explanation blue bibcodes open ADS page with paths to full text
Author name code: kato
ADS astronomy entries on 2022-09-14
=author:"Kato, Yoshiaki"
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Title: Comments on "Imaging Reanalyses of EHT Data"
Authors: Miyoshi, Makoto; Kato, Yoshiaki; Makino, Jun
2022arXiv220713279M Altcode:
On June 14, 2022, the EHT collaboration
(hereafter EHTC) made the web page
(https://eventhorizontelescope.org/blog/imaging-reanalyses-eht-data)
with the title "Imaging Reanalyses of EHT Data," in which they
made comments on our recent Miyoshi et al .2022 published in the
Astrophysical Journal. We investigated the EHTC comments and found
that all of the five points raised by the EHTC are subjective and
unsubstantiated claims. Thus they do not prove the correctness of
the result of EHTC. Sincerely we hope that the EHTC will publish,
not a collection of unsubstantiated claims, but a discussion based on
scientific arguments.
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Title: Radiation Magnetohydrodynamic Simulations of sub-Eddington
accretion flows in AGN
Authors: Igarashi, Taichi; Matsumoto, Yosuke; Matsumoto, Ryoji; Kato,
Yoshiaki; Takahashi, Hiroyuki; Ohsuga, Ken
2022cosp...44.2284I Altcode:
The origin of the soft X-ray excess in luminous active galactic
nuclei (AGN) such as type 1 Seyfert galaxies is thought to be the
warm Compton region with a temperature of $10^{6}-10^{7}$ K. This
temperature is higher than that of the optically thick standard disk
around a supermassive black hole and lower than the optically thin and
radiatively inefficient accretion flow (RIAF). Recently, transitions
between a hard X-ray dominant state and a soft X-ray dominant state have
been observed in changing look AGN, which changes between type 1 with
broad emission lines and type 2 without broad emission lines. This
hard-to-soft state transition is similar to the state transitions
observed in stellar-mass black hole candidates. The origin of the state
transition may be the cooling instability which grows when the accretion
rate exceeds the upper limit of RIAF. Igarashi et al. (2020) reported
the results of global three-dimensional radiation magnetohydrodynamic
simulations of the accretion flow into a supermassive black hole carried
out by using a radiation magnetohydrodynamic code CANS+R. They found
that when the accretion rate exceeds 10% of the Eddington accretion
rate, a warm Compton region with a temperature of $10^{7}-10^{8}$
K is formed outside the RIAF near the black hole and that radial
oscillation is excited in this region when the radiation pressure
becomes dominant. The critical luminosity for this transition is 0.5%
of the Eddington luminosity, which is consistent with observations
of changing look AGN. However, the temperature of the warm region is
higher than that of the soft X-ray emitting region of AGN obtained by
using the observed radiation spectra. Here we report the results of
radiation magnetohydrodynamic simulations carried out by implementing
the effects of Compton cooling to the CANS+R code. As a result, the
temperature of the Thomson thick, warm Compton region decreased to
$10^{6}-10^{7}$ K. We discuss the structure and time variation of the
warm Compton region.
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Title: The Jet and Resolved Features of the Central Supermassive
Black Hole of M87 Observed with the Event Horizon Telescope (EHT)
Authors: Miyoshi, Makoto; Kato, Yoshiaki; Makino, Junichiro
2022ApJ...933...36M Altcode: 2022arXiv220504623M
We report the result of our independent image reconstruction of the
M87 from the public data of the Event Horizon Telescope Collaborators
(EHTC). Our result is different from the image published by the
EHTC. Our analysis shows that (a) the structure at 230 GHz is consistent
with those of lower-frequency very long baseline interferometry
observations, (b) the jet structure is evident at 230 GHz extending
from the core to a few milliarcsecond, although the intensity rapidly
decreases along the axis, and (c) the "unresolved core" is resolved
into three bright features presumably showing an initial jet with
a wide opening angle of ~70°. The ring-like structures of the
EHTC can be created not only from the public data but also from the
simulated data of a point image. Also, the rings are very sensitive
to the field-of-view (FOV) size. The u-v coverage of the Event Horizon
Telescope (EHT) lacks ~ 40 μas fringe spacings. Combining with a very
narrow FOV, it created the ~40 μas ring structure. We conclude that
the absence of the jet and the presence of the ring in the EHTC result
are both artifacts owing to the narrow FOV setting and the u-v data
sampling bias effect of the EHT array. Because the EHTC's simulations
only take into account the reproduction of the input image models,
and not those of the input noise models, their optimal parameters can
enhance the effects of sampling bias and produce artifacts such as
the ~40 μas ring structure, rather than reproducing the correct image.
