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We present results from our magnetohydrodynamical simulations of accretion flows onto black holes. Our main focus is the interplay between inflows and related outflows. We consider applications of such flows to the Galatic center and low luminosity active galactic nuclei.

The X-ray spectra of NLS1, characterized by a significant soft excess and a steep hard tail analogous to the soft-state black-hole binaries, are systematically studied. If the soft component, best expressed by a blackbody model, is interpreted as an optically-thick disk emission, the X-ray luminosity and the disk temperature indicate, according to the slim disk concept, that these NLS1 are in general relatively low-mass (10–10) black holes accreting at highly supercritical rates with the luminosity near or higher than the Eddington limit. The mass-doubling time could be yrs, hence these black holes would be growing rapidly at the recent epoch.

We present an analysis of polarimetric observations of the water vapor masers in NGC 4258 obtained with the GBT at 22 GHz. We do not detect any circular polarization in the spectrum indicative of Zeeman-induced splitting of the maser lines of water. We have improved the 1- upper limit estimate on the toroidal component of the magnetic field in the circumnuclear disk of NGC 4258 at a radius of 0.2 pc from 300 mG to 90 mG. We have developed a new method for the analysis of spectra with blended features and derive a 1- upper limit of 30 mG on the radial component of the magnetic field at a radius of 0.14 pc. Assuming thermal and magnetic pressure balance, we estimate an upper limit on the mass accretion rate of ~10 Myr. We discuss the ramifications of our results on current accretion theories that have been used to explain low luminosity active galactic nuclei.

Over the past four years it has become increasingly clear that Sgr A* is not a steady source but is more or less continuously flaring. The first large flare was observed in X-rays on October 26, 2000 with  [1].

The cyclic activity of AGN may either be caused by the changes of fuel supply to the central engine, or by the processes intrinsic to the accretion disk. Here we consider the latter possibility, assuming that the mean accretion rate is constant, and the periodic changes of the disk luminosity appear due to the instability cycle operating in its inner parts, with possible transition to advective flow (ADAF).

By analogy to the different accretion states observed in black-hole X-ray binaries (BHXBs), it appears plausible that accretion disks in active galactic nuclei (AGN) undergo a state transition between a radiatively efficient and inefficient accretion flow. If the radiative efficiency changes at some critical accretion rate, there will be a change in the distribution of black hole masses and bolometric luminosities at the corresponding transition luminosity. To test this prediction, I consider the joint distribution of AGN black hole masses and bolometric luminosities for a sample taken from the literature. The small number of objects with low Eddington-scaled accretion rates and black hole masses constitutes tentative evidence for the existence of such a transition in AGN. Selection effects, in particular those associated with flux-limited samples, systematically exclude objects in particular regions of the plane. Therefore, they require particular attention in the analysis of distributions of black hole mass, bolometric luminosity, and derived quantities like the accretion rate. I suggest further observational tests of the BHXB-AGN unification scheme which are based on the jet domination of the energy output of BHXBs in the state, and on the possible equivalence of BHXB in the (or ) state showing ejections and efficiently accreting quasars and radio galaxies with powerful radio jets.

Among Active Galactic Nuclei (AGN), narrow line Seyfert 1 galaxies (NLS1s) are supposed to have high accretion rates [1,2]. The gas accretion rate for those objects is expected to be larger than 10 /, where is the Eddington luminosity.

We present the results of 3D global resistive magnetohydrodynamic (MHD) simulations of black hole accretion flows. General relativistic effects are simulated by using the pseudo-Newtonian potential. We included optically thin bremsstrahlung cooling to simulate the transition from low state to high state. We found that the outer disk cooled down by radiative cooling becomes dominated by magnetic pressure. Hard-state disks in the inner region co-exists with soft-state disks in the outer region. We computed radiation spectrum of the disk by including synchrotron emission and compton cooling, and reproduced the observed X-ray spectrum of black hole candidates.

In order to resolve the problem of apparent super-Eddington luminosities from accretion disks around black holes in super-luminous X-ray sources we propose that in strong magnetic fields under radiation pressure such disks fragment into individual magnetically aligned columns of cool disk gas and that radiation escapes through the gaps between them at super-Eddington luminosity.

We present a numerical study of the response of a thick accretion disk to a localized, external perturbation with the aim of exciting internal modes of oscillation. We find that the perturbations efficiently excite global modes recently identified as acoustic p–modes, and closely related to the epicyclic oscillations of test particles. The two strongest modes occur at eigenfrequencies which are in a 3:2 ratio. We have assumed a constant specific angular momenutm distribution within the disk. Our models are in principle scale–free and can be used to simulate accretion tori around stellar or super massive black holes.