Symposium 346 - Abstracts


HMXBs in the early Universe

Mirabel, Felix

Theoretical models and recent observations show the dependence of HMXB formation on decreasing metallicity and increasing redshift. These results support the idea that HMXBs were prolifically produced at cosmic dawn, and that feedback from HMXBs was an important source of heating and partial reionization of the Intergalactic medium far away from star forming galaxies. It will be shown that HMXBs have impact on cosmology because determine the early thermal history of the universe, impact on galaxy formation because determine the properties of the faintest galaxies at high redshifts and the smallest dwarf galaxies in the local universe, and HMXBs also have impact on the incipient gravitational wave astrophysics, because they are one of the possible channels for the formation of binary systems of stellar black holes and neutron stars. Theoretical models and recent observations show the dependence of HMXB formation on decreasing metallicity and increasing redshift. These results support the idea that HMXBs were prolifically produced at cosmic dawn, and that feedback from HMXBs was an important source of heating and partial reionization of the Intergalactic medium far away from star forming galaxies. It will be shown that HMXBs have impact on cosmology because determine the early thermal history of the universe, impact on galaxy formation because determine the properties of the faintest galaxies at high redshifts and the smallest dwarf galaxies in the local universe, and HMXBs also have impact on the incipient gravitational wave astrophysics, because they are one of the possible channels for the formation of binary systems of stellar black holes and neutron stars.

The Astrophysics of BH-BH/NS-NS Mergers with LIGO/Virgo

Belczynski, Krzysztof

I will discuss the astrophysical importance of the recent LIGO/Virgo direct detections of gravitational-waves. Despite majority of the expectations, it was not neutron star mergers being detected first, but the series of exotic massive black hole mergers. I will describe the leading theories of the formation of such black hole systems. I will also comment on a detection of NS-NS merger. This particular detection may provide striking constraints on binary evolution. Several astrophysical implications are beginning to emerge despite the fact that the exact origin of LIGO/Virgo sources is not yet known.

Massive star atmospheres and donor star winds in HMXBs

Sander, Andreas

Understanding the complex behavior of High Mass X-ray binaries (HMXBs) is not possible without proper information about their donor stars. While crucial, this turns out to be a challenge on multiple fronts. First, multi-wavelength spectroscopy is vital. As such systems can be highly absorbed, this is often already hard to accomplish. Secondly, even if the spectroscopic data is available, the determination of reliable stellar parameters requires sophisticated model atmospheres that accurately describe the outermost layers and the wind of the donor star.For early-type donors, the stellar wind is radiatively driven and there is a smooth transition between the outermost layers of the star and the wind. The intricate non-LTE conditions in the winds of hot stars complicate the situation even further, as proper model atmospheres need to account for a multitude of physics to accurately provide stellar and wind parameters. The latter are especially crucial for the so-called ``wind-fed'' HXMBs, where the captured wind of the supergiant donor is essentially the only source for the material accreted by the compact object.In this review I will briefly address the different approaches for treating stellar winds in the analysis of HMXBs. The fundamentals of stellar atmosphere modeling will be discussed, also addressing the limitations of modern models. Examples from recent analysis results for particular HMXBs will be presented. Furthermore, the path for the next generation of stellar atmosphere models will be outlined, where models can not only be used for measurement purposes, but also to make predictions and provide a laboratory for theoretical conclusions. Stellar atmospheres will be a key tool in understanding HMXBs, e.g. by providing insights about the accretion of stellar winds onto the compact object, or by placing the studied systems in the correct evolutionary context in order to identify potential gravitational wave (GW) progenitors.

The formation channels of gravitational wave source progenitors

Mapelli, Michela

What are the formation channels of merging black holes and neutron stars? The first two observing runs of Advanced LIGO and Virgo give us invaluable insights to address this question, but a new approach to theoretical models is required, in order to match the challenges posed by the new data. In my talk, I present a new model to study the progenitor stars of merging systems, by considering both dynamical and isolated binary evolution within a cosmological framework. I discuss the impact of stellar winds, core-collapse and pair instability supernovae on the formation of compact remnants in both isolated and dynamically formed binaries. Finally, I show that dynamical processes, such as the runaway collision scenario and the Kozai-Lidov mechanism, leave a clear imprint on the demography of merging systems.

High-Mass X-ray Binaries

van den Heuvel, Edward

A brief historical overview is given of the discovery of the different classes of High-Mass X-ray Binaries (HMXBs), and of the models for their evolutionary origins and fate. Already in the early 1970s it was expected that HMXBs will terminate their evolution as very close systems or will merge. This picture was confirmed by the 1974 discovery of the first close double neutron star system, showing that HMXBs which do not merge will terminate as close systems consisting of two compact objects.HMXBs are crucial astrophysical objects in many respects:As key probes of accretion processes, by stellar wind and Roche-lobe overflow or a combination of these.As probes of the winds of massive stars and of transient mass ejections of B-emission stars.As probes of interactions between matter flows and the magnetospheres of neutron stars, causing spin-down and spin-up.As probes of black-hole accretion and the formation of relativistic jets.As probes of the evolution of massive binary systems, leading to the formation of double neutron stars and black holes, and neutron star-black hole binaries.The present state of our knowledge and ignorance in these subjects is reviewed.

Massive Stars

Heger, Alexander

Stars more massive than about eight solar masses run through all exothermic nuclear burning phases and build up a massive iron core in their centre.  This core eventually collapses to leave behind a neutron star or black hole as compact remnant.  This collapse can drive explosions ranging from some of the most powerful explosions in the universe to quite weak events that are basically just a disappearance of the star.  The ejected mass may lead to bright displays we observe as GRBs, hypernovae, or supernovae.  Mass outside the core that is not ejected, falls back onto the compact remnant.  Both the explosion and the fallback may be asymmetric, imparting angular and linear momentum onto the remnant, even for black holes.  In binary star systems, large mass ejection can make the star unbound, whereas kicks may help to ‘repair’ this deficit in some cases, or aid to more and faster ejection.  Hence to understand binary star populations, one needs to understand the explosion, and for that, the final state of the star at the time of explosion.  This may include interactions the star has had as part of a binary system that affected its evolution and that of its binary star orbital parameters, as well as what the star just does by itself.  In particular, the late evolution stages, we assume today, are affected only little by being in a binary, once the helium, or certainly the carbon core size is set.  Once carbon burning is reached the remaining lifetime of the star is a thousand years or less.It would be misleading to assume stellar evolution even of single stars was a topic that is mostly set as we have been studying it since decades.  In contrast, there is many aspects of massive star evolution that still remain largely elusive, including internal transport processes of composition, angular momentum and energy, mass loss, the final structure of the stars at death, and which stars die what fate.  I shall provide a current view of at least a few of these aspects.