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Title: Wakefield Acceleration in a Jet from a Neutrino-driven
Accretion Flow around a Black Hole
Authors: Kato, Yoshiaki; Ebisuzaki, Toshikazu; Tajima, Toshiki
2022ApJ...929...42K Altcode: 2022arXiv220111755K
We have investigated electromagnetic (EM) wave pulses in a jet
from a neutrino-driven accretion flow (NDAF) around a black hole
(BH). NDAFs are massive accretion disks whose accretion rates are
$\dot{M}\,\approx $ 0.01-10 M <SUB>⊙</SUB> s<SUP>-1</SUP> for
stellar-mass BHs. Such an extreme accretion may produce a collimated
relativistic outflow like a magnetically driven jet in active galactic
nuclei and microquasars. When we consider strong toroidal magnetic field
stranded in the inner region of an NDAF disk and magnetic impulses
on the jet, we find that they lead to the emanation of high-energy
emissions for gamma-ray bursts, as well as high-energy cosmic rays. When
Alfvénic wave pulses are generated by episodic immense accretions,
they propagate along the large-scale structured magnetic field in the
jet. Once the Alfvénic wave pulses reach nearly the speed of light
in the underdense condition, they turn into EM wave pulses, which
produce plasma wakes behind them. These wakefields exert a collective
accelerating force synchronous to the motion of particles. As a result,
the wakefield acceleration premises various observational signatures,
such as pulsating bursts of high-energy gamma rays from accelerated
electrons, pulses of neutrinos from accelerated protons, and protons
with maximum energies beyond 10<SUP>20</SUP> eV.
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Title: X-ray laser development at the Institute of Laser Engineering,
Osaka University with worldwide collaboration
Authors: Kato, Yoshiaki; Daido, Hiroyuki
2021SPIE11886E..0DK Altcode:
This paper presents a brief review of the x-ray laser development at
the Institute of Laser Engineering, Osaka University, implemented with
worldwide collaboration. The scaling of the x-ray lasing toward shorter
wavelengths has been investigated in the recombination-pumped (RP) and
electron-collisional-excitation (CE) pumped x-ray lasers. Extension of
the RP x-ray laser close to the water window is described. With the
CE x-ray laser, intense lasing of the J = 0-1 line at 19.6 nm in the
neon-like Ge ion and lasing over 14.3 - 4.5 nm with the nickel-like ions
are reported. Spectroscopic studies of the x-ray lasers are described,
including the first observation of polarization of the x-ray laser
beam generated by amplified spontaneous emission. The perspective of
the plasma-based x-ray lasers is also presented.
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Title: Wakefield Acceleration in a Jet from a Neutrino Driven
Accretion Flow around a Black Hole
Authors: Kato, Yoshiaki; Ebisuzaki, Toshikazu; Tajima, Toshiki
2021APS..DPPJP1012K Altcode:
We have investigated electromagnetic pulses in a jet from a neutrino
driven accretion flow (NDAF) around a black hole. NDAFs are massive
accretion disks of accretion rates M ~ 0.01-10M⊙/s for black holes
of several solar masses M <= 10M⊙, such extreme accretions
are investigated as a model of gamma-ray bursts (GRBs) as well as
supernovae and hypernovae. Recently, Ebisuzaki & Tajima 2019
(ET19) have proposed a model of acceleration mechanism of charged
particles to very high energies ~ 10<SUP>20</SUP> eV by electromagnetic
wave-particle interaction. If episodic eruptive accretions generate Alfv
́enic pulses along large-scale structured magnetic field in the jet,
such Alfv ́enic pulses act as a driver of the collective accelerating
pondermotive forces which drive the wakes whose direction is parallel
to the motion of particles. Because the wakes propagate at the same
speed with the particles, the so-called wakefield acceleration has
a robust built-in coherence by the acceleration system itself. In
this study we extend a model of the accretion disk presented by ET19
into a NDAF. We estimate the energy flux of both the electro-magnetic
wave pulses and neutrino emissions from the NDAFs. We find that the
total luminosity of waves L<SUB>wave</SUB> = Mc<SUP>2</SUP> / ?? √
18α<SUP>2</SUP>β<SUP>3</SUP> and the that of neutrinos L<SUB>ν</SUB>
= Mc<SUP>2</SUP> / 4 where α and β are the viscosity parameter
and the plasma-β of the disk, respectively. The properties of the
NDAFs and magnetically driven jets as well as the maximum energy of
accelerated charged particles will be discussed.
