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1

On the Origin of Tilted Disks and Negative Superhumps

Smak J.

Acta Astronomica

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2009

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Vol. 59, No. 4

419--430

EN

The origin of tilted disks in cataclysmic variables is explained in terms of a model involving the stream-disk interactions. Tilted, precessing disk causes periodically variable asymmetry in the irradiation of the two hemispheres of the secondary component, resulting in variable vertical component of the velocity of the stream. The following stream-disk interactions provide additional vertical acceleration to disk elements needed to produce and maintain disk tilt. Predictions based on this model compare favorably with observations.

2

The Hot Spot in OY Car during its Superoutburst

Smak J.

Acta Astronomica

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2008

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Vol. 58, No. 1

65--67

EN

Detection of a "standard" hot spot in OY Car during one of its superoutbursts is reported. Its presence implies that the mass transfer rate was strongly enhanced.

3

Superoutbursts of Z Cha and their Interpretation

Smak J.

Acta Astronomica

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2008

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Vol. 58, No. 1

55--64

EN

Disk eclipses of Z Cha during its superoutbursts are analyzed. The resulting flux/temperature distributions in the outer parts of the disk show peculiarities which are interpreted as being due to absorption effects in the overflowing parts of the stream. The temperatures of the inner parts of the disk are used to determine the accretion rates as a function of time since superoutburst maximum. They are practically identical with mass transfer rates determined from the hot spot. Results are summarized in the form of the following, purely observational scenario: Superoutbursts are due and begin with a major enhancement in the mass transfer rate. During the "flat-top" part of the superoutburst the mass transfer rate decreases slowly, causing the observed luminosity to decline. The superoutburst ends when the mass transfer rate decreases below its critical value, resulting in a transition to the quiescent state of the dwarf nova cycle.

4

On the Origin of Clustering of Frequency Ratios in the Atoll Source 4U 1636-53

Török G., Abramowicz M. A., Bakala P., Bursa M., Horák J., Rebusco P., Stuchlík Z.

Acta Astronomica

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2008

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Vol. 58, No. 2

113--119

EN

A long discussion has been devoted to the issue of clustering of the kHz quasi periodic oscillation (QPO) frequency ratios in neutron star sources. While the distribution of ratios inferred from an occurrence of a single QPO seems to be consistent with a random walk, the distribution based on simultaneous detections of both peaks indicates a preference of ratios of small integers. Based on the public RXTE data we further investigate this issue for the source 4U 1636-53. Quality factors and rms amplitudes of both the QPOs nearly equal at the points where the frequencies are commensurable, and where the twin QPO detections cluster. We discuss a connection of the clustering with the varying properties of the two QPO modes. Assuming approximate relations for the observed correlations of the QPO properties, we attempt to reproduce the frequency and ratio distributions using a simple model of a random-walk evolution along the observed frequency-frequency correlation. We obtain results which are in qualitative agreement with the observed distributions.

5

Modeling the Twin Peak QPO Distribution in the Atoll Source 4U 1636-53

Török G., Bakala P., Stuchlík Z., Čech P.

Acta Astronomica

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2008

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Vol. 58, No. 1

1--14

EN

Relation between the lower and upper frequency mode of the twin peak quasi-periodic oscillations observed in the neutron star X-ray binaries is qualitatively well fitted by the frequency relation following from the relativistic precession model. Assuming this model with no preferred radius and the probability of an observable twin QPO excitation being uniform across the inner edge of an accretion disk we compare the expected and observed twin peak QPO distribution in the case of atoll source 4U 1636-53. We find these two distributions highly incompatible. We argue that the observed distribution roughly corresponds to the expected one if an additional consideration of preferred resonant orbits is included. We notice that our findings are relevant for some disk-oscillation QPO models as well.

6

Distribution of Kilohertz QPO Frequencies and Their Ratios in the Atoll Source 4U 1636-53

Török G., Abramowicz M. A., Bakala P., Bursa M., Horák J., Kluźniak W., Rebusco P., Stuchlík Z.

