Wyniki wyszukiwania - Biblioteka Nauki

1

Hydrodynamical Models of Accretion Disks in SU UMa Systems

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Kornet K. , Różyczka M.

Acta Astronomica

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2000

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tom Vol. 50, No. 2

163--175

EN

We numerically test the mode-coupling model (Lubow 1991a) of tidal instability in SU UMa systems. So far, all numerical models confirming it have been based on SPH codes and isothermal equation of state. In our paper we present Eulerian models, using both isothermal approximation and the full energy equation. We also investigate influence of different ways of mass transfer. While isothermal models behave similarly to SPH simulations, the behavior of models with full energy equation is quite different, and the mode-coupling model is not confirmed in this case.

2

Test of the Resonant Switch Model by Fitting the Data of Twin-Peak HF QPOs in the Atoll Source 4U 1636-53

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Stuchlìk Z. , Kotrlovà A. , Török G. , Goluchovà K.

Acta Astronomica

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2014

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tom Vol. 64, No. 1

45--64

EN

Resonant Switch (RS) model has recently been proposed as an alternative to the standard models of twin-peak high-frequency quasi-periodic oscillations (HF QPOs) observed in low-mass X-ray binaries containing a neutron star. The model assumes switch of twin oscillations at a resonant point, where frequencies of the upper and lower oscillations νU and νL become commensurable and one pair of the oscillating modes (corresponding to a specific model of HF QPOs) changes to some other pair due to non-linear resonant phenomena. We test the RS model for the atoll source 4U 1636-53, where we assume two resonant points observed at frequency ratios νU:νL=3:2, 5:4, by fitting the pairs of the oscillatory modes to the observed data in the regions related to the resonant points. Among acceptable variants of the RS model the most promising are those combining the relativistic precession (RP) and the total precession (TP) frequency relations or their modifications. The precision of the fits is shown to be strongly increased in comparison to fits realized by individual pairs along the whole data range. We demonstrate that the χ2 test is significantly improved. Fitting of the HF QPO data in the source 4U 1636-53 by the RP1-RP variant of the RS model gives the best results and implies that the neutron star mass and dimensionless spin are M≈2.2 Msun and a≈0.27.

3

Radiation Spectra of Advection Dominated Accretion Flows Around Kerr Black Holes

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Kurpiewski A. , Jaroszyński M.

Acta Astronomica

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2000

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tom 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.

4

Equations of State in the Hartle-Thorne Model of Neutron Stars Selecting Acceptable Variants of the Resonant Switch Model of Twin HF QPOs in the Atoll Source 4U 1636-53

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Stuchlìk Z. , Urbanec M. , Kotrlovà A. , Török G. , Goluchovà K.

Acta Astronomica

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2015

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tom Vol. 65, No. 2

169--195

EN

The Resonant Switch (RS) model of twin high-frequency quasi-periodic oscillations (HF QPOs) observed in neutron star binary systems, based on switch of the twin oscillations at a resonant point, has been applied to the atoll source 4U 1636-53 under assumption that the neutron star exterior can be approximated by the Kerr geometry. Strong restrictions of the neutron star parameters M (mass) and a (spin) arise due to fitting the frequency pairs admitted by the RS model to the observed data in the regions related to the resonant points. The most precise variants of the RS model are those combining the relativistic precession frequency relations with their modifications. Here, the neutron star mass and spin estimates given by the RS model are confronted with a variety of equations of state (EoS) governing structure of neutron stars in the framework of the Hartle-Thorne theory of rotating neutron stars applied for the observationally given rotation frequency frot≈580 Hz (or alternatively frot≈290 Hz) of the neutron star in 4U 1636-53. It is shown that only two variants of the RS model based on the Kerr approximation are compatible with two EoS applied in the Hartle-Thorne theory for frot≈580 Hz, while no variant of the RS model is compatible for frot≈290 Hz. The two compatible variants of the RS model are those giving the best fits of the observational data. However, a self-consistency test by fitting the observational data to the RS model with oscillation frequencies governed by the Hartle-Thorne geometry described by three spacetime parameters M,a and (quadrupole moment) q related by the two available EoS puts strong restrictions. The test admits only one variant of the RS model of twin HF QPOs for the Hartle-Thorne theory with the EoS predicting the parameters of the neutron star M≈2.10 M⊙, a≈0.208, and q/a2≈1.77.