Common evolutionary threads between low-mass X-ray binaries and merging binary compact objects

Fragos, Tassos

The recent detection of gravitational waves from coalescing binary black holes by LIGO allowed for the  first time the direct observation of stellar-mass black holes, while the simultaneous gravitational wave and electromagnetic signal from the merger of two neutron stars provided the first direct evidence for origin of short gamma-ray bursts. These gravitational wave events, complemented by a half-a-century-long history of indirect observations of accreting compact objects in X-ray binaries, can give us now a more complete picture of the formation and evolution of binary stellar systems containing compact objects. Low-mass X-ray binaries, despite having companion stars with masses too low to form a second compact object, have formation pathways that present many similarities with at least some of the proposed formation channels for merging double compact object, i.e. the isolated, common-envelope formation channel. In fact, the main uncertainties in the formation pathways for both types of systems are the same, namely the occurrence of natal kicks during the core-collapse phase leading to compact-object formation, the angular momentum content of the formed compact object and the phase of dynamically unstable mass-transfer also known as common envelope. In this talk, after briefly reviewing our current understanding of the evolutionary channels leading to the two types of compact object binaries, I will discuss what we can learn from studying these uncertain physical processes in the formation of low-mass X-ray binaries, especially the ones with a black hole accretor, and how we can transfer this knowledge to better understand the formation of double compact objects. Finally, I will present an outlook of how we can improve, in the near future, our understanding in all these fronts.

The Be stars in the Be X-ray binaries

Rivinius, Thomas

Rapidly rotating B-type stars with gaseous mass-loss disks in Keplerian rotation are common central objects in X-Ray binaries. After briefly introducing these Be stars and their physics in general, I will review their disk properties. The disks are well understood in a viscous framework, and their typical parameters have been established for a large number of (supposedly, this statistics will be discussed as well) single Be stars in the recent years. According to the current observational evidence, the Be stars and disks found in BeXRBs are well within the boundaries known from single Be stars, which provides important limits for modeling.  New results have also been obtained on the orbital disk truncation and other tidal and radiative effects of the companion objects on the disk.In turn, BeXRBs also provide constraints for understanding single Be star disks, as they make the outer regions of the disk observable, which are not usually accessible to observation except in the radio regime.

Ultraluminous Pulsars: Extremes of Accretion

Harrison, Fiona

Ultraluminous X-ray sources (ULXs) are binary systems in nearby galaxies with luminosities between 10^39 - 10^41 erg/s, or between ten to one thousand times the Eddington luminosity for a stellar mass compact object.    Long thought to harbor black hole accretors with masses between 10 - 1000 times the mass of the Sun, the discovery of three ultraluminos pulsars powered by accretion onto a neutron star presents many theoretical challengs.   I will present an overview of observations of pulsating ULXs, concentrating on the characteristics that successful theories must explain.  I will also summarize the range of theories proposed to explain the extreme observed  luminosities.

Clumpy wind accretion in Supergiant X-ray Binaries


Supergiant X-ray Binaries host a compact object, generally a neutron star, orbiting an evolved O/B star. Mass transfer proceeds through the intense radiatively-driven wind of the stellar donor, a fraction of which is captured by the gravitational field of the neutron star. The subsequent accretion process onto the neutron star is responsible for the abundant X-ray emission from those systems. They also display variations in time of the X-ray flux by a factor of a few 10, along with changes in the hardness ratios believed to be due to varying absorption along the line-of-sight. We used the most recent results on the inhomogeneities (aka clumps) in the non-stationary wind of massive hot stars to evaluate their impact on the time-variable accretion process. We ran three-dimensional simulations of the wind in the vicinity of the accretor to witness the formation of the bow shock and follow the inhomogeneous flow over several spatial orders of magnitude, down to the neutron star magnetosphere. In particular, we show that the impact of the clumps on the time-variability of the intrinsic mass accretion rate is severely damped by the crossing of the shock, compared to the purely ballistic Bondi-Hoyle-Lyttleton estimation. We also account for the variable absorption due to clumps passing by the line-of-sight and estimate the final effective variability of the mass accretion rate for different orbital separations. These results are confronted to recent analysis of Vela X-1 observations with Chandra by Grinberg et al (2017). It shows that clumps account well for time-variability at low luminosity but can not generate, per se, the high luminosity activity observed.

Multi messengers to gamma-ray XRB

Zhang, Shu

The Insight-HXMT is the China’s first astronomy X-ray satellite, which was selected in 2011 and launched on June 15th 2017 to an orbit of 550 km altitude and 43 dg inclination. It was designed to serve for at least 4 years in an energy band of 1-250 keV, for which the core science researches will be carried out in ways of: Galactic survey for transients, pointed observation on the bright XRBs, multi-wavelength campaigns and observations on burst events like GRB, GW EM and so on. I report our recent researches on the gamma-ray XRBs of e.g. LS5039, LS 61303, LS1259, with the multi-messengers of RXTE, Fermi, INTEGRAL, and Swift. The results will be addressed and discussed in the context of having the currently orbiting Insight-HXMT.

S-CUBE & RXTE X-ray surveys of the Small Magellanic Cloud

Coe, Malcolm

The Small Magellanic Cloud hosts the highest density known of Be/X-ray binary systems – over 100 objects. Long term studies of the SMC with RXTE for over a decade, now superseded by the weekly Swift/XRT monitoring (the S-CUBE project), present us with an opportunity to take an overview of these systems and explore some basic parameters. In particular we can gain an excellent insight into the duty cycles of these systems, the long-term pulse period changes, and the range of X-ray luminosities exhibited. From these observations we can infer some basic astrophysical parameters of the systems such as the limits to accretion and the magnetic field distribution of the neutron star components. This talk will review what we are learning about this class of objects from this long-term perspective.