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Title: Radiation Magnetohydrodynamic Simulations of Black Hole
Accretion Flows in Bright Hard State
Authors: Igarashi, Taichi; Matsumoto, Yosuke; Matsumoto, Ryoji; Kato,
Yoshiaki; Takahashi, Hiroyuki; Ohsuga, Ken
2021cosp...43E1550I Altcode:
Bright hard states are observed during the outbursts of stellar
mass black hole candidates. This state appears when the luminosity
exceeds 0.1% of the Eddington luminosity in the hard X-ray dominant
state. When the luminosity increase further, the black hole candidate
transit to the soft X-ray dominant state. Such a transition is called
hard-to-soft transitions. Similar state transitions are observed in
AGNs. In some Seyfert galaxies, soft X-ray excess component appears when
the luminosity exceeds 0.1% of the Eddington luminosity. Furthermore,
rapid time variabilities are observed during the hard-to-soft state
transition. Since the luminosity of the bright hard state exceeds
the upper limit for radiatively inefficient accretion flows (RIAFs),
radiative cooling should be taken into account to simulate the accretion
flow in this state. We carried out 3D radiation magnetohydrodynamic
simulations of the hard-to-soft state transition. The initial disk
density is determined, such that the mass accretion rate is around
10% of the Eddington accretion rate. We found that radiation pressure
dominant, relatively cool (T $<$$ 10^{8}$ K) region appears outside
the optically thin, hot accretion flow near the black hole. We
also found that the radiation pressure dominant region oscillates
quasi-periodically. The possible mechanism of the oscillation is
the radial pulsational instability (e.g., Kato 1978; Blumenthal
et al. 1984). In this talk, we discuss the possibility that the
non-axisymmetric radial pulsation in the radiation pressure dominant
region is the origin of low frequency QPOs in black hole candidates
and rapid time variabilities in changing look AGNs.
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Title: Radiation Magnetohydrodynamic Simulations of Sub-Eddington
Accretion Flows in AGNs: Origin of Soft X-Ray Excess and Rapid
Time Variabilities
Authors: Igarashi, Taichi; Kato, Yoshiaki; Takahashi, Hiroyuki R.;
Ohsuga, Ken; Matsumoto, Yosuke; Matsumoto, Ryoji
2020ApJ...902..103I Altcode: 2020arXiv200905813I
We investigate the origin of the soft X-ray excess component in Seyfert
galaxies observed when their luminosity exceeds 0.1% of the Eddington
luminosity ( ${L}_{\mathrm{Edd}}$ ). The evolution of a dense blob in
radiatively inefficient accretion flow (RIAF) is simulated by applying
a radiation magnetohydrodynamic code, CANS+R. When the accretion rate
onto a ${10}^{7}\,{M}_{\odot }$ black hole exceeds 10% of the Eddington
accretion rate ( ${\dot{M}}_{\mathrm{Edd}}={L}_{\mathrm{Edd}}/{c}^{2}$
, where c is the speed of light), the dense blob shrinks vertically
because of radiative cooling and forms a Thomson thick, relatively
cool (∼10<SUP>7-8</SUP> K) region. The cool region coexists with
the optically thin, hot ( $T\sim {10}^{11}\,{\rm{K}}$ ) RIAF near the
black hole. The cool disk is responsible for the soft X-ray emission,
while hard X-rays are emitted from the hot inner accretion flow. Such
a hybrid structure of hot and cool accretion flows is consistent
with the observations of both hard and soft X-ray emissions from
"changing-look" active galactic nuclei (CLAGNs). Furthermore, we
find that quasi-periodic oscillations (QPOs) are excited in the soft
X-ray-emitting region. These oscillations can be the origin of rapid
X-ray time variabilities observed in CLAGNs.
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Title: Chromospheric and Coronal Wave Generation in a Magnetic
Flux Sheath
Authors: Kato, Yoshiaki; Steiner, Oskar; Hansteen, Viggo; Gudiksen,
Boris; Wedemeyer, Sven; Carlsson, Mats
2016ApJ...827....7K Altcode: 2016arXiv160608826K
Using radiation magnetohydrodynamic simulations of the solar
atmospheric layers from the upper convection zone to the lower corona,
we investigate the self-consistent excitation of slow magneto-acoustic
body waves (slow modes) in a magnetic flux concentration. We
find that the convective downdrafts in the close surroundings of
a two-dimensional flux slab “pump” the plasma inside it in
the downward direction. This action produces a downflow inside the
flux slab, which encompasses ever higher layers, causing an upwardly
propagating rarefaction wave. The slow mode, excited by the adiabatic
compression of the downflow near the optical surface, travels along the
magnetic field in the upward direction at the tube speed. It develops
into a shock wave at chromospheric heights, where it dissipates,
lifts the transition region, and produces an offspring in the form
of a compressive wave that propagates further into the corona. In the
wake of downflows and propagating shock waves, the atmosphere inside
the flux slab in the chromosphere and higher tends to oscillate with a
period of ν ≈ 4 mHz. We conclude that this process of “magnetic
pumping” is a most plausible mechanism for the direct generation
of longitudinal chromospheric and coronal compressive waves within
magnetic flux concentrations, and it may provide an important heat
source in the chromosphere. It may also be responsible for certain
types of dynamic fibrils.