5

On Light Curves Modeling of Low Inclination Binary Systems with Accretion Disks

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Zoła S.

Acta Astronomica

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1998

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tom Vol. 48, No. 2

373--382

EN

Computer generated light curves of a low inclination, semidetached binary system containing an accretion disk, were analyzed with the Wilson-Devinney code (which does not account for the disk effects), to search for the best solution. The following inclinations were considered: 77°.4, 75°.0 and 60°.0 as well as three different disk contributions to the total light: 2%, 15% and 30%. For the disk contribution being small (2%), the resulting configuration agreed with the input one. For higher disk light detached geometry was derived. The parameters obtained from modeling greatly differ from the input ones and the fits are quite good.

6

Mount Suhora High Cadence Photometric Survey of T Tauri-Type Stars

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Siwak M. , Dróżdż M. , Gut K. , Winiarski M. , Ogłoza W. , Stachowski G.

Acta Astronomica

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2019

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tom Vol. 69, No. 3

227--260

EN

Results of high-cadence multi-color observations of 121 pre-main sequence stars available from the northern hemisphere are presented. The aim of this survey was to detect transit-like signatures caused by occultation of these young stars and their accretion-induced hot spots by close-in planets and/or dusty clumps. Although none planetary transits were detected, our data allow the determination of rotational periods for some T Tau stars and characterization of the accretion processes operating in classical T Tau-type stars in time scales ranging from a few minutes to days, as well as characterization of the large-scale dips caused by dusty warped disks.

7

Binary Coalescence of a Strange Star with a Black Hole: Newtonian Results

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Lee W. H. , Kluźniak W. , Nix J.

Acta Astronomica

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2001

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tom 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.

8

The Influence of Bulk Motion on Comptonization in Advectively Dominated Accretion Flows

100%

Jaroszyński M.

Acta Astronomica

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2001

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tom 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.

9

Pre-low-mass X-ray binaries containing a black hole: investigating a detection mechanism

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Bleach J.

Open Physics

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2007

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tom 5

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nr 1

1-10

EN

This work investigates the feasibility of detecting close, detached, black hole-red dwarf binaries, which are expected to be evolutionary precursors of low-mass X-ray binaries (LMXBs). Although this pre-low-mass X-ray binary (pre-LMXB) phase of evolution is predicted theoretically, as yet no such systems have been identified observationally. The calculations presented here suggest that the X-ray luminosity of black hole wind accretion in a pre-LMXB system could exceed the intrinsic X-ray luminosity of the red dwarf secondary star, thereby providing a detection mechanism. However, there is significant uncertainty regarding the efficiency of the conversion of gravitational potential energy to X-ray luminosity resulting from accretion onto a black hole, for example energy may be lost via advection across the event horizon. Still, sources with X-ray luminosities greater than that expected for a red dwarf star, but whose positions coincide with that of a red dwarf would represent candidate pre-LMXB systems. These candidates should be surveyed for the radial velocity shifts that would occur as a result of the orbital motion of a red dwarf star within a close binary system containing a black hole.

10

On the Periods and Nature of Superhumps

100%

Smak J.

Acta Astronomica

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2020

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tom Vol. 70, No. 4

317--321

EN

It is commonly accepted that the periods of superhumps can be satisfactorily explained within a model involving apsidal motion of the accretion disk provided the frequency of the apsidal motion in addition to the dynamical term includes also the pressure effects. Using a larger sample of systems with reliable mass ratios it is shown, however, that this view is not true and the model requires further modifications.

11

The Appearance of Non-Spherical Systems. Application to LMXB

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Różańska A. , Bełdycki B. , Madej J. , Adhikari T. P. , You B.

Acta Astronomica

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2017

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tom Vol. 67, No. 1

51--62

EN

We study the appearance of the neutron star-accretion disk system as seen by a distant observer in the UV/X-ray domain. The observed intensity spectra are computed assuming non-spherical geometry of the whole system, in which outgoing spectrum is not represented by the flux spectrum, the latter being valid for spherically symmetric objects. Intensity spectra of our model display double bumps in UV/X-ray energy domains. Such structure is caused by the fact that the the source is not spherically symmetric, and the proper integration of intensity over emitted area is needed to reproduce observed spectral shape. Relative normalization of double bump is self consistently computed by our model. X-ray spectra of such a type were often observed in LMXB with accretion disk, ultra luminous X-ray sources, and accreting black hole systems with hot inner compact corona. Our model naturally explains high energy broadening of the disk spectrum observed in some binaries. We attempted to fit our model to X-ray data of XTE J1709-267 from XMM-Newton. Unfortunately, the double intensity bump predicted by our model for LMXB is located in soft X-ray domain, uncovered by existing data for this source.