Acta Astronomica

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2008

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Vol. 58, No. 1

15--21

EN

A recently published study on long term evolution of the frequencies of the kilohertz quasi-periodic oscillations (QPOs) in the atoll source 4U 1636-53 concluded that there is no preferred frequency ratio in a distribution of twin QPOs that was inferred from the distribution of a single frequency alone. However, we find that the distribution of the ratio of actually observed pairs of kHz QPO frequencies is peaked close to the 3/2 value, and possibly also close to the 5/4 ratio. To resolve the apparent contradiction between the two studies, we examine in detail the frequency distributions of the lower kHz QPO and the upper kHz QPO detected in our data set. We demonstrate that for each of the two kHz QPOs (the lower or the upper), the frequency distribution in all detections of a QPO differs from the distribution of frequency of the same QPO in the subset of observations where both the kHz QPOs are detected. We conclude that detections of individual QPOs alone should not be used for calculation of the distribution of the frequency ratios.

7

Black Holes Admitting Strong Resonant Phenomena

Stuchlík Z., Kotrlová A., Török G.

Acta Astronomica

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2008

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Vol. 58, No. 4

441--461

EN

High-frequency twin peak quasiperiodic oscillations (QPOs) are observed in four microquasars, i.e., Galactic black hole binary systems, with frequency ratio very close to 3:2. In the microquasar GRS 1915+105 the structure of QPOs exhibits additional frequencies and more than two frequencies are observed in the Galaxy nuclei Sgr A* or in some extragalactic sources (NGC 4051, MCG-6-30-15 and NGC 5408 X-1). The observed QPOs can be explained by a variety of the orbital resonance model versions assuming resonance of oscillations with the Keplerian frequency νK or the vertical epicyclic frequency νθ, and the radial epicyclic frequency νr, or some combinations of these frequencies. Generally, different resonances could arise at different radii of an accretion disk. However, we have shown that for special values of dimensionless black hole spin a strong resonant phenomena could occur when different resonances can be excited at the same radius, as cooperative phenomena between the resonances may work in such situations. The special values of a are determined for triple frequency ratio sets νK:νθ:νr=s:t:u with s,t,u being small integers. The most promising example of such a special situation arises for black holes with extraordinary resonant spin a=0.983 at the radius r=2.395 M, where νK:νθ:νr=3:2:1. We also predict that when combinations of the orbital frequencies are allowed, QPOs with four frequency ratio set 4:3:2:1 could be observed in the field of black holes with a=0.866,0.882 and 0.962. Assuming the extraordinary resonant spin a=0.983 in Sgr A*, its QPOs with observed frequency ratio ≈3:2:1 imply the black hole mass in the interval 4.3×106 Mʘ< M < 5.4×106 Mʘ, in agreement with estimates given by other, independent, observations.

8

Superoutbursts of Z Chameleontis. Detection of the Hot Spot

Smak J.

Acta Astronomica

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2007

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Vol. 57, No. 1

87--101

EN

Eclipse light curves of Z Cha collected during its superoutbursts are decomposed into the disk eclipse and spot eclipse components. It is found that the presence (or absence) of the hot spot and all its parameters depend on the phase φb of the beat period (related to the orbital and superhump periods). At beat phases 0.40<φb<0.60 the "standard" hot spot is present, the analysis of its parameters shows that (a) the mass transfer rate is strongly enhanced and (b) the radius of the disk is roughly equal to the tidal radius. No hot spot can be detected during eclipses located close to the superhump (beat phases 0.80<φb<0.35). The spot observed at intermediate beat phases (φb≈0.35 and φb≈0.75) shows peculiarities which are interpreted as being due to the stream overflow. Rediscussion of system parameters, based on the mass ratio obtained from the analysis of the spot eclipses during quiescence (Smak 1996) and K2 from Wade and Horne (1988), gives: M1=0.93±0.10 Msolar, M2=0.186±0.030 Msolar, q=0.20±0.01 and i=80°.2±0°.3.

9

Modulation of the Neutron Star Boundary Layer Luminosity by Disk Oscillations

Abramowicz M. A., Horák J., Kluźniak W.

Acta Astronomica

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2007

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Vol. 57, No. 1

1--10

EN

Paczyński (1987) pointed out that any modulation of the rate at which mass is accreted by a neutron star from the innermost part of a relativistic accretion disk will lead to a modulation of the luminosity of the boundary layer. Following this reasoning, we demonstrate that variability of the boundary layer X-ray flux must necessarily show frequencies of certain global accretion disk oscillations. This theoretical paradigm - clock in the disk, modulation at the boundary layer - resolves one puzzling paradox in the neutron star quasi periodic oscillation (QPO) data.