IGR J17329-2731: the birth of a symbiotic X-ray binary

Bozzo, Enrico

I will report on the results of the multi-wavelength campaign carried out after the discovery of the INTEGRAL transient IGR J17329-2731. The optical data collected with the SOAR telescope allowed us to identify the donor star in this system as a late M giant at a distance of 2.7(+3.4,-1.2) kpc. The data collected quasi-simultaneously with XMM–Newton and NuSTAR showed the presence of a modulation with a period of 6680±3 s in the X-ray lightcurves of the source. This unveils that the compact object hosted in this system is a slowly rotating neutron star. The broad-band X-ray spectrum showed the presence of a strong absorption (10^23 cm^−2 ) and prominent emission lines at 6.4 keV, and 7.1 keV. These features are usually found in wind-fed systems, with the emission lines resulting from the fluorescence of the X-rays from the accreting compact object on the surrounding stellar wind. The presence of a strong absorption line around ∼21 keV in the NuSTAR spectrum suggests a cyclotron origin, thus allowing us to estimate the neutron star magnetic field as ∼2.4×10^12 G. All evidence thus points to IGR J17329-2731 being a symbiotic X-ray binary. As no X-ray emission was ever observed from the location of IGR J17329-2731 by INTEGRAL during the past 15 yr in orbit and considering that symbiotic X-ray binaries are known to be variable but persistent X-ray sources, we concluded that IGR J17329-2731 was most likely caught by INTEGRAL when the source shined as a symbiotic binary in X-rays for the very first time. The Swift XRT monitoring performed up to ∼3 months after the discovery of the source, showed that it maintained a relatively stable X-ray flux and spectral properties. This supports the idea that IGR J17329-2731 might have become a persistent X-ray source in our Galaxy.

Wind inhibition in HMXBs: the effect of clumping and implications for X-ray luminosity

Krticka, Jirí

Winds of hot stars are driven by the radiative force due to absorption of light in the lines of heavier elements. Consequently, the amount of mass lost per unit of time by the wind (mass-loss rate) and the wind velocity depend on the ionization state of the wind. As a result of this, there is a feedback between the ionizing X-ray source and the stellar wind in HMXBs powered by wind accretion. We study the influence of the small-scale wind structure (clumping) on this feedback using our NLTE hydrodynamical wind models. We find that clumping weakens the effect of X-ray irradiation. Moreover, we show that the observed X-ray luminosities of HMXBs can not be explained by wind accretion scenario without introducing the X-ray feedback. Taking into account the feedback, the observed and estimated X-ray luminosities nicely agree. We identify two cases of X-ray feedback with low and high X-ray luminosities that can explain the dichotomy between SFXTs and sgXBs.

Phase connected X-ray light curve and He II radial velocity measurements of NGC 300 X-1

Carpano, Stefania

NGC 300 X-1 and IC 10 X-1 are currently the only two robust extragalactic candidates for being Wolf-Rayet/black hole X-ray binaries, the Galactic analogue being Cyg X-3. These systems are believed to be a late product of high-mass X-ray binary evolution and direct progenitor of black hole mergers. From the analysis of Swift data only, the orbital period of the NGC 300 X-1 was found to be 32.8 h. We here merge the full set of existing data of NGC 300 X-1, using XMM-Newton, Chandra and Swift observations to derive a more precise value of the orbital period of 32.794+-0.004 h above a confidence level of 99.99%. This allows us to phase connect the X-ray light curve of the source with radial velocity measurements of He II lines performed in 2010. We show that, as for IC 10 X-1 and Cyg X-3, the X-ray eclipse corresponds to maximum of the blueshift of the He II lines, instead of the expected zero velocity. This indicates that for NGC 300 X-1 as well, the wind of the WR star is completely ionised by the black hole radiation and that the emission lines come from the region of the WR star that is in the shadow. We also present a more detailed analysis of two recent very long XMM-Newton observations of the source, performed on 2016 December 17 to 20.

Using High-Mass X-ray Binaries to Probe Massive Binary Evolution

Garofali, Kristen

High-mass X-ray binaries (HMXBs) provide an exciting window into the underlying processes of both binary as well as massive star evolution. Because HMXBs are systems containing a compact object accreting from a high-mass star at close orbital separations they are also likely progenitors of gamma-ray bursts and gravitational wave sources. I will present work on the classification and age measurements of HMXBs in M33 using a combination of deep Chandra X-ray imaging, and archival Hubble Space Telescope data. I am able to constrain the ages of the HMXB candidates by fitting the color-magnitude diagrams of the surrounding stars, which yield the star formation histories of the surrounding region. Unlike the age distributions measured for HMXB populations in the Magellenic Clouds, the age distribution for the HMXB population in M33 contains a number of extremely young (< 5 Myr) sources, including M33 X-7, an eclipsing binary composed of a ~15 Msun black hole accreting from a 70 Msun O star companion. I will discuss these results for M33 in the context of the effect of host galaxy properties on the observed HMXB population. 

Orbital period and pulsed spectrum of the super-Eddington neutron star NGC 7793 P13

Fuerst, Felix

Ultra-luminous X-ray pulsars (ULPs) are a recently discovered class of neutron stars accreting orders of magnitude above their Eddington limit. Currently only about 5 of these systems have been identified, either through the discovery of pulsations or cyclotron resonant scattering features. Their distance, large extinction, and source confusion makes detailed observations challenging. An exception is NGC 7793 P13 which shows a high X-ray flux and a clearly identified optical companion, a B9I supergiant. Furthermore, the pulsations with a period around 415ms have been detected in all X-ray observations so far, allowing us to study the long-term spin evolution. I will also present timing and spectral analysis of a recent XMM and NuSTAR campaign. Through X-ray timing, we can unambiguously measure the orbital period to be ~64d based. This period is consistent with the optical photometric period, but significantly shorter than the periodicity of the X-ray flux (P_X ~ 66.5d). I will discuss possible explanations for this difference, like a very long super-orbital period. The spectral analysis reveals two components, the harder of which we identify with the emission from the accretion column. I will compare the timing and spectral properties of P13 with Galactic HMXBs and discuss what properties of the compact object are necessary to form an ultra-luminous pulsar.

