Marek A. Abramowicz

Thermal equilibria of accretion disks

We show that most of hot, optically thin accretion disk models which ignore advective cooling are not self-consistent. We have found new types of optically thin disk solutions where cooling is dominated by radial advection of heat. These new solutions are thermally and viscously stable.

A precise determination of black hole spin in GRO J1655-40

We note that the recently discovered 450 Hz frequency in the X-ray flux of the black hole candidate GRO J1655-40 is in a 3:2 ratio to the previously known 300 Hz frequency of quasi-periodic oscillations (QPO) in the same source. If the origin of high frequency QPOs in black hole systems is a resonance between orbital and epicyclic motion of accreting matter, as suggested previously, the angular momentum of the black hole can be accurately determined, given its mass. We nd that the dimensionless angular momentum is in the range 0:2 <j< 0:67 if the mass is in the (corresponding) range of 5.5 to 7.9 solar masses.We note that the recently discovered 450 Hz frequency in the X-ray flux of the black hole candidate GRO J1655-40 is in a 3:2 ratio to the previously known 300 Hz frequency of quasi-periodic oscillations (QPO) in the same source. If the origin of high frequency QPOs in black hole systems is a resonance between orbital and epicyclic motion of accreting matter, as suggested previously, the angular momentum of the black hole can be accurately determined, given its mass. We find that the dimensionless angular momentum is in the range

Three-dimensional Magnetohydrodynamic Simulations of Radiatively Inefficient Accretion Flows

0.2<j<0.65

Is the Accretion Flow in NGC 4258 Advection Dominated

if the mass is in the (corresponding) range of 5.5 to 7.9 solar masses.

Unified description of accretion flows around black holes

We present three-dimensional MHD simulations of rotating radiatively inefficient accretion flows onto black holes. We continuously inject magnetized matter into the computational domain near the outer boundary and run the calculations long enough for the resulting accretion flow to reach a quasi-steady state. We have studied two limiting cases for the geometry of the injected magnetic field: pure toroidal field and pure poloidal field. In the case of toroidal field injection, the accreting matter forms a nearly axisymmetric, geometrically thick, turbulent accretion disk. The disk resembles in many respects the convection-dominated accretion flows found in previous numerical and analytical investigations of viscous hydrodynamic flows. Models with poloidal field injection evolve through two distinct phases. In an initial transient phase, the flow forms a relatively flattened, quasi-Keplerian disk with a hot corona and a bipolar outflow. However, when the flow later achieves steady state, it changes in character completely. The magnetized accreting gas becomes two-phase, with most of the volume being dominated by a strong dipolar magnetic field from which a thermal low-density wind flows out. Accretion occurs mainly via narrow slowly rotating radial streams that diffuse through the magnetic field with the help of magnetic reconnection events.

Two-dimensional Models of Hydrodynamical Accretion Flows into Black Holes

The mass of the central black hole in the active galaxy NGC 4258 (M106) has been measured to be

Foundations of Black Hole Accretion Disk Theory

M=3.6\times10^7\Msun

Magnetically Arrested Disk: an Energetically Efficient Accretion Flow

(Miyoshi et al. 1995). The Eddington luminosity corresponding to this mass is

The orbital resonance model for twin peak kHz quasi periodic oscillations in microquasars

L_E=4.5\times10^{45}

Advection-dominated Accretion: Global Transonic Solutions

erg s