 7 figures, accepted for publication in Ap
Suzaku Observations of the Black Hole H1743-322 in Outburst
We observed the Galactic black hole candidate H1743-322 with Suzaku for
approximately 32 ksec, while the source was in a low/hard state during its 2008
outburst. We collected and analyzed the data with the HXD/PIN, HXD/GSO and XIS
cameras spanning the energy range from 0.7-200 keV. Fits to the spectra with
simple models fail to detect narrow Fe XXV and Fe XXVI absorption lines, with
90% confidence upper limits of 3.5 eV and 2.5 eV on the equivalent width,
respectively. These limits are commensurate with those in the very high state,
but are well below the equivalent widths of lines detected in the high/soft
state, suggesting that disk winds are partially state-dependent. We discuss
these results in the context of previous detections of ionized Fe absorption
lines in H1743-322 and connections to winds and jets in accreting systems.
Additionally, we report the possible detection of disk reflection features,
including an Fe K emission line.Comment: 16 pages, 4 figures, 4 tables. Accepted for publication in ApJ
Observable Signatures of EMRI Black Hole Binaries Embedded in Thin Accretion Disks
We examine the electromagnetic (EM) and gravitational wave (GW) signatures of
stellar-mass compact objects (COs) spiraling into a supermassive black hole
(extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure
dominated, accretion disk. At large separations, the tidal effect of the
secondary CO clears a gap. We show that the gap refills during the late
GW-driven phase of the inspiral, leading to a sudden EM brightening of the
source. The accretion disk leaves an imprint on the GW through its angular
momentum exchange with the binary, the mass increase of the binary members due
to accretion, and its gravity. We compute the disk-modified GWs both in an
analytical Newtonian approximation and in a numerical effective-one-body
approach. We find that disk-induced migration provides the dominant
perturbation to the inspiral, with weaker effects from the mass accretion onto
the CO and hydrodynamic drag. Depending on whether a gap is present, the
perturbation of the GW phase is between 10 and 1000 radians per year,
detectable with the future Laser Interferometer Space Antenna (LISA) at high
significance. The Fourier transform of the disk-modified GW in the stationary
phase approximation is sensitive to disk parameters with a frequency trend
different from post-Newtonian vacuum corrections. Our results suggest that
observations of EMRIs may place new sensitive constraints on the physics of
accretion disks.Comment: 42 pages, 8 figures, 3 tables, submitted to Phys. Rev.
Low angular momentum flow model of Sgr A* activity
Sgr A* is the closest massive black hole and can be observed with the highest
angular resolution. Nevertheless, our current understanding of the accretion
process in this source is very poor. The inflow is almost certainly of low
radiative efficiency and it is accompanied by a strong outflow and the flow is
strongly variable but the details of the dynamics are unknown. Even the amount
of angular momentum in the flow is an open question. Here we argue that low
angular momentum scenario is better suited to explain the flow variability. We
present a new hybrid model which describes such a flow and consists of an outer
spherically symmetric Bondi flow and an inner axially symmetric flow described
through MHD simulations. The assumed angular momentum of the matter is low,
i.e. the corresponding circularization radius in the equatorial plane of the
flow is just above the innermost stable circular orbit in pseudo-Newtonian
potential. We compare the radiation spectrum from such a flow to the broad band
observational data for Sgr A*.Comment: Proceedings of the AHAR 2008 Conference: The Universe under the
Microscope; Astrophysics at High Angular Resolution, Bad Honef
Nuclear Polarization of Molecular Hydrogen Recombined on a Non-metallic Surface
The nuclear polarization of molecules formed by recombination
of nuclear polarized H atoms on the surface of a storage cell initially coated
with a silicon-based polymer has been measured by using the longitudinal
double-spin asymmetry in deep-inelastic positron-proton scattering. The
molecules are found to have a substantial nuclear polarization, which is
evidence that initially polarized atoms retain their nuclear polarization when
absorbed on this type of surfac
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