Our current picture of the physics of wind-fed massive X-ray binary systems

Martínez-Núñez, Silvia

S. Martínez-Núñez (IFCA), P. Kretschmar (ESAC) on behalf of a large collaboration.A large number of massive X-ray binaries consist of a neutron star accreting matter from the strong stellar wind of a massive companion and producing a powerful X-ray source.  These wind-fed systems can play an important and complementary role to characterize the stellar wind properties of massive stars. During a collaborative effort of astronomers from the stellar wind and the X-ray communities, we have found that to advance the current knowledge we have to combine the physics of stellar winds in massive stars in general, accretion theory for compact objects and the physics of X-ray emission from accreting neutron stars and to confront the results with detailed, multi-wavelength observations.In this talk, we will review our current state of knowledge on wind-fed massive X-ray binaries, including recent results and indicate some avenues for future progress.

Wolf Rayet binaries in the Magellanic Clouds as progenitors of high mass X-ray binaries

Shenar, Tomer

Wolf-Rayet stars are evolved, hydrogen depleted stars that exhibit strong mass-loss. Non-degenerate Wolf-Rayet binaries are thought be progenitors of high mass X-ray binaries hosting a black hole. Understanding the origin and interaction physics of Wolf-Rayet binaries at different metallicities is therefore crucial for constraining the properties and incidence of high mass X-ray binaries. It is generally not known whether the majority of Wolf-Rayet stars in the Magellanic Clouds originate via stripping in binary systems or via intrinsic stripping due to mass-loss. We performed a complete spectral analysis of all known Wolf-Rayet binaries in the Small and Large Magellanic Clouds (SMC, LMC), as well as additional orbital analyses, and constrained the evolutionary histories of these important stars. In my talk, I will summarize our findings regarding the origin of Wolf-Rayet stars in the Magellanic Clouds and their feedback on their environments. I will further describe our study's implications on gravitational wave progenitors, high mass X-ray binaries, and the initial mass function as a function of metallicity.

Investigating High Mass X-ray Binaries at hard X-rays with INTEGRAL

Sidoli, Lara

The INTEGRAL archive developed at INAF-IASF Milano with the available public observations from 2003 to 2016 is investigated to extract the hard X-ray properties of about 60 High Mass X-ray Binaries (HMXBs).  This sample consists of both persistent and transient sources, hosting either a Be star (Be/XRBs) or an early-type supergiant companion (SgHMXBs), including the most extreme systems, the Supergiant Fast X-ray Transients. INTEGRAL X-ray light curves (18-50 keV), sampled at a bin time of about 2 ks, are used to build the cumulative distributions of their hard X-ray luminosities, their duty cycles (defined as the percentage of detections at this same bin time), the range of variability of their hard X-ray luminosities. Putting the phenomenology observed with INTEGRAL into context with other known source properties (e.g. orbital parameters, pulsar spin periods), together with observational contraints coming from softer X-rays (below 10 keV), allowed us to investigate how the different HMXB sub-classes behave (and sometimes overlap). We will present the results of this research, aimed at comparing HMXB subclasses (Be/XRBs, SgHMXBs and SFXTs) and investigating how intermediate systems that link them.

Long-term Properties of Superorbital Periodic Modulation in Wind-Accretion High-Mass X-ray Binaries

Corbet, Robin

Superorbital modulation of X-ray flux has been detected in several high-mass X-ray binaries where accretion takes place from the primary star's wind.  Unlike those HMXBs where accretion is driven by Roche-lobe overflow such as LMC X-4, the driving mechanism for superorbital modulation in wind-accretion systems remains elusive, although several models have been proposed.  Here, we systematically investigate the long-term properties of superorbital modulation in 2S 0114+650, IGR J16418-4532, IGR J16479-4514, IGR J16493-4348, and 4U 1909+07 using over thirteen years of data from the Swift Burst Alert Telescope. We examine long-term period stability and changes in modulation profile and phasing, and compare these with predictions of the various proposedmodels.

Empirical Constraints on the Evolution of X-ray Binaries from Chandra Deep Fields and Local Galaxies

Lehmer, Bret

Recently, we have found, in the Chandra Deep Field-South, that the emission from X-ray binary (XRB) populations in galaxies evolves significantly with cosmic time, most likely due to changes in the physical properties of galaxies like star-formation rate, stellar mass, stellar age, and metallicity.  These  observations have shown that the X-ray emissivity from HMXBs likely outpowered active galactic nuclei in the z > 6-8 Universe, implicating HMXBs as an important source of heating to the IGM at z ~ 10-20.  Furthermore, these observations provide an important benchmark for self-consistently modeling the processes leading to the gravity-wave emitting populations now detected by aLIGO and VIRGO.  I will review results from the Chandra Deep Fields and provide new results based on Chandra and multiwavelength observations of local spiral galaxies that allow us to extract XRB X-ray  luminosity function histories.  I will conclude by discussing next steps in developing a comprehensive empirical model for how XRB populations evolve with age, depending on the physical properties of their host stellar populations.

Constraining the progenitor evolution of GW 150914

Vink, Jorick

One of the largest surprises from the LIGO results regarding the first gravitational wave detection was the fact the black holes were "heavy", of order 30 - 40 solar masses. The most promising explanation for this obesity is that the merger occurred at low metallicity: when the iron (Fe) contents is lower, this is expected to result in weaker mass loss & more rapid stellar rotation, allowing Wolf-Rayet and black hole progenitor evolution in a chemically homogeneous manner. However, there is as yet no empirical evidence for more rapid rotation amongst Wolf-Rayet stars. Due to the intrinsic challenge of determining the rotation rates of Wolf-Rayet stars from their emission lines, the most promising way to constrain rotation rate distributions amongst the various subgroups of Wolf-Rayet stars in various metallicity regimes is through the utilisation of their emission lines in polarised light. I will thus provide an overview of spectropolarimetry observations of both single star and binary Wolf-Rayet stars in the Galaxy, as well as the Large and Small Magellanic Clouds, at 1/2 and 1/5th solar metallicity, respectively. I will use these results to argue which evolutionary routes are most promising to produce heavy black holes.