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Title: The statistical properties of vortex flows in the solar
atmosphere
Authors: Wedemeyer, Sven; Kato, Yoshiaki; Steiner, Oskar
2015IAUGA..2256852W Altcode:
Rotating magnetic field structures associated with vortex flows
on the Sun, also known as “magnetic tornadoes”, may serve
as waveguides for MHD waves and transport mass and energy upwards
through the atmosphere. Magnetic tornadoes may therefore potentially
contribute to the heating of the upper atmospheric layers in quiet
Sun regions.Magnetic tornadoes are observed over a large range
of spatial and temporal scales in different layers in quiet Sun
regions. However, their statistical properties such as size, lifetime,
and rotation speed are not well understood yet because observations
of these small-scale events are technically challenging and limited
by the spatial and temporal resolution of current instruments. Better
statistics based on a combination of high-resolution observations and
state-of-the-art numerical simulations is the key to a reliable estimate
of the energy input in the lower layers and of the energy deposition
in the upper layers. For this purpose, we have developed a fast and
reliable tool for the determination and visualization of the flow
field in (observed) image sequences. This technique, which combines
local correlation tracking (LCT) and line integral convolution (LIC),
facilitates the detection and study of dynamic events on small scales,
such as propagating waves. Here, we present statistical properties
of vortex flows in different layers of the solar atmosphere and try
to give realistic estimates of the energy flux which is potentially
available for heating of the upper solar atmosphere
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Title: Measuring energy flux of magneto-acoustic wave in the magnetic
elements by using IRIS
Authors: Kato, Yoshiaki; De Pontieu, Bart; Martinez-Sykora, Juan;
Hansteen, Viggo; Pereira, Tiago; Leenaarts, Jorritt; Carlsson, Mats
2014cosp...40E1423K Altcode:
NASA's Interface Region Imaging Spectrograph (IRIS) has opened a new
window to explore the chromospheric/coronal waves that potentially
energize the solar atmosphere. By using an imaging spectrograph covering
the Si IV and Mg II h&k lines as well as a slit-jaw imager centered
at Si IV and Mg II k onboard IRIS, we can determine the nature of
propagating magneto-acoustic waves just below and in the transition
region. In this study, we compute the vertically emergent intensity of
the Si IV and Mg II h&k lines from a time series of snapshots of
a magnetic element in a two-dimensional Radiative MHD simulation from
the Bifrost code. We investigate the synthetic line profiles to detect
the slow magneto-acoustic body wave (slow mode) which becomes a slow
shock at the lower chromosphere in the magnetic element. We find that
the Doppler shift of the line core gives the velocity amplitude of the
longitudinal magneto-acoustic body wave. The contribution function of
the line core indicates that the formation of Mg II h&k lines is
associated with the propagating shocks and therefore the time evolution
of the line core intensity represents the propagating shocks projected
on the optical surface. We will report on measurement of the energy
flux of slow modes in the magnetic elements by using IRIS observations.
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Title: Saturation of Stellar Winds from Young Suns
Authors: Suzuki, Takeru K.; Imada, Shinsuke; Kataoka, Ryuho; Kato,
Yoshiaki; Matsumoto, Takuma; Miyahara, Hiroko; Tsuneta, Saku
2013PASJ...65...98S Altcode: 2012arXiv1212.6713S
We investigated mass losses via stellar winds from Sun-like
main-sequence stars with a wide range of activity levels. We performed
forward-type magnetohydrodynamical numerical experiments for Alfvén
wave-driven stellar winds with a wide range of input Poynting flux
from the photosphere. Increasing the magnetic field strength and
the turbulent velocity at the stellar photosphere from the current
solar level, the mass-loss rate rapidly at first increases, owing
to suppression of the reflection of the Alfvén waves. The surface
materials are lifted up by the magnetic pressure associated with
the Alfvén waves, and the cool dense chromosphere is intermittently
extended to 10%#8211;20% of the stellar radius. The dense atmospheres
enhance the radiative losses, and eventually most of the input Poynting
energy from the stellar surface escapes by radiation. As a result, there
is no more sufficient energy remaining for the kinetic energy of the
wind; the stellar wind saturates in very active stars, as observed in
Wood et al. (2002, ApJ, 574, 412; 2005, ApJ, 628, L143). The saturation
level is positively correlated with B<SUB>r,</SUB><SUB>0</SUB>
f<SUB>0</SUB>, where B<SUB>r,</SUB><SUB>0</SUB> and f<SUB>0</SUB>
are the magnetic field strength and the filling factor of open flux
tubes at the photosphere. If B<SUB>r,</SUB><SUB>0</SUB> f<SUB>0</SUB>
is relatively large gtrsim 5 G, the mass-loss rate could be as high as
1000 times. If such a strong mass loss lasts for ∼ 1 billion years,
the stellar mass itself would be affected, which could be a solution to
the faint young Sun paradox. We derived a Reimers-type scaling relation
that estimates the mass-loss rate from an energetics consideration of
our simulations. Finally, we derived the evolution of the mass-loss
rates, dot;{M} ∝ t<SUP>-1.23</SUP>, of our simulations, combining
with an observed time evolution of X-ray flux from Sun-like stars,
which are shallower than dot;{M} ∝ t<SUP>-2.33±0.55</SUP> in Wood
et al. (2005).
