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1
Content available remote Possible Suppression of Magnetorotational Instability by Rapid Radial Flow
Abramowicz M. A., Horák J., Kluźniak W.
Acta Astronomica
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2013
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Vol. 63, No. 2
267--273
EN
By investigating the magnetorotational instability (MRI) in a region of rapidly varying accretion-disk thickness, e.g., in the ISCO region, we find a limit to the applicability of a recent finding by Balbus (2012) that the MRI is present also in the region of Rayleigh-unstable flow.
2
Content available remote Multi-Resonance Orbital Model Applied to High-Frequency Quasi-Periodic Oscillations Observed in Sgr A*
Kotrlová A., Stuchlík Z., Török G.
Acta Astronomica
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2013
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Vol. 63, No. 2
275--292
EN
The multi-resonance orbital model of high-frequency quasi-periodic oscillations (HF QPOs) enables precise determination of the black hole dimensionless spin a if observed set of oscillations demonstrates three (or more) commensurable frequencies. The black hole spin a is related to the frequency ratio only, while its mass M is related to the frequency magnitude. The model is applied to the triple frequency set of HF QPOs observed in Sgr A* source with frequency ratio 3:2:1. Acceptable versions of the multi-resonance model are determined by the restrictions on the Sgr A* supermassive black hole mass. The version of strong resonances related to the black hole "magic" spin a=0.983 is acceptable but the version demonstrating the best agreement with the mass restrictions predicts spin a=0.980.
3
Content available remote The Influence of Bulk Motion on Comptonization in Advectively Dominated Accretion Flows
Jaroszyński M.
Acta Astronomica
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2001
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Vol. 51, No. 2
91--101
EN
We consider Comptonization of soft photons inside advection dominated accretion flows (ADAFs). We use Monte Carlo simulations to follow the individual photon paths. We investigate the influence of the bulk motion on the spectra and find it to be weak. We find that the slope of the spectrum from radio to gamma range can be estimated if the probability and average energy gain in the first photon scattering are known.
4
Content available remote Binary Coalescence of a Strange Star with a Black Hole: Newtonian Results
Lee W. H., Kluźniak W., Nix J.
Acta Astronomica
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2001
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Vol. 51, No. 4
331--346
EN
We present Newtonian three-dimensional hydrodynamical simulations of the merger of quark stars with black holes. The initial conditions correspond to non-spinning stars in Keplerian orbits, the code includes gravitational radiation reaction in the quadrupole approximation for point masses. We find that the quark star is disrupted, forming transient accretion structures around the black hole, but 0.03 of the original stellar mass survives the initial encounter and remains in an elongated orbit as a rapidly rotating quark starlet, in all cases. No resolvable amount of mass is dynamically ejected during the encounters - the black hole eventually accretes 99.99%±0.01% of the quark matter initially present.
5
Content available remote Stellar Kinematics Near the Galactic Center - Possible Test for a Binary Black Hole
Jaroszyński M.
Acta Astronomica
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2000
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Vol. 50, No. 1
67--78
EN
We consider consequences of the presence of a binary black hole system in the center of our Galaxy. We show that the existence of such a system with total mass ≈2.6×106 Msolar and semimajor axis ≈0.001 pc is not excluded on theoretical grounds and it may have characteristic evolution time of a few billions years. We simulate astrometric measurements of proper motion of stars in the vicinity of the black hole system and check whether such observations are sufficient to discover the binary nature of the central mass. We show that such findings require astrometric accuracy about ten times higher than presently available; the Keck Interferometer, will be able to place stringent observational limits on the presence of a black hole binary in the Galactic center.
6
Content available remote Radiation Spectra of Advection Dominated Accretion Flows Around Kerr Black Holes
Kurpiewski A., Jaroszyński M.
