arXiv subjects

Grzegorz Wiktorowicz

Publications and source records attributed to Grzegorz Wiktorowicz.

30 records · Page 2Linked to original sources

arXiv2019

The connection between merging double compact objects and the Ultraluminous X-ray Sources

We explore the different formation channels of merging double compact objects (DCOs: BH-BH/BH-NS/NS-NS) that went through a ultraluminous X-ray phase (ULX: X-ray sources with apparent luminosity exceeding $10^{39}$ erg s$^{-1}$). There are many evolutionary scenarios which can naturally explain the formation of merging DCO systems: isolated binary evolution, dynamical evolution inside dense clusters and chemically homogeneous evolution of field binaries. It is not clear which scenario is responsible for the majority of LIGO/Virgo sources. Finding connections between ULXs and DCOs can potentially point to the origin of merging DCOs as more and more ULXs are discovered. We use the StarTrack population synthesis code to show how many ULXs will form merging DCOs in the framework of isolated binary evolution. Our merger rate calculation shows that in the local Universe typically 50% of merging BH-BH progenitor binaries have evolved through a ULX phase. This indicates that ULXs can be used to study the origin of LIGO/Virgo sources. We have also estimated that the fraction of observed ULXs that will form merging DCOs in future varies between 5% to 40% depending on common envelope model and metallicity.

astro-ph.HE↗

arXiv2019

Populations of stellar mass Black holes from binary systems

In large and complicated stellar systems like galaxies it is difficult to predict the number and characteristics of a black hole population. Such populations may be modelled as an aggregation of homogeneous (i.e. having uniform star formation history and the same initial chemical composition) stellar populations. Using realistic evolutionary models we predict the abundances and properties of black holes formed from binaries in these environments. We show that the black hole population will be dominated by single black holes originating from binary disruptions and stellar mergers. Furthermore, we discuss how black hole populations are influenced by such factors as initial parameters, metallicity, initial mass function, and natal kick models. As an example application of our results, we estimate that about 26 microlensing events to happen every year in the direction of the Galactic Bulge due to black holes in a survey like OGLE-IV. Our results may be used to perform in-depth studies related to realistic black hole populations, e.g. observational predictions for space survey missions like Gaia, or Einstein Probe. We prepared a publicly available database with the raw data from our simulations to be used for more in-depth studies.

astro-ph.HE↗

arXiv2019

The observed vs total population of ULXs

We have analyzed how anisotropic emission of radiation affects the observed sample of ultraluminous X-ray sources (ULXs) by performing simulations of the evolution of stellar populations, employing recent developments in stellar and binary physics, and by utilizing a geometrical beaming model motivated by theory and observation. Whilst ULXs harboring black hole accretors (BH ULXs) are typically emitting isotropically, the majority of ULXs with neutron star accretors (NS ULXs) are found to be beamed. These findings confirm previous assertions that a significant fraction of ULXs are hidden from view due to a substantial misalignment of the emission beam and the line-of-sight. We find the total number of NS ULXs in regions with constant star formation, solar metallicity, and ages above ~1 Gyr to be higher than the BH ULXs, although observationally both populations are comparable. For lower metallicities BH ULX dominate both the total and observed ULX populations. As far as burst star-formation is concerned, young ULX populations are dominated by BH ULXs, but this changes as the population ages and, post star-formation, NS ULXs dominate both the observed and total population of ULXs. We also compare our simulation output to a previous analytical prediction for the relative ratio of BH to NS ULXs in idealized flux-limited observations and find broad agreement for all but the lowest metallicities. In so doing we find that in such surveys the observed ULX population should be heavily dominated by black-hole systems rather than by systems containing neutron stars.