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Title: First phase observations of the enormous flare due to vast
mass infall onto the Sgr A* black hole
Authors: Asaki, Yoshiharu; Dodson, Richard; Miyoshi, Makoto; Tsuboi,
Masato; Oka, Tomoharu; Horiuchi, Shinji; Nishiyama, Shogo; Yonekura,
Yoshinori; Kato, Yoshiaki; Takaba, Hiroshi; Miyamoto, Yusuke;
Takahashi, Masaaki; Saida, Hiromi; Takekawa, Shunya; Sekido, Mamoru;
Kameya, Osamu
2013atnf.prop.5778A Altcode:
We propose to perform the first phase observations of the enormous flare
due to vast mass infall onto the Sgr A* black hole using the ATCA at
15, 7, and 3 mm bands in order to record the spectacular event as soon
as possible after the Sgr A* flare-up event observed from our daily
monitor systems in radio using the Japanese VLBI and in near-infrared
(NIR) using a 1.4-m telescope (J, H, Ks) in South Africa.
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Title: First phase observations of the enormous flare due to vast
mass infall onto the Sgr A* black hole
Authors: Asaki, Yoshiharu; Miyoshi, Makoto; Tsuboi, Masato; Horiuchi,
Shinji; Nishiyama, Shogo; Yonekura, Yoshinori; Kato, Yoshiaki; Takaba,
Hiroshi; Miyamoto, Yusuke; Takahashi, Masaaki; Saida, Hiromi
2013atnf.prop.5501A Altcode:
We propose to perform the first phase observations of the enormous
flare due to vast mass infall onto the Sgr A* black hole using the ATCA
at 15, 7, and 3~mm bands in order to record the spectacular event as
soon as possible after our detection of the flare-up event of Sgr A*
from daily monitoring in radio using the Japanese short-baseline VLBI
and in near-infrared (NIR) using a 1.4-m telescope (J, H, Ks) in South
Africa. We have obtained Target-of-Opportunity (ToO) time of telescopes
from radio to infrared for the same science target, so that we will
be tackling on the first phase of the event in the wide frequency range.
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Title: First phase observations of the enormous flare due to vast
mass infall onto the Sgr A* black hole
Authors: Miyoshi, Makoto; Tsuboi, Masato; Yonekura, Yoshinori;
Asaki, Yoshiharu; Kato, Yoshiaki; Takaba, Hiroshi; Miyamoto, Yusuke;
Takahashi, Masaaki; Saida, Hiromi
2012atnf.prop.5114M Altcode:
We propose to perform the first phase observations of the enormous flare
due to vast mass infall onto the Sgr A* black hole using the ATCA at 15,
7, and 3 mm bands in order to record the spectacular event as soon as
possible after our detection of the increase in flux density of Sgr A*
from the daily monitoring using the short-baseline VLBI in Japan.
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Title: Chromospheric Lyman-alpha spectro-polarimeter (CLASP)
Authors: Kano, Ryouhei; Bando, Takamasa; Narukage, Noriyuki; Ishikawa,
Ryoko; Tsuneta, Saku; Katsukawa, Yukio; Kubo, Masahito; Ishikawa,
Shin-nosuke; Hara, Hirohisa; Shimizu, Toshifumi; Suematsu, Yoshinori;
Ichimoto, Kiyoshi; Sakao, Taro; Goto, Motoshi; Kato, Yoshiaki; Imada,
Shinsuke; Kobayashi, Ken; Holloway, Todd; Winebarger, Amy; Cirtain,
Jonathan; De Pontieu, Bart; Casini, Roberto; Trujillo Bueno, Javier;
Štepán, Jiří; Manso Sainz, Rafael; Belluzzi, Luca; Asensio Ramos,
Andres; Auchère, Frédéric; Carlsson, Mats
2012SPIE.8443E..4FK Altcode:
One of the biggest challenges in heliophysics is to decipher the
magnetic structure of the solar chromosphere. The importance of
measuring the chromospheric magnetic field is due to both the key role
the chromosphere plays in energizing and structuring the outer solar
atmosphere and the inability of extrapolation of photospheric fields to
adequately describe this key boundary region. Over the last few years,
significant progress has been made in the spectral line formation
of UV lines as well as the MHD modeling of the solar atmosphere. It
is found that the Hanle effect in the Lyman-alpha line (121.567 nm)
is a most promising diagnostic tool for weaker magnetic fields in
the chromosphere and transition region. Based on this groundbreaking
research, we propose the Chromospheric Lyman-Alpha Spectro-Polarimeter
(CLASP) to NASA as a sounding rocket experiment, for making the first
measurement of the linear polarization produced by scattering processes
and the Hanle effect in the Lyman-alpha line (121.567 nm), and making
the first exploration of the magnetic field in the upper chromosphere
and transition region of the Sun. The CLASP instrument consists
of a Cassegrain telescope, a rotating 1/2-wave plate, a dual-beam
spectrograph assembly with a grating working as a beam splitter, and
an identical pair of reflective polarization analyzers each equipped
with a CCD camera. We propose to launch CLASP in December 2014.
