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1

Search for Dormant Black Holes in the OGLE Data

Kapusta M., Mróz P.

Acta Astronomica

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2023

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Vol. 73, No. 3

197--226

EN

Light curves of ellipsoidal variables collected by the Optical Gravitational Lensing Experiment (OGLE) were analyzed, in order to search for dormant black hole candidates. After the preselection based on the amplitude of ellipsoidal modulation, each object was investigated by means of the spectral energy distribution fit, which allowed us to select objects that are in close agreement with the spectrum of a single stellar object. After this final step of the preselection process, we were left with only fourteen objects that were then investigated in detail. For each candidate, we estimated basic physical parameters such as temperature, mass, luminosity, and, in some cases, radial velocity semi-amplitude. One of the objects turned out to be a spotted star while the rest are considered black-hole binary candidates. In the end, we present an alternative explanation for the ellipsoidal modulation in the form of contact binaries, which are not only vast in number, contrary to black-hole binaries, but are also in much better agreement with the radial velocity estimates for some of the systems analyzed. Even if the presented arguments suggest a noncompact character of the companion objects, each of them should be observed spectroscopically in order to verify the compact object hypothesis.

2

Measuring the Mass Function of Isolated Stellar Remnants with Gravitational Microlensing I. Revisiting the OGLE-III Dark Lens Candidates

Mróz P., Wyrzykowski Ł.

Acta Astronomica

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2021

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Vol. 71, No. 2

89--102

EN

Gravitational microlensing may detect dark stellar remnants - black holes or neutron stars - even if they are isolated. However, it is challenging to estimate masses of isolated dark stellar remnants using solely photometric data for microlensing events. A recent analysis of OGLE-III long-timescale microlensing events exhibiting the annual parallax effects claimed that a number of bright events were due to "mass-gap" objects (with masses intermediate between those of neutron stars and black holes). Here, we present a detailed description of the updated and corrected method that can be used to estimate masses of dark stellar remnants detected in microlensing events given the light curve data and the proper motion of the source. We use this updated method, in combination with new proper motions from Gaia EDR3, to revise masses of dark remnant candidates previously found in the OGLE-III data. We demonstrate that masses of "mass-gap" and black hole events identified in the previous work are overestimated and, hence, these objects are most likely main-sequence stars, white dwarfs, or neutron stars.

3

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.

4

How Li and Paczyński Model of Kilonova Fits GW170817 Optical Counterpart

Kruszewski A.

Acta Astronomica

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2018

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Vol. 68, No. 3

205--211

EN

The original Li and Paczyński model of kilonova was compared with the observed bolometric optical light curve of the GW170817 electromagnetic counterpart. Perfect agreement is obtained for early observations up to 1.5 d since the time of merger.

5

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.

6

Constraints on the Formation of Double Neutron Stars from the Observed Eccentricities and Current Limits on Merger Rates

Chruslinska M., Belczynski K., Bulik T., Gladysz W.

Acta Astronomica

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2017

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Vol. 67, No. 1

37--50

EN

We employ population synthesis method to model the double neutron star (DNS) population and test various possibilities on natal kick velocities gained by neutron stars after their formation. We first choose natal kicks after standard core collapse supernovae (CCSN) from a Maxwellian distribution with velocity dispersion of σ = 265 km/s as proposed by Hobbs and then modify this distribution by changing σ toward smaller and larger kick values. We also take into account the possibility of NS formation through electron capture supernova. In this case we test two scenarios: zero natal kick or small natal kick, drawn from Maxwellian distribution with σ = 26.5 km/s. We calculate the present-day orbital parameters of binaries and compare the resulting eccentricities with those known for observed DNSs. As an additional test we calculate Galactic merger rates for our model populations and confront them with observational limits. We do not find any model unequivocally consistent with both observational constraints simultaneously. The models with low kicks after CCSN for binaries with the second NS forming through core collapse SN are marginally consistent with the observations. This means that either 14 observed DNSs are not representative of the intrinsic Galactic population, or that our modeling of DNS formation needs revision.

7

Accuracy of Mass and Radius Determination for Neutron Stars in X-ray Bursters from Simulated LOFT Spectra

Majczyna A., Madej J., Różańska A., Należyty M.