HMXBs as progenitors of gravitational wave mergers

Bulik, Tomasz

Formation of merging binary compact objects must be preceded by a stage including a compact object and a non compact massive companion on a relatively tight orbit. At this stage the compact object will be acreting matter from the wind and should be visible as a high mass X-ray binary. I will review the properties of the known high mass X-ray binaries and discuss the possibility that they will lead to formation of coalescing compact object binaries. 

Optical Interferometry of HMXBs: Resolving Wind, Disk and Jet Ouflows at Sub-milliarcsecond Scale

Waisberg, Idel

Because of their small angular size < few mas, spatial information on High-mass X-ray binaries (HMXB) has typically been inferred from photometry or spectroscopy. Optical interferometry offers the possibility to spatially resolve such systems, but has been traditionally limited to bright targets or low spectral resolution. The VLTI instrument GRAVITY, working in the near-infrared K band, achieves unprecedented precision in differential interferometric quantities at high spectral resolution, allowing to study many HMXBs through the lens of optical interferometry for the first time. Here we will present GRAVITY observations on two X-ray binaries: the microquasar SS 433 and the supergiant HMXB BP Cru. The former is the only known super-Eddington accretor in the Galaxy in a unique stage of binary evolution, with probable ties to at least part of the ULX population. With GRAVITY, we resolve its massive winds and optical baryonic jets for the first time, finding evidence for both a circumbinary disk and polar outflow and characterizing the activity of the central jet in both space and time. BP Cru harbors an X-ray pulsar accreting from the wind of its early-blue hypergiant companion Wray 977. By comparing the properties of its spatially resolved wind to those of isolated blue hypergiants zeta1 Sco and HD 169454, also observed with GRAVITY, we characterize the influence of the pulsar on the wind of Wray 977. We also compare our observations to predictions from stellar atmosphere models, highlighting the potential of optical interferometry to study winds of massive stars in general. 

The donor stars of ultraluminous X-ray sources

Heida, Marianne

Ultraluminous X-ray sources (ULXs) are the most extreme X-ray binaries in the Universe. As the large majority of ULXs are extragalactic objects and their optical emission is in most cases dominated by the accretion disc, spectroscopic identification of ULX donors is challenging. Many ULXs are believed to have high mass donor stars as they are found in or near star forming regions. This makes them possible progenitors for the black hole mergers discovered by LIGO. To learn where they fit in the picture of massive binary star evolution, knowledge of the donor stars in these systems is crucial. Detecting stellar absorption lines is also necessary to obtain dynamical mass measurements, which is the only direct way to determine the mass of any black hole accretors in ULXs. Of the five ULX donor stars that have been identified spectroscopically, three are red supergiants discovered in our near-IR survey of nearby ULXs. The photometric part of this survey is now finished and our spectroscopic follow-up campaign, to classify the counterparts and monitor the ones that are confirmed to be stellar, is well underway. I will show the latest results of this campaign and discuss the opportunities afforded by upcoming facilities such as JWST and 30-m class telescopes.

CI Camelopardalis: The first sgB[e]-High Mass X-ray Binary Revisited Twenty Years on

Bartlett, Elizabeth

 Supergiant B[e] (sgB[e]) stars are a rare type of evolved, very massive star with complex hybrid spectra. The standard explanation for these spectra is a two component stellar wind which includes a dense, slowly expanding disc and/or torus, many times larger than that of a classical Be star. Binary interactions are thought to be the formative agent for this torus, but identifying bona fide sgB[e] binaries has proved extremely difficult due to the extreme stellar winds and high levels of local extinction (attributed to the torus). The Galactic supergiant B[e] star CI Camelopardalis (CI Cam) was the first sgB[e] star detected during an X-ray outburst. The star brightened by 5 orders of magnitude within hours before decaying to quiescence in less than 2 weeks. Such X-ray activity firmly establishes this source as a High-Mass X-ray Binary but the nature of the compact object, as well as binary system parameters, are still a topic of debate 20 years on. Since the outburst of CI Cam, a number of sgB[e] stars have been identified as X-ray overluminous for a single star (i.e., LX > 10-7Lbol). This small population is highly hetergeneous and has recently expanded to include two Ultra Luminous X-ray sources (ULX), Holmberg II X-1 and NGC300 ULX-1/supernova imposter SN2010da. In this talk I will report on our recent discovery of dramatic variability in the quiescent X-ray spectrum of CI Cam in the context of binarity, and discuss the X-ray bright sgB[e] stars as a population and their importance for other fields of astronomy.

X-ray binaries with neutron stars at accreting and non-accreting stages

Postnov, Konstantin

Results of the population synthesis calculations of X-ray binaries with rotating magnetized neutron stars are presented. A modified version of the BSE population synthesis code is used, supplemented with a detailed treatment of interaction of neutron stars with matter gravitationally captured from stellar wind of the massive companion.  Specifically, we discuss several observed classes, including symbiotic X-ray binaries, Be X-ray binaries with X-ray pulsars and with propelling neutron stars at quasi-spherical stage (possible gamma-Cas analogs). 

Local merger rates of double neutron star systems

Chruslinska, Martyna

The first ever detection of gravitational waves from a merging double neutron star (DNS) binarywas registered on August 17, 2017. This amazing, yet unexpected discovery challenged our understanding of evolution of its potential progenitor systems, implying a much higher rate of DNS coalescences in the local Universe than predicted on theoretical grounds.In light of this detection, we revisited the topic of DNS formation in the classical isolated binary evolution scenario. We employed population synthesis method, aiming to pinpoint the factors that affect the predicted merger rates the most. Apart from being particularly sensitive to the common envelope treatment, DNS merger rates appear rather robust against variations of several of the key factors probed in our study (e.g. conservativeness of the mass transfer, angular momentum loss, and natal kicks).Binaries that merge within the local Universe originate from progenitor systems that formed at different redshifts and in various environments. It appears that the efficiency of formation of double compact objects is highly sensitive to metallicity at which star formation proceeds. Therefore, to confront the theoretical predictions with observational limits resulting from gravitational waves observations  one has to account for the formation and evolution of progenitor stars in chemically evolving Universe. In particular, this requires knowledge of the amount of star formation taking place at different metallicities and at different redshifts. This introduces another layer of uncertainties to such considerations, besides the factors relating directly to binary evolution. We combine the available observational relations connecting metallicity, star formation and stellar masses of galaxies to quantify those uncertainties.

