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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

Comparison of the Hartle-Thorne Model of Neutron Stars with its Kerr Approximation by using Resonant Switch Model

Stuchlík Z., Klimovičová K., Schee J., Török G., Kotrlová A.

Acta Astronomica

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2021

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Vol. 71, No. 4

311--141

EN

The Hartle-Thorne (H-T) models of slowly rotating neutron or quark stars, characterized by the mass M, dimensionless spin a, and reduced quadrupole moment q, are constructed for the observationally given rotational frequency frot=580 Hz (290 Hz) of the compact star in the atoll source 4U 1636-53, and a wide range of equations of state (EoS) giving sequences of allowed states governed by the relations a(M), q(M). These sequences are used in the framework of the resonant switch (RS) model combining pairs of geodesic oscillation models to match the data of the twin high-frequency quasi-periodic oscillations observed in the 4U 1636-53 source. The results of the matching procedure using the H-T models are compared to those based on the Kerr approximation of the exterior of the neutron stars. The best H-T matches fix the only variant of the RS model combining particular modifications of the relativistic precession model, exclude the rotation frequency frot=290 Hz, restrict the considered EoS to six of them, excluding the strange quark stars, and significantly improve precision of the matches given by any single geodesic oscillations model. The Kerr matching allows two variants of the RS model, thus, giving false information, and only three EoS, thus, giving insufficient information. Our results demonstrate that in the matching procedure, the Kerr approximation can be used only for neutron stars governed by the H-T models with q<2, implying an important restriction on the applicability of the Kerr approximation for description of the oscillatory phenomena around neutron stars. On the other hand, the RS model is sufficiently discriminating for the spacetime metric to be largely determined by fitting to the data. The ranges of the external spacetime parameters of the neutron stars related to the best H-T matches are determined to be M≈2.10-2.13 Mo, a≈0.21-0.25, q≈1.8-2.3. Most compact neutron star is predicted by the Gandolfi EoS, when M≈2.10 Mo, a≈0.21, q≈1.8, with the equatorial radius R≈10.83 km and eccentricity ε=0.03.

3

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.

4

Optical and X-ray Behavior of the High Mass X-ray Transient A0535+26/HDE245770 in 2014

Giovannelli F., Bisnovatyi-Kogan G. S., Bruni I., Corfini G., Martinelli F., Rossi C.

Acta Astronomica

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2015

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Vol. 65, No. 1

107--116

EN

The optical behavior of the Be star in the high-mass X-ray transient A0535+26/HDE245770 shows that at periastron the luminosity is typically enhanced by 0.02 mag to a few tenths mag, and the X-ray outburst occurs eight days after the periastron. Indeed, at the periastron an increase of the mass flux occurs. This sort of flush reaches the external part of the temporary accretion disk around the neutron star and moves to the hot central parts of the accretion disk and the neutron star's surface. The time necessary for this process is dependent on the turbulent viscosity in the accretion disk. In this paper we will show the behavior of this system in optical band around the predicted periastron passages in 2014, by using the ephemeris - JDopt-outb = JD0(2 444 944)±n(111.0±0.4) days - that we used to schedule our spectroscopic and photometric optical observations. Spectroscopic unusual activity detected in the Balmer lines and the enhancement in the emission in B, V, and R bands around the 106th periastron passage, and in V-band around the 108th periastron passage after the "zero event" 811205-E at JD 2 444 944, and the subsequent X-ray events definitively demonstrate the existence of a ≈8-day delay between optical and X-ray flares.

5

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.

6

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.

7

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.

8

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.

9

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.

10

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.

11

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.

12

Coronal Activity from the ASAS Eclipsing Binaries

Szczygieł D. M., Socrates A., Paczyński B., Pojmański G., Pilecki B.

Acta Astronomica

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2008

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Vol. 58, No. 4

405--418

EN

We combine the catalog of eclipsing binaries from the All Sky Automated Survey (ASAS) with the ROSAT All Sky Survey (RASS). The combination results in 836 eclipsing binaries that display coronal activity and is the largest sample of active binary stars assembled to date. By using the (V-I) colors of the ASAS eclipsing binary catalog, we are able to determine the distances and thus bolometric luminosities for the majority of eclipsing binaries that display significant stellar activity. A typical value for the ratio of soft X-ray to bolometric luminosity is L_X/L_bol≈ a few ×10^-4, similar to the ratio of soft X-ray to bolometric flux F_X/F in the most active regions of the Sun. Unlike rapidly rotating isolated late-type dwarfs - stars with significant outer convection zones - a tight correlation between Rossby number and activity of eclipsing binaries is absent. We find evidence for the saturation effect and marginal evidence for the so-called ``super-saturation'' phenomena. Our work shows that wide-field stellar variability searches can produce a high yield of binary stars with strong coronal activity.