18 research outputs found
Global Operation of Resistive, Radiation-Inefficient, Accretion Flows in Preparing Jet-Driving Circumstances
In our recent paper, we have obtained a model solution to the problem of
radiation-inefficient accretion flows (RIAFs) in a global magnetic field (so
called, resistive RIAF model), which is asymptotically exact in outer regions
of such flows forming accretion disks. When extrapolated inwardly, the model
predicts a local enhancement of the vertical Poynting flux within a small
radius that may be regarded as the disk inner-edge. This fact has been
interpreted as the origin of power source for the astrophysical jets
observationally well-known to be ejected from this type of accretion disks.
Since the accuracy of the solution may become rather poor in such inner
regions, however, the ground of this assertion may not seem to be so firm. In
the present paper, we develop a sophisticated discussion for the appearance of
jet-driving circumstances, based on a much more firm ground by deriving a
global solution in the same situation. Although the new solution still has an
approximate nature, it becomes exact in the limits not only of large radius but
also of small radius. The analytic results clarify that the electrodynamic
power is gathered by the Poynting flux, from outer main-disk region to feed the
innermost part of an accretion disk. The injected power largely exceeds the
local supply of work by the fluid motion.Comment: 10 pages, 3 figures, accepted for publication in PAS
Two Types of Magnetohydrodynamic Sheath Jets
Recent observations of astrophysical jets emanating from various galactic
nuclei strongly suggest that a double layered structure, or a spine-sheath
structure, is likely to be their common feature. We propose that such a sheath
jet structure can be formed magnetohydrodynamically within a valley of the
magnetic pressures, which is formed between the peaks due to the poloidal and
toroidal components, with the centrifugal force acting on the rotating sheath
plasma is balanced by the hoop stress of the toroidal field. The poloidal field
concentrated near the polar axis is maintained by a converging plasma flow
toward the jet region, and the toroidal field is developed outside the jet cone
owing to the poloidal current circulating through the jet. Under such
situations, the set of magnetohydrodynamic (MHD) equations allows two main
types of solutions, at least, in the region far from the footpoint. The first
type solution describes the jets of marginally bound nature. This type is
realized when the jet temperature decreases like viral one, and neither the
pressure-gradient nor the MHD forces, which are both determined consistently,
cannot completely overcome the gravity even at infinity. The second type is
realized under an isothermal situation, and the gravity is cancelled exactly by
the pressure-gradient force. Hence, the jets of this type are accelerated
purely by the MHD force. It is suggested also that these two types correspond,
respectively, to the jets from type I and II radio galaxies in the
Fanaroff-Riley classification.Comment: 12 pages, 1 figure, to appear in PASJ, Vol.61, No.
Appearance of Jet-Driving Poynting Flux in Hot, Tenuous Accretion Disks Threaded by an Ordered Magnetic Field
In a series of our previous works, a model of radiatively inefficient
accretion flows (RIAFs) in a global magnetic field (so called resistive RIAF
model) has proved its ability to account for many physical processes taking
place in such accretion flows as realized in the nuclei of the galaxies
believed to be accreting at a very small fraction of each Eddinton accretion
rate. Within the present status of this model, however, the model cannot
describe the launch of a self-confined bipolar jet from the vicinity of disk's
inner edge, although it allows the existence of a thermal wind widely
distributed over the disk surfaces. This is because the electric field (and
hence the Poynting flux) vanishes everywhere in the disk, whereas such a jet in
a globally ordered magnetic field is most likely to be accelerated
electrodynamically. We show in the present paper that this defect can be
overcome naturally if we reformulate the problem so as to admit a
quasi-stationary change of the magnetic field (and hence the appearance of a
non-irrotational electric field), and also restore all the terms of order
epsilon ~ (v_r/v_{varphi})^2 < 1 (where v_r and v_{varphi} denote radial and
azimuthal components, respectively, of the fluid velocity) which have been
neglected altogether in the previous treatments. The restored effects are the
inertial and magnetic draggings on the infalling matter. As an illustrative
example, a model solution which is correct up to the first order in epsilon is
derived under a set of plausible restrictions. The new solution predicts the
appearance of a localized Poynting flux in a region near the disk inner edge,
suggesting strongly that a jet is launched from this region. Another
interesting prediction is the appearance of a rapid change of the magnetic
field also localized to this region.Comment: 13 pages, 1 figure, accepted for publication in PAS
Analytic Model for Advection-Dominated Accretion Flows in a Global Magnetic Field
A model for advection-dominated accretion flows (ADAFs) in a global magnetic
field is proposed. In contrast to the well known ADAF models in which the
viscosity of a fluid determines both angular momentum transfer and energy
dissipation in the flow, the magnetic field and the electric resistivity,
respectively, control them in this model. A manageable set of analytic
solutions for the flow and the magnetic field is obtained to vertically
non-integrated basic equations. This set describes mathematically a fully
advective accretion flow and, in physically plausible situations for most AGNs,
it is also confirmed that the radiation cooling estimated on this solution is
really negligible compared with the internal energy of the flow.Comment: 27pages, 1 figure, to appear in ApJ vol 529, Feb.1, 200
Criterion for Generation of Winds from Magnetized Accretion Disks
An analytic model is proposed for non-radiating accretion flows accompanied
by up or down winds in a global magnetic field. Physical quantities in this
model solution are written in variable-separated forms, and their radial parts
are simple power law functions including one parameter for wind strength.