12

Superspinning Quark Stars Limited by Twin High-Frequency Quasiperiodic Oscillations

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Stuchlìk Z. , Schee J. , Šràmkovà E. , Török G.

Acta Astronomica

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2017

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tom Vol. 67, No. 2

181--201

EN

We study properties of Keplerian disks and their high-frequency quasi-periodic oscillations (HF QPOs) in the field of quark stars with dimensionless spin a breaking the black-hole spin limit of a=1 up to a≈1.3. Using the external geometry of the superspinning quark stars approximated by the Kerr geometry, we show that the Keplerian disks have to touch the surface of such quark stars and their accretion efficiency η≈18% significantly exceeds the efficiency related to the Schwarzschild black holes. Using the geodesic oscillation models, we test possible existence of the superspinning quark stars in atoll sources demonstrating the twin HF QPOs with resonant frequency ratios 3:2, 4:3, 5:4. For explanation of the twin HF QPOs we consider the standard relativistic precession model and its modifications, the tidal distortion model, the resonance epicyclic and the warped disk model. In a given model, we assume occurrence of the twin oscillatory modes at a common resonant dimensionless radius x=r/M determined by the frequency ratio and the quark star spin a. The theoretical limit R>3M on the quark star surface radius puts strong restrictions on the relations between the resonant radii x and the quark star spin a. These restrictions imply that all the considered geodesic oscillation models can be excluded, except for one variant of the relativistic precession model, or alternatively the tidal distortion and warped disk models, that allow for appearance of the twin HF QPOs with frequency ratio 3:2 at radii slightly above the theoretical limit on the radius of the quark star surface, but exclude the smaller frequency ratios (4:3, 5:4).

13

Advection Dominated Accretion Flows. A Toy Disk Model

100%

Paczyński B.

Acta Astronomica

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1998

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tom 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.

14

Radiative Corrections to the Neutron Star Mass Inferred from QPO Frequencies

100%

Abramowicz M. A. , Kluźniak W. , Yu W.

Acta Astronomica

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2011

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tom Vol. 61, No. 4

319--324

EN

The frequencies of kHz QPOs are widely interpreted as being indicative of the values of characteristic frequencies related to orbital motion around neutron stars, e.g., the radial epicyclic frequency. In regions directly exposed to the radiation from the luminous neutron star these frequencies change with the luminosity. Including radiative corrections will change the neutron star mass value inferred from the QPO frequencies. Radiative forces may also be behind the puzzling phenomenon of parallel tracks.

15

On the Critical Mass Ratio for the 3:1 Resonance in Accretion Disks

100%

Smak J.

Acta Astronomica

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2020

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tom Vol. 70, No. 4

313--315

EN

The 3:1 resonance between the orbital frequency of the binary system and the orbital frequency of the outer parts of the disk can occur only in systems with mass ratio smaller than certain critical value. It is shown that qcrit=0.22, i.e. smaller than obtained earlier by other authors (qcrit=0.25-0.39). This implies that the tidal-resonance model fails to explain superhumps in systems with orbital periods above the period gap.

16

On the Structure of the Outer Parts of Accretion Disks in Close Binary Systems

100%

Smak J.

Acta Astronomica

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2002

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tom Vol. 52, No. 3

263--272

EN

A simple model for the outer parts of the disk in steady-state accretion is proposed in which (1) the flux emitted from the surface of the disk is due to the viscous dissipation within the disk body; (2) the tidal removal of the angular momentum occurs in the disk's outer edge which has negligibly small radial extent; (3) the energy dissipated by tidal effects is radiated exclusively from the disk's edge. The model predicts the temperature of the edge to be comparable to that of the outermost parts of the disk. The edge contributes only about 1 percent to the mean bolometric luminosity of the disk. Its contribution in the V-band, however, is much larger: about 15% at i=60° and more then 50% at i>80°. Application of this model to disks in dwarf novae implies that the crude, approximate description of tidal effects used in earlier models should be replaced with a rigid condition Rd≤Rtid. Accordingly the tidal effects are important only during outbursts.