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Title: Oscillation Phenomena in the Disk around the Massive Black
Hole Sagittarius A<SUP>*</SUP>
Authors: Miyoshi, Makoto; Shen, Zhi-Qiang; Oyama, Tomoaki; Takahashi,
Rohta; Kato, Yoshiaki
2011PASJ...63.1093M Altcode:
We report the detection of radio QPOs with structure changes using
the Very Long Baseline Array (VLBA) at 43 GHz. We found conspicuous
patterned changes of the structure with P = 16.8±1.4, 22.2±1.4,
31.4±1.5, 56.4±6 min roughly in a 3:4:6:10 ratio. The first two
periods show a rotating one-arm structure, while P = 31.4 min shows a
rotating 3-arm structure, as if viewed edge-on. At the central 50 μas
the P = 56.4 min period shows a double-amplitude variation of those in
its surroundings. The spatial distributions of the oscillation periods
suggest that the disk of Sgr A* is roughly edge-on, rotating around an
axis with PA = -10°. Presumably, the observed VLBI images of Sgr A*
at 43 GHz retain several features of the black hole accretion disk
of Sgr A* in spite of being obscured and broadened by scattering of
surrounding plasma.
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Title: Excitation of magneto-acoustic waves in network magnetic
elements
Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu,
Yoshinori
2011IAUS..273..442K Altcode:
From radiation magnetohydrodynamic (RMHD) simulations we track the
temporal evolution of a vertical magnetic flux sheet embedded in a
two-dimensional non-stationary atmosphere that reaches all the way
from the upper convection zone to the low chromosphere. Examining its
temporal behavior near the interface between the convection zone and
the photosphere, we describe the excitation of propagating longitudinal
waves within the magnetic element as a result of convective motion in
its surroundings.
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Title: Excitation of Slow Modes in Network Magnetic Elements Through
Magnetic Pumping
Authors: Kato, Yoshiaki; Steiner, Oskar; Steffen, Matthias; Suematsu,
Yoshinori
2011ApJ...730L..24K Altcode: 2011arXiv1102.5164K
From radiation magnetohydrodynamic simulations of the solar atmosphere,
we find a new mechanism for the excitation of longitudinal slow modes
within magnetic flux concentrations. We find that the convective
downdrafts in the immediate surroundings of magnetic elements are
responsible for the excitation of slow modes. The coupling between
the external downdraft and the plasma motion internal to the flux
concentration is mediated by the inertial forces of the downdraft that
act on the magnetic flux concentration. These forces, in conjunction
with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric
and chromospheric layers of the magnetic element. Subsequent to the
transient pumping phase, the atmosphere rebounds, causing a slow
mode traveling along the magnetic flux concentration in the upward
direction. It develops into a shock wave in chromospheric heights,
possibly capable of producing some kind of dynamic fibril. We propose
an observational detection of this process.
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Title: Theoretical Interpretation of X-ray Spectra from Active
Galactic Nuclei and Low/Hard State of X-ray Binaries with an Accretion
Disk-Corona Model
Authors: Kawanaka, Norita; Kato, Yoshiaki; Mineshige, Shin
2010ecsa.conf..304K Altcode:
No abstract at ADS
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Title: Global Radiation-Magnetohydrodynamic Simulations of Black-Hole
Accretion Flow and Outflow: Unified Model of Three States
Authors: Ohsuga, Ken; Mineshige, Shin; Mori, Masao; Kato, Yoshiaki
2009PASJ...61L...7O Altcode: 2009arXiv0903.5364O
Black-hole accretion systems are known to possess several distinct
modes (or spectral states), such as low/hard state, high/soft state,
and so on. Since the dynamics of the corresponding flows is distinct,
theoretical models were separately discussed for each state. We here
propose a unified model based on our new, global, two-dimensional
radiation-magnetohydrodynamic simulations. By controlling a density
normalization we could for the first time reproduce three distinct
modes of accretion flow and outflow with one numerical code. When
the density is large (model A), a geometrically thick, very luminous
disk forms, in which photon trapping takes place. When the density is
moderate (model B), the accreting gas can effectively cool by emitting
radiation, thus generating a thin disk, i.e., the soft-state disk. When
the density is too low for radiative cooling to be important (model C),
a disk becomes hot, thick, and faint; i.e., the hard-state disk. The
magnetic energy is amplified within the disk up to about twice, 30%,
and 20% of the gas energy in models A, B, and C, respectively. Notably,
the disk outflows with helical magnetic fields, which are driven either
by radiation pressure force or magnetic pressure force, are ubiquitous
in any accretion modes. Finally, our simulations are consistent with
the phenomenological alpha-viscosity prescription, that is, the disk
viscosity is proportional to the pressure.