astro-ph.HE↗

arXiv2019

CG X-1: an eclipsing Wolf-Rayet ULX in the Circinus galaxy

We investigated the time-variability and spectral properties of the eclipsing X-ray source Circinus Galaxy X-1 (GG X-1), using Chandra, XMM-Newton and ROSAT. We phase-connected the lightcurves observed over 20 years, and obtained a best-fitting period $P = (25,970.0 \pm 0.1)$ s $\approx$7.2 hr, and a period derivative $\dot{P}/P = ( 10.2\pm4.6) \times 10^{-7}$ yr$^{-1}$. The X-ray lightcurve shows asymmetric eclipses, with sharp ingresses and slow, irregular egresses. The eclipse profile and duration vary substantially from cycle to cycle. We show that the X-ray spectra are consistent with a power-law-like component, absorbed by neutral and ionized Compton-thin material, and by a Compton-thick, partial-covering medium, responsible for the irregular dips. The high X-ray/optical flux ratio rules out the possibility that CG X-1 is a foreground Cataclysmic Variable; in agreement with previous studies, we conclude that it is the first example of a compact ultraluminous X-ray source fed by a Wolf-Rayet star or stripped Helium star. Its unocculted luminosity varies between $\approx$4 $\times 10^{39}$ erg s$^{-1}$ and $\approx$3 $\times 10^{40}$ erg s$^{-1}$. Both the donor star and the super-Eddington compact object drive powerful outflows: we suggest that the occulting clouds are produced in the wind-wind collision region and in the bow shock in front of the compact object. Among the rare sample of Wolf-Rayet X-ray binaries, CG X-1 is an exceptional target for studies of super-critical accretion and close binary evolution; it is also a likely progenitor of gravitational wave events.

astro-ph.HE↗

arXiv2019

Merger of compact stars in the two-families scenario

We analyse the phenomenological implications of the two-families scenario on the merger of compact stars. That scenario is based on the coexistence of both hadronic stars and strange quark stars. After discussing the classification of the possible mergers, we turn to detailed numerical simulations of the merger of two hadronic stars, i.e., "first family" stars in which delta resonances and hyperons are present, and we show results for the threshold mass of such binaries, for the mass dynamically ejected and the mass of the disk surrounding the post-merger object. We compare these results with those obtained within the one-family scenario and we conclude that relevant signatures of the two-families scenario can be suggested, in particular: the possibility of a rapid collapse to a black hole for masses even smaller than the ones associated to GW170817; during the first milliseconds, oscillations of the postmerger remnant at frequencies higher than the ones obtained in the one-family scenario; a large value of the mass dynamically ejected and a small mass of the disk, for binaries of low total mass. Finally, based on a population synthesis analysis, we present estimates of the number of mergers for: two hadronic stars; hadronic star - strange quark star; two strange quark stars. We show that for unequal mass systems and intermediate values of the total mass, the merger of a hadronic star and a strange quark star is very likely (GW170817 has a possible interpretation into this category of mergers). On the other hand, mergers of two strange quark stars are strongly suppressed.

astro-ph.HE↗

arXiv2018

Wind Roche Lobe Overflow as a way to make type Ia supernova from the widest symbiotic systems

Symbiotic stars are interacting binaries with one of the longest orbital periods. Since they can contain a massive white dwarf with a high accretion rate they are considered a possible type Ia supernovae (SNe Ia) progenitors. Among symbiotic binaries there are systems containing a Mira donor, which can have orbital periods of a few tens of years and more. This subclass of symbiotic stars due to their very large separation usually was not considered promising SNe Ia progenitors. We analysed evolution of one of the well studied symbiotic star with a Mira donor, V407 Cyg. We showed that the standard evolution model predicts that the system will not become a SN Ia. However, by simply adding a Wind Roche Lobe Overflow as one of the mass transfer modes we predict that the white dwarf in V407 Cyg will reach the Chandrasekhar limit in 40-200 Myr.