Acta Astronomica
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2000
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Vol. 50, No. 1
79--91
EN
We study the formation of the spectra in advection dominated accretion flows (ADAFs) around Kerr black holes. We use a Monte Carlo approach and fully general relativistic treatment to follow the paths of individual photons and model their scattering with mildly relativistic, thermal electrons of the two temperature plasma present in the flow. We study the influence of the accretion rate, black hole mass, black hole angular momentum, and the strength of the small scale magnetic field present in the plasma on the resulting spectra. The impact of the black hole angular momentum on the spectra is present and can be distinguished from the influence of changes in other parameters. This property of the models should be taken into account when modeling the individual sources and the population of inefficiently accreting black holes in the Universe.
7
Content available remote Erratum: Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center
Jaroszyński M.
Acta Astronomica
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1999
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Vol. 49, No. 1
117
EN
In the paper "Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center" (Acta Astron., 48, p. 653) the highest astrometric accuracy of the Keck Interferometer (≈ 20 μas) was assumed to objects as faint as K ≈ 22 mag.
8
Content available remote Relativistic Effects in Proper Motions of Stars Surrounding the Galactic Center
Jaroszyński M.
Acta Astronomica
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1998
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Vol. 48, No. 4
653--665
EN
We simulate the astrometric observations of stars moving close to the black hole in the Galactic center. We show that for orbits ≤103 a.u. and position measurements with the accuracy of the Keck Interferometer, the periastron motion of elliptical orbits will be measurable. The models of star trajectories neglecting the periastron motion will be easy to reject with the high confidence level. The measurement of orbital elements and the periastron motion can be effectively used as an independent estimate the distance to the Galactic center. The effects of orbit precession may be visible in some cases. The effects of gravitational radiation are completely negligible as well as influence of the black hole rotation on the propagation of light.
9
Content available remote Gravitational Lensing and Proper Motions of Stars Surrounding the Galactic Center
Jaroszyński M.
Acta Astronomica
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1998
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Vol. 48, No. 3
413--429
EN
We examine lensing by the black hole in the Galactic center. We use the new photometric data in the infrared to estimate the density of stars in the star cluster surrounding the black hole. We calculate the probability of lensing by the black hole, the rate of the new events and the influence of lensing on the velocity distribution of proper motions of stars. The relative importance of lensing has its maximum at distances ≈10 mas from the star cluster center. The measurement of the proper motion of stars on the scale of ≈3 mas would be necessary to investigate the orbits of stars at distances ≈103rg from the black hole. In principle the deformation of the projected orbit shape by lensing may serve as a way to estimate the angular momentum of the black hole.
10
Content available remote Advection Dominated Accretion Flows. A Toy Disk Model
Paczyński B.
Acta Astronomica
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1998
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Vol. 48, No. 4
667--676
EN
A toy model of a disk undergoing steady state accretion onto a black hole is presented. The disk is in a hydrostatic equilibrium for all radii r>rin, with the inner disk radius located between the marginally stable and marginally bound orbits: rms>rin>rmb. Matter flows from the disk through a narrow cusp at rms and falls freely into the black hole carrying with it no thermal energy. At radii larger than rout the disk is assumed to radiate away all locally generated heat, and therefore the disk is geometrically thin for r>rout. We assume that no heat generated in the inner disk, with rout>r>rin can be radiated away, i.e., the disk is 100% advective, and it becomes geometrically thick in this range of radii. All enthalpy of the thick disk is used up to press the inner disk radius towards the marginally bound orbit, and to lower the efficiency of conversion of accreted mass into radiation generated only for r>rout, by assumption. Conservation laws of mass, angular momentum and energy make it possible to calculate the inner thick disk radius rin for any specified value of its outer radius rout. As the nature of disk viscosity is not known there is some freedom in choosing the shape of the thick disk, subject to several general conditions which include the hydrostatic equilibrium everywhere for r>rin. The main purpose of this toy model is to emphasize the effect the disk thickness has on lowering the energetic efficiency of a black hole accretion.
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