Accreting magnetars: stellar winds and accretion

Torrejon, Jose Miguel

Magnetars are isolated neutron stars with extreme magnetic fields of up to 10^15 gauss. Their X-ray emission is powered by the decay of this gigantic field. There is a growing, albeit indirect, evidence on the existence of such a highly magnetised neutron stars (10^13-14 gauss) in many High Mass X-ray Binaries accreting from the powerful winds of their massive companions: the accreting magnetars. In this talk I will present X-ray observations of some candidates and explore what are their main observational properties and how do they help us to understand the stellar winds from their massive companions. 

The black hole spin in coalescing binary black holes and high-mass X-ray binaries

QIN, Ying

Binary stellar systems are unique astrophysical laboratories for the study of black holes (BHs). Accretion of matter from a binary companion gives rise to X-ray emission, bringing them to the X-ray binary phase, and the recent gravitational wave observations enabled us to witness the last few seconds of the lives of coalescing binary BHs. In fact, some types of high-mass X-ray binaries are considered as the potential immediate progenitors of Gravitational-Wave sources. The BH spin is one of the observable quantities in both types of systems, and it carries significant information about their past evolutionary history. In the last decade, the measurement of the spin of BHs has become possible for about a dozen of X-ray binaries. The observed BH spins in Galactic field in low-mass X-ray binaries can be explained through accretion onto the BH after its formation. But for three well-known high-mass X-ray binary sources (Cyg X-1, LMC X-1 and M33 X-7), their high spins (a_* > 0.8) are believed to have a different origin. In our proposed model, such high spins can be well explained when the BH progenitor star looses its hydrogen envelope via a case-A mass-transfer, and never expands to become a giant star, while an inefficient angular momentum transport mechanism seems to also be required. For the case of coalescing binary BHs, we study the immediate progenitor of the binary BH is in a close binary system composed of a BH and a helium star. Upon core collapse, the helium star produces a BH  (the second-born BH in the system) with a spin that can span the entire range from zero to maximally spinning. We find an anti-correlation between the merging timescale of the two BHs, T$_{\rm merger}$, and the effective spin $\chi_{\rm eff}$. We predict that, with future improvements to AdLIGO's sensitivity, the sample of merging binary BH systems will show an overdensity of sources with positive but small $\chi_{\rm eff}$ originating from lower mass BH mergers born at low redshift.

Massive binary stars with relativistic companions: Studying donor winds with the HST

Hainich, Rainer

High-mass X-ray binaries (HMXBs) are unique massive star laboratories. These objects represent an advanced stage in the evolution of massive binary systems, after the initially more massive star has already collapsed in a supernova explosion, but its remnant, a neutron star or a black hole, remains gravitationally bound to the system. In the so-called wind-fed HMXBs, the stellar wind from the OB-type donor is partially accreted onto its compact companion, powering its high X-ray luminosity. Recently, the number of known Galactic wind-fed HMXBs was more than doubled by the unexpected discovery of a new type of objects: the supergiant fast X-ray transients with OB supergiant donors. It was suggested that the physics of these objects is governed by their donor's stellar winds. To correctly model the populations of relativistic binaries in galaxies, it is imperative to understand the reasons for the existence of different types of HMXBs with OB-type donors. But, up to now, only a few donor stars were analyzed by means of sophisticated stellar atmosphere models. Therefore, using the HST we performed a UV spectroscopic survey of donor stars, covering both the newly discovered as well as the classical HMXB systems. The UV spectra were complemented by optical and simultaneously obtained X-ray data. The analysis was performed using PoWR non-LTE stellar atmospheres. In this talk, I will present results of this survey, including an up-to-date overview of our knowledge about OB supergiant donors with relativistic companions.

On the nature of SFXTs

Negueruela, Ignacio

After more than a decade of investigation, it is clear that the differences between SFXTs and classical SGXBs cannot be due to any single parameter. Explanations based on orbital size, wind clumping or neutron star properties have been proposed, but none of them seems to provide the full answer. The X-ray properties of systems do not seem to show any clear divide between the two behaviours. Observation of the optical/infrared countarparts to SFXT can help us understand the source of some of these differences. I will present detailed modelling of the infrared counterpart to IGR J16479-4514, which has an orbital period of only 3.32 d, showing that it is a typical supergiant. The combination of stellar parameters and orbital size implies the need for a gating mechanism close to the neutron star.I will also present full characterisation for a number of optically visible counterparts. I will show that the existing population of counterparts, though not statistically significant, gives strong hints as to the reasons why some objects behave as SGXBs while others are SFXTs, as well as why there are intermediate objects.

The Cartwheel galaxy as a stepping stone for binaries' formation

Wolter, Anna

Ultraluminous X-ray sources (ULXs) are end points of stellar evolution.They are mostly interpreted as binary systems with a massive donor. They are also the most probable progenitors for BH-BH, and even more, for BH-NS coalescence. This last event has not been detected yet. Many recent estimates of expected rates (e.g. Inoue et al 2016) do take into consideration the ULX properties (Luminosity Function, active time, frequency by galaxy mass or SFR). However, many of these parameters are very uncertain or not known. Especially crucial is the link with the metallicity of the environment (e.g. Mapelli et al 2010) which has been invoked frequently but not proven strongly. We have tackled this problem by using a MUSE DEEP mosaic of the Cartwheel galaxy and applying a newly developed Monte Carlo code that jointly fits spectroscopy and photometry (Fossati et al. 2018).The Carthweel is the archetypal ring galaxy and the location and formation time of new stellar populations is easier to reconstruct than in more normal galaxies. It has the largest population of ULXs ever observed in a single galaxy (16 sources have been classified as ULXs in Chandra and XMM-Newton data).The Cartwheel galaxy is therefore the ideal laboratory to study the relation between Star Formation (SF Rates and SF History) and number of ULXs and also their final fate.We will show the derived SFHs and the formation timescale for three different zones of the galaxy, tracing the impact that created the ring. We aim at measuring the ionization properties and metallicity of the emitting gas at the positions of X-ray sources by constructing spatially resolved emission line ratio maps and BPT diagnostic maps.The findings for the Cartwheel will be a testbed for further modelization of binary formation and evolution paths.