astro-ph.SR↗

arXiv2018

The merger of two compact stars: a tool for dense matter nuclear physics

We discuss the different signals, in gravitational and electromagnetic waves, emitted during the merger of two compact stars. We will focus in particular on the possible contraints that those signals can provide on the equation of state of dense matter. Indeed, the stiffness of the equation of state and the particle composition of the merging compact stars, strongly affect e.g. the life time of the post-merger remnant and its gravitational wave signal, the emission of the short gamma-ray-burst, the amount of ejected mass and the related kilonova. The first detection of gravitational waves from the merger of two compact stars in August 2017, GW170817, and the subsequent detections of its electromagnetic counterparts, GRB170817A and AT2017gfo, is the first example of the era of "multi-messenger astronomy": we discuss what we have learned from this detection on the equation of state of compact stars and we provide a tentative interpretation of this event, within the two families scenario, as due to the merger of a hadronic star with a quark star.

astro-ph.HE↗

arXiv2017

Strange quark stars in binaries: formation rates, mergers and explosive phenomena

The existence of strange quark stars has been proposed many years ago. More recently, the possible co-existence of a first family composed of "normal" neutron stars with a second family of strange quark stars has been proposed as a solution of problems related to the maximum mass and to the minimal radius of these compact stellar objects. In this paper we study the mass distribution of compact objects formed in binary systems and the relative fractions of quark and neutron stars in different subpopulations. We incorporate the strange quark star formation model provided by the two-families scenario and we perform a large-scale population synthesis study in order to obtain the population characteristics. In our model, below a critical gravitational mass $M_\mathrm{max}^H- ΔM$ only normal (hadron) neutron stars exist. Then in the mass range $(M_\mathrm{max}^H- ΔM) \leqslant M \leqslant M_\mathrm{max}^H$ strange quark stars and neutron stars coexist. Finally, above $M_\mathrm{max}^H$ all compact objects are strange quark stars. We argue that $M_\mathrm{max}^H$ is in the range $\sim 1.5-1.6 M_\odot$. According to our results, the main channel for strange quark star formation in binary systems is accretion from a secondary companion on a neutron star.This opens the possibility of having explosive GRB-like phenomena not related to supernovae and not due to the merger of two neutron stars. The enhancement in the number of compact objects in the co-existence mass range $(M_\mathrm{max}^H- ΔM) \leqslant M \leqslant M_\mathrm{max}^H$ is not very pronounced. The number of double strange quark star's systems is rather small with only a tiny fraction which merge within a Hubble time. This drastically limits the flux of strangelets produced by the merger, which turns out to be compatible with all limits stemming from Earth and lunar experiments.

astro-ph.HE↗

arXiv2017

The origin of the Ultraluminous X-ray Sources

Recently, several ultraluminous X-ray (ULX) sources were shown to host a neutron star (NS) accretor. We perform a suite of evolutionary calculations which show that, in fact, NSs are the dominant type of ULX accretor. Although black holes (BH) dominate early epochs after the star-formation burst, NSs outweigh them after a few 100 Myr and may appear as late as a few Gyr after the end of the star formation episode. If star formation is a prolonged and continuous event (i.e., not a relatively short burst), NS accretors dominate ULX population at any time in solar metallicity environment, whereas BH accretors dominate when the metallicity is sub-solar. Our results show a very clear (and testable) relation between the companion/donor evolutionary stage and the age of the system. A typical NS ULX consists of a $\sim1.3\,M_\odot$ NS and $\sim1.0\,M_\odot$ Red Giant. A typical BH ULX consist of a $\sim8\,M_\odot$ BH and $\sim6\,M_\odot$ main-sequence star. Additionally, we find that the very luminous ULXs ($L_X\gtrsim10^{41}$ erg/s) are predominantly BH systems ($\sim9\,M_\odot$) with Hertzsprung gap donors ($\sim2\,M_\odot$). Nevertheless, some NS ULX systems may also reach extremely high X-ray luminosities ($\gtrsim10^{41}$ erg/s).

astro-ph.HE↗

arXiv2017

Dynamical formation of Black Hole Low-Mass X-Ray Binaries in the field - an alternative to common envelope