Acta Astronomica

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2017

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Vol. 67, No. 2

171--180

EN

We present a simulation of an X-ray spectrum of a hot neutron star, as would be seen by the LAD detector on board of LOFT satellite. We also compute a grid of theoretical spectra corresponding to a range of effective temperatures Teff and surface gravities log g with values corresponding to compact stars in Type I X-ray bursters. A neutron star with the mass M=1.64 ,M☉ and the radius R=11.95 km (which yields the surface gravity log g=14.30 [cgs] and the surface redshift z=0.30) is used in simulation. Accuracy of mass and radius determination by fitting theoretical spectra to the observed one is found to be M=1.64+0.16 -0.02 M☉ and R=11.95+1.57 -0.40 km (2σ). The confidence contours for these two variables are narrow but elongated, and therefore the resulting constraints on the EOS cannot be strong. Note, that in this paper we aim to discuss error contours of NS mass and radius, whereas discussion of EOS is beyond the scope of this work.

8

P13 an ULX that is a Potential Progenitor of Merging BH-NS System

Beldycki B., Belczynski K.

Acta Astronomica

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2016

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Vol. 66, No. 3

347--355

EN

We have studied the future evolution of a recently discovered ULX source P13 in NGC 7793. This source was shown to contain a 5-15 M⊙ black hole and a massive 18-23 M⊙ B9Ia companion on a 64 day orbit. For low black hole mass (5-10 M⊙) and high companion mass ≳20 M⊙ the binary is predicted to initiate a common envelope evolution in near future and significantly decrease the orbital separation (6 hr orbit). This leads to a high probability (≈70%) of the system surviving a supernova explosion that will form a neutron star out of the companion. About one third of the surviving BH-NS systems will merge within Hubble time and be a source of high frequency gravitational radiation. We estimate that the chances of detection of BH-NS systems with Advanced LIGO/Virgo that form via P13-evolutionary channel are at the level of 0.1 yr-1 with wide range of allowed probability 0÷0.6 yr-1. This is the fourth empirical estimate of BH-NS merger rate.

9

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.

10

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.

11

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.

12

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.

13

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.

14

Observational Tests of Neutron Star Relativistic Mean Field Equations of State

Urbanec M., Beták E., Stuchlík Z.

Acta Astronomica

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2010

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Vol. 60, No. 2

149--163

EN

Set of neutron star observational results is used to test some selected equations of state of dense nuclear matter. The first observational result comes from the mass-baryon number relation for pulsar B of the double pulsar system J0737-3039. The second one is based on the mass-radius relation coming from observation of the thermal radiation of the neutron star RX J1856.35-3754. The third one follows the population analysis of isolated neutron star thermal radiation sources. The last one is the test of maximum mass. The equation of state of asymmetric nuclear matter is given by the parametrized form of the relativistic Brueckner-Hartree-Fock mean field, and we test selected parametrization that represent fits of full relativistic mean field calculation. We show that only one of them is capable to pass the observational tests. This equation of state represents the first equation of state that is able to explain all the mentioned observational tests, especially the very accurate test given by the double pulsar even if no mass loss is assumed.

15

Mass and Radius Determination for the Neutron Star in X-ray Burst Source 4U/MXB 1728-34

Majczyna A., Madej J.

Acta Astronomica

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2005

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Vol. 55, No. 4

349--366

EN

We analyzed archival X-ray spectra of MXB 1728-34 obtained in 1996-99 by the Proportional Counter Array on board of the RXTE satellite. X-ray spectra were fitted to our extensive grids of model atmosphere spectra to determine the effective temperature Teff on the neutron star surface, logarithm of surface gravity log g, and simultaneously the gravitational redshift z. We chose fitting of numerical model spectra plus broad Gaussian line, modified by interstellar absorption and the absorption on dust. We arbitrarily assumed either hydrogen-helium chemical composition of a model atmosphere, or H-He-Fe mixture in solar proportion. The statistically best values of log g and z were subsequently used to determine mass and radius of the neutron star. We obtained the best values of parameters for the neutron star in X-ray burst source MXB1728-34: mass either M=0.40 Msolar or 0.63 Msolar (for H-He or H-He-Fe models, respectively), radius R=4.6 km or 5.3 km, log g=14.6 or 14.6 and the gravitational redshift z=0.14 or 0.22. All the above parameters have very wide 1σ confidence limits. Their values strongly support the equation of state for strange matter in MXB 1728-34.

16

The Influence of Reprocessing in the Column on the Light Curves of Accretion Powered Neutron Stars

Čemeljić M., Bulik T.

Acta Astronomica

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1998

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Vol. 48, No. 1

65--75

EN

Flow of matter onto strongly magnetized neutron stars in X-ray binaries proceeds through accretion funnels that roughly follow geometry of the magnetic field. X-rays originate near the surface of the neutron star, and it may happen that the accretion flow passes through the line of sight as the star rotates. We consider the effects of such accretion flow eclipses on the X-ray light curves of accretion powered pulsars, and present a set of X-ray light curves measured by BATSE for A0535+262 for which this phenomenon is very likely to take place.