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

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.

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

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

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.

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