Very wide binaries (> 500 AU) are subject to numerous encounters with flying-by stars in the Galactic field and can be perturbated into highly eccentric orbits (e ~ 0.99). For such systems tidal interactions at close pericenter passages can lead to orbit circularization and possibly mass transfer, consequently producing X-Ray binaries without the need for common envelope. We test this scenario for the case of Black Hole Low-Mass X-Ray Binaries (BH LMXBs) by performing a population synthesis from primordial binaries with numerical treatment of random stellar encounters. We test various models for the threshold pericenter distance under which tidal forces cause circularization. We estimate that fly-by interactions can produce a current population of ~ 60$-$220 BH LMXBs in the Galactic field. The results are sensitive to the assumption on tidal circularization efficiency and zero to very small BH natal kicks of a few km/s are required. We show that the most likely donors are low-mass stars (< 1 Msun; at the onset of mass transfer) as observed in the population of known sources (~ 20). However, the low number of systems formed along this route is in tension with most recent observational estimate of the number of dormant BH LMXBs in the Galaxy 10$^4-$10$^8$ (Tetarenko et al. 2016). If indeed the numbers are so high, alternative formation channels of BHs with low-mass donors need to be identified.

astro-ph.HE↗

arXiv2015

Nature of the Extreme Ultraluminous X-ray Sources

In this proof-of-concept study we demonstrate that in a binary system mass can be transferred toward an accreting compact object at extremely high rate. If the transferred mass is efficiently converted to X-ray luminosity (with disregard of the classical Eddington limit) or if the X-rays are focused into a narrow beam then binaries can form extreme ULX sources with the X-ray luminosity of Lx>10^42 erg/s. For example, Lasota & King argued that the brightest known ULX (HLX-1) is a regular binary system with a rather low-mass compact object (a stellar-origin black hole or a neutron star). The predicted formation efficiencies and lifetimes of binaries with the very high mass transfer rates are large enough to explain all observed systems with extreme X-ray luminosities. These systems are not only limited to binaries with stellar-origin black hole accretors. Noteworthy, we have also identified such objects with neutron stars. Typically, a 10 Msun black hole is fed by a massive (10 Msun) Hertzsprung gap donor with Roche lobe overflow rate of 10^-3 Msun/yr (2600 MEdd). For neutron star systems the typical donors are evolved low-mass (2 Msun) helium stars with Roche lobe overflow rate of 10^-2 Msun/yr. Our study does not prove that any particular extreme ULX is a regular binary system, but it demonstrates that any ULX, including the most luminous ones, may potentially be a short-lived phase in the life of a binary star.

astro-ph.HE↗

arXiv2014

Black Hole X-ray Transients: The Formation Puzzle

There are 19 confirmed BH binaries in the Galaxy. 16 of them are X-ray transients hosting a ~5-15 Msun BH and a Roche-lobe overflowing low-mass companion. Companion masses are found mostly in 0.1-1 Msun mass range with peak at 0.6 Msun. The formation of these systems is believed to involve a common envelope phase, initiated by a BH progenitor, expected to be a massive star >20 Msun. It was realized that it may be very problematic for a low-mass companion to eject a massive envelope of the black hole progenitor. It invoked suggestions that an intermediate-mass companion ejects the envelope, and then is shredded by the Roche-lobe overflow to its current low-mass. But this creates another issue; a temperature mismatch between hot models and the observed cool low-mass donors. Finally, the main driver of Roche-lobe overflow that is believed to be magnetic braking does not seem to follow any theoretically calculated models. Number of ideas were put forward to explain various parts of this conundrum; pre-main sequence donor nature, alternative approach to magnetic braking and common envelope energy was revisited. We test various proposals and models to show that no overall solution exists so far. We argue that common envelope physics is not crucial in the understanding BH transient physical properties, but may affect significantly their formation rates. Our failure most likely indicates that either the current evolutionary models for low-mass stars and magnetic braking are not realistic or that the intrinsic population of BH transients is quite different from the observed one.

astro-ph.HE↗