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Title: X-Ray Emissions from Three-Dimensional Magnetohydrodynamic
Coronal Accretion Flows
Authors: Kawanaka, Norita; Kato, Yoshiaki; Mineshige, Shin
2008PASJ...60..399K Altcode: 2008arXiv0801.4613K
We calculated the radiation spectrum and its time variability of a
black-hole accretion disk-corona system based on a three-dimensional
magnetohydrodynamic (MHD) simulation. In explaining the spectral
properties of active galactic nuclei, it is often assumed that they
consist of a geometrically thin, optically thick disk and hot, optically
thin corona surrounding the thin disk. As for a model of the corona,
we adopted the simulation data of three-dimensional, non-radiative
MHD accretion flows calculated by Y. Kato and coworkers, while for a
thin disk we assumed a standard-type disk. We performed Monte-Carlo
radiative transfer simulations of the corona, while taking into account
the Compton scattering of soft photons from the thin disk by hot
thermal electrons and coronal irradiation heating of the thin disk,
which emits blackbody radiation. By adjusting the density parameter
of the MHD coronal flow, we could produce emergent spectra that are
consistent with those of typical Seyfert galaxies. Moreover, we found
rapid time variability in the X-ray emission spectra, originating from
a density fluctuation produced by a magnetorotational instability in
the MHD corona. The features of reflection component including iron
fluorescent line emission are also briefly discussed.
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Title: 3-D MHD Simulations of Astrophysical Jets
Authors: Kato, Yoshiaki
2008APS..APR3HE002K Altcode:
Jets and outflows are ubiquitous among the accretion process in a
variety of astrophysical objects. Global three-dimensional (3-D)
magnetohydrodynamic (MHD) simulations of accretion flows have
revealed the generation of jets by the emergence of a magnetic
tower. In other words, the magnetic interaction associated with
stars and/or accretion disks is a promising universal mechanism of
launching jets. However, photon spectra of accretion flows and jets
in some microquasars display some serious problems. When spetrum is
dominated by non-thermal emissions, a mildly relativistic steady
jet is observed. On the other hand, when spectrum is dominated by
thermal emissions, no jet is observed. Remarkably, during a transition
from a non-thermal state to a thermal state, a ultra-relativistic
transient jet is observed. Therefore, the radiation in a magnetized
accretion flow is a key to understand the formation of jets. In this
talk, we present 3-D MHD simulations of magnetic tower jets and also
3-D radiation transfer (RT) simulations of magnetic tower jets. The
possible connection between the emergence of magnetic tower jets and
the evolution of radiation properties will be discussed.
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Title: Magnetic-Tower Jet Solution for Launching Astrophysical Jets
Authors: Kato, Yoshiaki
2007Ap&SS.307...11K Altcode: 2006Ap&SS.tmp..531K
In spite of the large number of global three-dimensional (3-D)
magnetohydrodynamic (MHD) simulations of accretion disks and
astrophysical jets, which have been developed since 2000, the launching
mechanisms of jets is somewhat controversial. Previous studies of jets
have concentrated on the effect of the large-scale magnetic fields
permeating accretion disks. However, the existence of such global
magnetic fields is not evident in various astrophysical objects,
and their origin is not well understood. Thus, we study the effect
of small-scale magnetic fields confined within the accretion disk. We
review our recent findings on the formation of jets in dynamo-active
accretion disks by using 3-D MHD simulations. In our simulations,
we found the emergence of accumulated azimuthal magnetic fields from
the inner region of the disk (the so-called magnetic tower) and also
the formation of a jet accelerated by the magnetic pressure of the
tower. Our results indicate that the magnetic tower jet is one of
the most promising mechanisms for launching jets from the magnetized
accretion disk in various astrophysical objects. We will discuss the
formation of cosmic jets in the context of the magnetic tower model.
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Title: Magnetized accretion flow, jets, and coronae
Authors: Mineshige, Shin; Kato, Yoshiaki; Ohsuga, Ken; Kawanaka, Norita
2005AIPC..801..147M Altcode:
Magnetic fields seem to play critical roles in accretion flow,
jets, and coronae. We overview their roles in connection with
observations. Special attentions will be paid on the spectral properties
of magnetized flow and corona, and emergence of magnetic-tower jets from
localized fields. Despite rapid progress in numerical simulations we are
still in a state far from being complete in understanding. Outstanding
issues are also presented.