17

Resonant Switch Model of Twin Peak HF QPOs Applied to the Source 4U 1636-53

100%

Stuchlík Z. , Kotrlová A. , Török G.

Acta Astronomica

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2012

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tom Vol. 62, No. 4

389--407

EN

Resonant Switch (RS) model of twin peak high-frequency quasi-periodic oscillations (HF QPOs) assumes switch of twin oscillations at a resonant point where frequencies of the upper and lower oscillations νU and νL become commensurable and the twin oscillations change from one pair of the oscillating modes (corresponding to a specific model of HF QPOs) to some other pair due to non-linear resonant phenomena. The RS model is used to determine range of allowed values of spin a and mass M of the neutron star located in the atoll source 4U 1636-53 where two resonant points are observed at frequency ratios νU:νL=3:2, 5:4. We consider the standard specific models of the twin oscillations based on the orbital and epicyclic geodetical frequencies. The resonant points are determined by the energy switch effect exhibited by the vanishing of the amplitude difference of the upper and lower oscillations. The predicted ranges of the neutron star parameters are strongly dependent on the twin modes applied in the RS model. We demonstrate that for some of the oscillatory modes used in the RS model the predicted parameters of the neutron star are unacceptable. Among acceptable RS models the most promising are those combining the Relativistic Precession and the Total Precession frequency relations or their modifications.

18

RW Tri - its Negative Superhumps and System Parameters

88%

Smak J.

Acta Astronomica

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2019

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tom Vol. 69, No. 1

79--85

EN

Negative superhumps are detected in the light curves of RW Tri observed in September 1994 and November/December 1957. New system parameters, obtained using K2 and V2,rotsin i and q (estimated from PnSH), are M1=0.60±0.20 M☉, M2=0.48±0.15 M☉, A=1.13±0.09×1011 cm and i=72°.5±2°.5.

19

Doppler Tomograms from Hydrodynamical Models of Dwarf Nova Disks

88%

Gawryszczak A. J. , Różyczka M.

Acta Astronomica

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2003

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tom Vol. 53, No. 1

73--83

EN

We present three-dimensional models of accretion disks in U Gem-like systems and calculate their Doppler tomograms. The tomograms are based on two different assumptions concerning the origin of line emission from the disk. The assumption of lines originating due to irradiation of the surface layer of the disk by the central source leads to a better agreement with observations. We argue that fully three-dimensional modeling is necessary to properly interpret the observed tomograms.

20

A Survey Length for AGN Variability Studies

88%

Kozłowski S.

Acta Astronomica

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2021

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tom Vol. 71, No. 2

103--112

EN

The damped random walk (DRW) process is one of the most commonly used and simplest stochastic models to describe variability of active galactic nuclei (AGN). An AGN light curve can be converted to just two DRW model parameters - the signal decorrelation timescale τ and the asymptotic amplitude SF∞. In principle, these two model parameters may be correlated with the physical parameters of AGN. By simulation means, we have recently shown that in order to measure the decorrelation timescale accurately, the experiment or the light curve length must be at least 10 times the underlying decorrelation timescale. In this paper, we investigate the origin of this requirement and find that typical AGN light curves do not sufficiently represent the intrinsic stationary process. We simulated extremely long (10 000τ) AGN light curves using DRW, and then measured the variance and the mean of short light curves spanning 1-1000τ. We modeled these light curves with DRW to obtain both the signal decorrelation timescale τ and the asymptotic amplitude SF∞. The variance in light curves shorter than ≈30τ is smaller than that of the input process, as estimated by both a simple calculation from the light curve and by DRW modeling. This means that while the simulated stochastic process is intrinsically stationary, short light curves do not adequately represent the stationary process. Since the variance and timescale are correlated, underestimated variances in short light curves lead to underestimated timescales as compared to the input process. It seems, that a simulated AGN light curve does not fully represent the underlying DRW process until its length reaches even ≈30 decorrelation timescales. Modeling short AGN light curves with DRW leads to biases in measured parameters of the model - the amplitude being too small and the timescale being too short.