Common Envelope Evolution of Massive Binaries

Ricker, Paul

The discovery via gravitational waves of binary black hole systems with total masses greater than 60M? has raised interesting questions for stellar evolution theory. Among the most promising formation channels for these systems is one involving a common envelope binary containing a low metallicity, core helium burning star with mass ~ 80 – 90M? and a black hole with mass ~ 30 – 40M?. For this channel to be viable, the common envelope binary must eject more than half the giant star's mass and reduce its orbital separation by as much as a factor of 80. I will discuss issues faced in numerically simulating the common envelope evolution of such systems and present a 3D AMR simulation of the dynamical inspiral of a low-metallicity red supergiant with a massive black hole companion.

Clumpy accretion in high mass X-ray binaries: the showcase Vela X-1

Grinberg, Victoria

The winds that the compact objects in high mass X-ray binaries accrete from are intrinsically highly structured, with colder, denser clumps embedded in tenuous hot gas. Understanding this clumpy winds and their interaction with the compact object is key to understanding the observed X-ray variability of HMXBs.Persistent, bright and eclipsing, the HMXB Vela X-1 offers the best opportunity to study accretion onto a neutron star and to disentangle the complex accretion geometry of these systems. Various lines of research have established the presence of large-scale accretion structures such as accretion and photoionization wakes that lead to highly variable absorption along the ~9-day orbit. However, at orbital phase 0.25, when our line of sight towards the source does not pass through the large-scale accretion structure, we observe strong changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars alone. In absorption-resolved high resolution X-ray spectra taken with Chandra-HETGS, we detect line features from high and low ionization species of multiple elements whose strengths and presence depend on the overall level of absorption. These features imply a co-existence of cool and hot gas phases in the system, which we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.  Our results show how indispensable time- and absorption resolved X-ray high resolution spectroscopy observations with today's and future instruments are for understanding the clumpy accretion environment in HMXBs.

The dark side of HMXB

Chaty, Sylvain

High Mass X-ray Binaries (HMXB) have been revealed by a wealth of multi-wavelength observations, from X-ray to optical and infrared domain. We will review here what these observations have brought to light concerning our knowledge of HMXB, and what part of HMXB still remains in the dark side.Intensive programs, including imaging, photometry, low and high resolution spectroscopy, stellar spectra modeling, spectral energy distribution (SED) fitting, timing and interferometry, have shown that properties of HMXB are mainly dictated by the nature of their massive host stars. Imaging and photometry allow us to identify various types of HMXB, low and high resolution spectroscopy, combined to stellar spectra modeling, lead us to derive accurate parameters of the companion star (interstellar absorption, metallicity, rotation, gravity, etc), SED fitting gives us information on intrinsic absorption and characteristics of circumstellar enveloppe, mid-infrared imaging allows us to explore the impact of these active stars on their environment, timing brings us orbital and spin periods, and finally interferometry opens the way to directly imaging the dust cocoon surrounding HMXB…The INTEGRAL satellite has launched the revival of HMXB studies, extending the population of supergiant HMXB (from only 5 in 1986 to 35 today), revealing previously unknown highly obscured and transient HMXB (so-called supergiant Fast X-ray Transients, SFXT). The first detections of gravitational waves has confirmed the interest of studying compact binaries hosting massive stars, the obscured HMXB being the precursors of common enveloppe systems.Many questions are still pending, related to the accretion processes, the wind properties in these massive and active stars, and the overall evolution due to transfer of mass and angular momentum between the companion star and the compact object. We will see how future observations should be able to answer to these questions, which constitute the dark side of HMXB.

MWC 656: the first HMXB containing a BH orbiting a Be star

Ribó, Marc

High Mass X-ray Binaries (HMXBs) have been found in a veriety of configurations depending on the donor and the accretor properties, as well as on the binary orbit parameters and mass transfer channel. Indeed, a significant fraction of HMXBs are composed of Be stars orbited by neutron stars (NSs). However, binary population synthesis models predicted the existence of Be stars orbited by black holes (BHs) as well, although their detection had been elusive until recently. After a possible gamma-ray flare detected by the AGILE satellite, the Be star MWC 656 was discovered to be a member of a binary system. Subsequent radial velocity studies revealed that the orbiting compact object is in fact a BH. X-ray observations showed the binary system to be in quiescence, allowing thus studies of quiescent BHs in HMXBs for the first time. Simultaneous X-ray/radio observations have allowed us to study the accretion/ejection coupling in BH HMXB for the first time. Our results show that the obtained X-ray/radio luminosities for this quiescent BH HMXB are fully compatible with those of the X-ray/radio correlations derived from quiescent BH low mass X-ray binaries. These results reveal that the accretion/ejection coupling in stellar-mass BHs is independent of the nature of the donor star and the mass transfer channel. We note that this is the first stellar-mass BH that has not been discovered by its prominent X-ray emission. Finally, studies of the formation and evolution of Be/BH systems have been conducted, revealing that evolved descendents of these systems in nearby galaxies can lead to detectable gravitational waves from NS-BH mergers. We will review all these discoveries related to the first Be/BH system MWC 656 and provide an outlook of the current observational efforts aimed at better understanding Be/BH systems.

Cyclic evolution of misaligned Be disks in Be/X-ray binaries and its observational implications

Okazaki, Atsuo

About a half of high-mass X-ray binaries has a Be star as the mass donor. These Be/X-ray binaries are mostly transient X-ray sources, because the binary orbit is generally eccentric and in addition the orientation of the circumstellar disk of the Be star is not always favorable for the compact object to capture disk gas around periastron. In order to understand the complicated, long-term X-ray behavior of Be/X-ray binaries, including the origin of giant X-ray outbursts, it is thus important to study the long-term evolution of Be-star disks, under the influence of the tidal force of the compact object.In this contribution, first I propose a mechanism for cyclic evolution of misaligned Be disks in Be/X-ray binaries, on the basis of 3D hydrodynamic simulations. Each evolutionary cycle consists of the following stages: 1) As the Be disk grows with time, the initially circular disk becomes eccentric by the Kozai-Lidov mechanism. 2) At some point, the tidal torque on the disk prevails against the torque via mass ejection from the central star, due either to the increase in the former or to the decrease in the latter. Then, the disk starts precession. 3) Thanks to precession, a gap opens between the disk base and mass ejection region of the central star. This allows the formation of a new disk in the stellar equatorial plane. 4) The newly formed circumstellar disk finally replaces the precessing old disk.Then, I discuss the observational features expected from such cyclic disk evolution.