Several, mathematically equivalent but physically different expressions of the
criterion for wind generation are obtained. It is suggested also that the
generation of wind is a consequence of the intervention of some mechanism that
redistributes the locally available gravitational energy, and that the
Bernoulli sum can be a good indicator of the existence of such mechanisms.Comment: 24 pages, 0 figures, ApJ accepte
What Kinds of Accretion Disks Are There in the Nuclei of Radio Galaxies?
It seems to be a widely accepted opinion that the types of accretion disks
(or flows) generally realized in the nuclei of radio galaxies and in further
lower mass-accretion rate nuclei are inner, hot, optically thin, radiatively
inefficient accretion flows (RIAFs) surrounded by outer, cool, optically thick,
standard type accretion disks. However, observational evidence for the
existence of such outer cool disks in these nuclei is rather poor. Instead,
recent observations sometimes suggest the existence of inner cool disks of
non-standard type, which develop in the region very close to their central
black holes. Taking NGC 4261 as a typical example of such light eating nuclei,
for which both flux data ranging from radio to X-ray and data for the
counterjet occultation are available, we examine the plausibility of such a
picture for the accretion states as mentioned above, based on model
predictions. It is shown that the explanation of the gap seen in the counterjet
emission in terms of the free-free absorption by an outer standard disk is
unrealistic, and moreover, the existence itself of such an outer standard disk
seems very implausible. Instead, the model of RIAF in an ordered magnetic field
(so called resistive RIAF model) can well serve to explain the emission gap in
terms of the synchrotron absorption, as well as to reproduce the observed
features of the overall spectral energy distribution (SED). This model also
predicts that the RIAF state starts directly from an interstellar hot gas phase
at around the Bondi radius and terminates at the inner edge whose radius is
about 100 times the Schwartzschild radii. Therefore, there is a good
possibility for a cool disk to develop within this innermost region.Comment: 8 pages, 3 figures, to appear in PASJ, Vol.62, No.
Effects of wind on radiation spectra from magnetized accretion disks
The effects of a wind on the emerging spectrum from an
inefficiently-radiating accretion flow in a global magnetic field are examined,
based on the analytic solution obtained recently by one of the present authors.
The results exhibit the steepening of the negative slope appearing in the
intermediate frequency range of bremsstrahlung spectrum and the decrease in the
luminosity ratio of thermal synchrotron to bremsstrahlung, in accordance with
the increasing wind strength. Both effects are due to a suppressed mass
accretion rate in the inner disk, caused by a mass loss in terms of wind.
In order to demonstrate the reliability of this model, Sagittarius A^* (Sgr
A^*) and the nucleus of M 31, both of which have been resolved in an X-ray band
by Chandra, are taken up as the best candidates for the broadband spectral
fittings. Although the observed X-ray data are reproduced for these objects by
both of the inverse-Compton and the bremsstrahlung fittings, some evidence of
preference for the latter are recognized. The wind effects are clearly seen in
the latter fitting case, in which we can conclude that a widely extending
accretion disk is present in each nucleus, with no or only weak wind in Sgr A^*
and with a considerably strong wind in the nuclear region of M 31. Especially
in Sgr A^*, the inferred mass accretion rates are much smaller than the Bondi
rate whose estimate has become reliable due to Chandra. This fact strongly
suggests that the accretion in this object does not proceed like Bondi's
prediction, though its extent almost reaches the Bondi radius.Comment: Accepted for publication in MNRA
On the Global Structure of Stellar Magnetospheres with Stellar Winds
The solutions for the magnetic field and stellar wind in axially symmetric stellar magnetospheres are obtained through perturbational method in three artificial extreme situations, i.e., the limits of weak magnetic field, strong magnetic field and weak electromagnetic coupling. The set of basic equations are derived from MHD equations in the two-fluid approximation of a plasma by assuming quasi-neutrality and small mass ratio of electrons to protons. It is emphasized in this treatment that, since the inertial term in the generalized Ohm\u27s law has generally a non-zero rotation, the \u27violation of flux-freezing\u27 arises even in a perfectly conducting plasma. This fact makes it possible for a stellar wind to blow across the magnetic lines of force. The global structure of a stellar magnetosphere is inferred from the results obtained in the three extreme situations. It is suggested that the magnetosphere have generally the closed magnetic lines of force and the stellar wind blows across them forming a current sheet which may result in a very elongated shape of the lines of force at about the equatorial plane
Radiation Spectra from Advection-Dominated Accretion Flows in a Global Magnetic Field
We calculate the radiation spectra from advection-dominated accretion flows
(ADAFs), taking into account the effects of a global magnetic field.
Calculation is based on the analytic model for magnetized ADAFs proposed by
Kaburaki, where a large-scale magnetic field controls the accretion process.
Adjusting a few parameters, we find that our model can well reproduce the
observed spectrum of Sagittarius A. The result is discussed in comparison
with those of well-known ADAF models, where the turbulent viscosity controls
the accretion process.Comment: Accepted for publication in Ap
Magnetohydrodynamical winds with Finite Electrical Conductivities
We discuss a numerical treatment of the resistive. MHD winds which are driven by strongly magnetized, rotating stars. For simplicity, the axis of rotation is assumed to coincide with that of magnetization. It is shown that rotation-driven wind solutions surely exist, to the set of resistive MHD equations even when the stellar magnetic fields remain of closed type