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Title: Formation of magnetic-tower jets in magnetohydrodynamic
accretion flows
Authors: Kato, Yoshiaki
2005AstHe..98..525K Altcode:
The previous study of astrophysical jets has been concentrated on
the effects of large-scale magnetic fields permeating accretion
disks. However, the existence of such global magnetic fields is
not evident in many astrophysical objects. Instead, the magnetic
fields, generated in the disks as a result of magneto-rotational
instability, can create jets accelerated by the magnetic pressure of the
magnetic-tower. We describe our attempt to explore the magnetic-tower
jet solution as a unified model of astrophysical jets.
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Title: Spectral Properties of Three-dimensional Magnetohydrodynamic
Accretion Flows
Authors: Ohsuga, Ken; Kato, Yoshiaki; Mineshige, Shin
2005ApJ...627..782O Altcode: 2005astro.ph..3686O
In spite of a large number of global three-dimensional MHD simulations
of accretion flows and jets being made recently, their astrophysical
relevance for realistic situations is not well known. In order to
examine to what extent the simulated MHD flows can account for the
observed SED of Sgr A*, for the first time we calculate the emergent
spectra from three-dimensional MHD flows in a wide range of wavelengths
(from radio to X-ray) by solving the three-dimensional radiative
transfer equations. We use the simulation data by Kato and coworkers and
perform Monte Carlo radiative transfer simulations, in which synchrotron
emission/absorption, free-free emission/absorption, and Compton/inverse
Compton scattering are taken into account. We assume two-temperature
plasmas and calculate electron temperatures by solving the electron
energy equation. Only thermal electrons are considered. It is found
that the three-dimensional MHD flow generally overproduces X-rays by
means of bremsstrahlung radiation from the regions at large radii. A
flatter density profile, ρ~r<SUP>-a</SUP> with a<1, than that of the
ADAF, ρ~r<SUP>-3/2</SUP>, is the main reason for this. If we restrict
the size of the emission region to be as small as ~10r<SUB>S</SUB>,
where r<SUB>S</SUB> is the Schwarzschild radius, the MHD model can
reproduce the basic features of the observed SED of Sgr A* during its
flaring state. Yet, the spectrum in the quiescent state remains to
be understood. We discuss how to resolve this issue in the context of
MHD flow models. Possibilities include modifications of the MHD flow
structure by the inclusion of radiative cooling and/or significant
contributions by nonthermal electrons. It is also possible that the
present spectral results may be influenced by particular initial
conditions. We also calculate the time-dependent spectral changes,
finding that the fluxes fluctuate in a wide range of the frequency
and the flux at each wavelength does not always vary coherently.
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Title: 2:3 Twin Quasi-Periodic Oscillations in Magnetohydrodynamic
Accretion Flows
Authors: Kato, Yoshiaki
2004PASJ...56..931K Altcode: 2004astro.ph..8018K
We have studied the radial and vertical oscillations in
three-dimensional magnetohydrodynamic (MHD) accretion flows around
black holes. General-relativistic effects were taken into account by
using a pseudo-Newtonian potential. We found that the structure of MHD
flows is changed at 3.8r<SUB>s</SUB> ≤ r ≤ 6.3r<SUB>s</SUB>, and
that two pairs of quasi-periodic oscillations (twin QPOs) are excited
in that region. The time evolution of the power spectrum density (PSD)
indicates that these twin QPOs are most likely to be produced by a
resonance between the Keplerian frequency, ν<SUB>K</SUB>, and the
epicyclic frequency, ν<SUB>K</SUB>. The PSD shows that the lower peak
frequency, ν<SUB>l</SUB> corresponds to, ν<SUB>K</SUB>, while the
upper peak frequency, ν<SUB>u</SUB>, corresponds to ν<SUB>K</SUB>
+ ν<SUB>κ</SUB>. The ratio of two peak frequencies is close to 2 :
3. The results provide the first direct evidence for the excitation
of resonant disk oscillation in MHD accretion flows.
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Title: XUV laser research at the Rutherford Appleton Laboratory
Authors: Key, Michael H.; Rose, S. J.; Grande, Manuel; Tallents,
Gregory J.; Ramsden, Stuart A.; Rogoyski, A.; Lewis, Ciaran L.;
O'Neill, D. M.; Regan, C.; Evans, Brian L.; Al-Arab, Ali M.; Pert,
Geoffrey J.; Henshaw, M.; Jaegle, Pierre; Carillon, Antoine; Klisnick,
Annie; Jamelot, Gerard; Sureau, Alain; Guennou, M.; Kato, Yoshiaki;
Yamanaka, Masanobu; Dido, H.; Tachi, T.; Nishimura, Hiroaki; Shiraga,
H.; Herman, P.; Miura, E.; Takabe, Hideaki; McLean, Edgar A.; Lee, Tim
1989SPIE.1140...21K Altcode:
No abstract at ADS