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1

Spectral and Temporal Analysis of Ultraluminous X-ray Sources in NGC 2276

Singha A. Ch., Devi A. S.

Acta Astronomica

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2021

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Vol. 71, No. 4

261--279

EN

We present the results of spectral and temporal analysis of Ultraluminous X-ray sources (ULXs) present in the relatively nearby spiral galaxy NGC 2276 which has been observed five times by the Chandra ACIS-S over a time period of 16 yr. We detect seven X-ray point sources from the recent Chandra observations of 2019-2020 and compare the results with earlier observations from 2004 and 2013. The spectra of these sources are fitted using two empirical models- an absorbed power law and an absorbed disk blackbody. The X-ray luminosity of all these sources are estimated to be Lx≥ 1039 erg/s, thus falling under the ULX category, with five sources having Lx≈1040 erg/s. The power law photon index (Γ) and the inner disk temperature (kTin) of these sources (except for one - Src6) are found to be in the hard states implying the radiative mechanism of these sources to be the inverse comptonization of soft photons and the emission is dominated by the inner regions of the accretion disk. Comparing the long term luminosity of these sources, we observe variations in two sources while three sources have gradually increasing luminosity and two other sources have almost consistent luminosity. Short term variability is absent in these sources. Assuming the emission to be isotropic, we estimate the BH mass from the disk blackbody model. The upper limit of the BH mass of these seven sources are found to be in the stellar mass BHs range with MBH<70 M☉, accreting at higher rates than their respective Eddington limit.

2

A Survey Length for AGN Variability Studies

Kozłowski S.

Acta Astronomica

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2021

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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.

3

On the Periods and Nature of Superhumps

Smak J.

Acta Astronomica

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2020

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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.

4

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

Smak J.

Acta Astronomica

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2020

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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.

5

RW Tri - its Negative Superhumps and System Parameters

Smak J.

Acta Astronomica

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2019

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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.

6

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

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

Acta Astronomica

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2019

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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

Contemporaneous Broad-Band Photometry and Hα Observations of T Tau Stars

Frasca A., Montes D., Alcalá J. M., Klutsch A., Guillout P.

Acta Astronomica

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2018

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Vol. 68, No. 4

403--424

EN

In this paper we present the results of a simultaneous BVRI and Hα photometric monitoring, contemporaneous to medium-resolution spectroscopy of six T Tau stars in the Taurus-Auriga star forming region. We have characterized the Hα photometric system using synthetic templates and the contemporaneous spectra of the targets. We show that we can achieve a precision corresponding to 2–3 Å in the Hα equivalent width, in typical observing conditions. The spectral analysis has allowed us to determine the basic stellar parameters and the values of quantities related to the accretion. In particular, we have measured a significant veiling only for the three targets with the strongest Hα emission (T Tau, FM Tau, and DG Tau). The broad-band photometric variations are found to be in the range of 0.05–0.70 mag and are often paired to variations in the Hα intensity, which becomes stronger when the stellar continuum is weaker. In addition, we have mostly observed a redder V-I and a bluer B-V color as the stars become fainter. For most of the targets, the timescales of these variations seem to be longer than the rotation period. One exception is T Tau, for which the broad-band photometry varies with the rotation period. The most plausible interpretation of these photometric and Hα variations is that they are due to non-stationary mass accretion onto the stars, but rotational modulation can play a major role in some cases.

8

The Appearance of Non-Spherical Systems. Application to LMXB

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

Acta Astronomica

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2017

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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.

9

Superspinning Quark Stars Limited by Twin High-Frequency Quasiperiodic Oscillations

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

Acta Astronomica

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2017

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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).

10

DW UMa and the Irradiation Modulated Mass Transfer Model for Superhumps

Smak J.

Acta Astronomica

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2017

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Vol. 67, No. 3

273--280

EN

The light curves of the permanent superhumper DW UMa are analyzed in order to determine the amplitudes of its superhumps, A SH, and the amplitudes of the periodic light variations with the beat period - the irradiation amplitudes - Airr. The resulting values of ASH and Airr, together with other values from the literature, turn out to be correlated thereby confirming the irradiation modulated mass transfer model for superhumps.

11

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

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

Acta Astronomica

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2015

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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.

12

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

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

Acta Astronomica

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2014

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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.

13

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.

14

On the Periods of Negative Superhumps and the Nature of Superhumps

Smak J.

Acta Astronomica

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2013

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Vol. 63, No. 1

109--114

EN

Osaki and Kato interpreted variations of the negative superhump periods, discovered by them in dwarf nova V1504 Cyg, as evidence in favor of the thermal-tidal instability model for superoutbursts. It is shown that their interpretation was incorrect. The observational evidence is recalled showing that superoutbursts are due to the enhanced mass transfer rate.

15

On the Nature of Superhumps

Smak J.

Acta Astronomica

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2013

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Vol. 63, No. 3

369--377

EN

Further evidence is presented supporting the alternative interpretation of superhumps as being due to irradiation modulated periodically variable mass transfer rate. NZ Boo, HT Cas and PU UMa are added to the sample of high inclination dwarf novae showing - during their superoutbursts - modulation of the observed brightness of the disk with beat period. Simple model calculations confirm earlier hypothesis that this modulation is due to a non-axisymmetric structure of the outer parts of the disk, involving the azimuthal dependence of their geometrical thickness, rotating with the beat period. The modulation amplitude Amod is found to decrease during superoutbursts. In particular, it is found that during two superoutbursts of OY Car the rate of decline of the superhump amplitude dASH/dt was correlated with the rate of decline dAmod/dt. This leads to a simple explanation for the decreasing amplitudes of superhumps: it is a consequence of the decreasing modulation amplitude.

16

On the Eclipsing Cataclysmic Variable Star HBHA 4705-03

Rutkowski A., Ak T., Marsh T. R., Eker Z.

Acta Astronomica

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2013

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Vol. 63, No. 2

225--238

EN

We present observations and analysis of a new eclipsing binary HBHA 4705-03. Using decomposition of the light curve into accretion disk and hot spot components, we estimated photometrically the mass ratio of the studied system to be q=0.62±0.07. Modeling of the system yields the following fundamental parameters: white dwarf mass M1=(0.8±0.2) Msun, secondary mass M2=(0.497±0.05) Msun, orbital radius a=1.418 Rsun, orbital inclination i=(81.°58±0.°5), accretion disk radius rd/a=0.366±0.002, and accretion rate M=(2.5±2)x1018 [g/s] (3×10-8 Msun/yr). Power spectrum analysis revealed a set of possible low-period Quasi Periodic Oscillations centered at the frequencies f1=0.00076 Hz, f2=0.00048 Hz and f3=0.00036 Hz. The system B-V=0.04 mag corresponds to a dwarf nova color during an outburst. The light curves of HBHA 4705-03 suggest that it is a nova-like variable star.

17

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.

18

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

Stuchlík Z., Kotrlová A., Török G.

Acta Astronomica

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2012

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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.

19

Radiative Corrections to the Neutron Star Mass Inferred from QPO Frequencies

Abramowicz M. A., Kluźniak W., Yu W.

Acta Astronomica

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2011

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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.

20

Dwarf Nova V1040 Centauri and Variable Stars in its Vicinity

Rutkowski A., Pietrukowicz P., Olech A., Ak T., Złoczewski K., Poleski R., Tappert C., Eker Z.

Acta Astronomica

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2011

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Vol. 61, No. 4

345--357

EN

We present the results of a photometric campaign of the dwarf nova V1040 Cen. The light curve shows two normal outbursts with recurrence time ≈40 days and amplitude ≈2.5 mag. Quiescence data show oscillations with periods in the range ≈0.1 days (2.4 h) to ≈0.5 days (12 h) of unknown origin. We measured the orbital period of V1040 Cen to be Porb=0.060458(80) days (1.451±0.002 h). Based on the MV-Porb relation we found the distance of V1040 Cen to be 137±31 pc. In this paper we also report the detection of eleven new variable stars in the field of the monitored dwarf nova.