Different generations of HMXBs: clues about their formation efficiency from Magellanic Clouds studies

Antoniou, Vallia

Nearby star-forming galaxies offer a unique environment to study the populations of young (<100 Myr) X-ray binaries, which consist of a compact object - typically a neutron star or a black hole - powered by accretion from a companion star. These systems are tracers of past populations of massive stars that heavily affect their immediate environment and parent galaxies. The SMC is the ideal environment for population studies of young X-ray binaries by providing us with what the Milky Way cannot: A complete sample of X-ray sources within a galaxy. Using a Chandra X-ray Visionary program, we investigate the young neutron-star binary population in this low-metallicity, nearby, star-forming galaxy by reaching quiescent X-ray luminosity levels (~few times 1032 erg/s). In this talk, I will present the first measurement of the formation efficiency of high-mass X-ray binaries (HMXBs) as a function of the age of their parent stellar populations. We use three indicators of the formation efficiency of young accreting binaries in the low SMC metallicity: the number ratio of the HMXBs, N(HMXBs), to the number of OB stars, to the star-formation rate (SFR), and to the stellar mass produced during the specific star-formation burst they are associated with, all as a function of the age of their parent stellar populations. In all cases, we find that the HMXB formation efficiency increases as a function of time up to ~40-60 Myr, and then gradually decreases. The peak formation efficiency N(HMXB)/SFR is in good agreement with previous estimates of the average formation efficiency in the broad ~20-60 Myr age range. I will also present the deepest luminosity function ever recorded for a galaxy, and discuss the X-ray properties of the largest sample of extragalactic accreting pulsars as well. Moreover, I will present the formation efficiency of HMXBs in the LMC based on the N(HMXBs)/SFR ratio, and discuss how these findings are compared to the lower metallicity environment of the SMC.

Is the redshift evolution of HMXBs driven by metallicity?

Fornasini, Francesca

In nearby galaxies, it is observed that the integrated X-ray luminosity (LX) of HMXB populations is correlated with the host galaxy’s star formation rate (SFR) due to the fact that HMXBs form shortly (~10 Myr) after an episode of star formation.  Recent X-ray studies of high-redshift galaxies indicate a positive evolution of this correlation with redshift. The higher LX/SFR of HMXBs at high redshift has been attributed to the formation of more luminous HMXB populations in lower metallicity (Z) environments, a trend predicted by population synthesis models.  While there is observational evidence that HMXB populations in nearby low-Z dwarf galaxies have enhanced LX/SFR, the correlation between LX, SFR, and Z is still poorly constrained and it remains to be proven that the redshift evolution of LX/SFR is driven by the Z-dependence of HMXBs.  Better constraints on the variations of HMXB luminosity with Z and redshift will improve our understanding of the likelihood that HMXBs in low-Z environments are progenitors of the heavy BH binaries discovered by GW observatories and of the contribution of HMXBs to the X-ray heating of intergalactic gas during the Epoch of Reionization.  We present the results of an X-ray stacking study of 120,000 star-forming galaxies between 0.1<z<5.0 in="" the="" chandra="" cosmos="" legacy="" survey.  comparing="" stacks="" to="" previous="" relations,="" we="" find="" that="" redshift="" evolution="" of="" lx="" sfr="" hmxbs="" is="" weaker.  we="" also="" present="" preliminary="" results="" about="" z-dependence="" at="" different="" redshifts="" based="" on="" x-ray="" stacking="" archival="" data="" and="" oxygen="" abundance="" measurements="" from="" sdss="" z~0.3,="" zcosmos="" z~0.7,="" mosdef="" survey="" z~2.  these="" studies="" will="" provide="" first="" direct="" tests="" connection="" between="" hmxbs.<="" p="">

Characterization of compact object populations in nearby galaxies

Zezas, Andreas

The demographics of compact objects is a key parameter for constraining models of massive stellar evolution and understanding the populations of gravitational wave progenitors and short gamma-ray bursts. NuSTAR has opened a new window in this field by enabling the characterization of the compact objects in X-ray binary systems in nearby galaxies.  We present a diagnostic tool for the classification of X-ray binaries on the basis of their compact objects and their accretion state. We apply this tool on NuSTAR observations of a sample of nearby galaxies spanning a wide range of specific star-formation rates, and hence star-formation environments. Our analysis indicates that more actively star-forming galaxies host a larger fraction of black-hole systems. Furthermore, we see a clear preference for accreting  pulsars to be associated with star-forming regions, in good agreement with similar studies in our Galaxy. Finally, we compare these results with predictions from population synthesis models for different star-formation scenarios. 

An eclipsing Wolf-Rayet ULX in the Circinus galaxy

Soria, Roberto

Ultraluminous X-ray sources with a Wolf-Rayet donor star are the most likely progenitors for LIGO merger events. They are accreting at a super-Eddington rate for ~1e5 yr, likely via Roche lobe overflow; after core collapse of the donor star, the system can turn into a double black hole binary with an initial orbital period of a few hours. By contrast, HMXBs with a supergiant or main sequence donor can also produce black hole-black hole binaries, but with binary separations that are too large for inspiral and merger within a Hubble time. Only a handful of ultraluminous Wolf-Rayet HMXBs have been identified so far. We will present the X-ray spectral and timing properties of the best-known example, located in the Circinus Galaxy. Its intriguing dipping and eclipse behaviour is different from the type of orbital modulations seen in other classes of HMXBs. We argue that such lightcurves are a defining property of this class of super-Eddington HMXBs, in which both the primary and the secondary launch dense, fast outflows with similar kinetic power. We suggest that the occulting material is dense shocked gas between black hole and donor star, and in a bow shock ahead of the black hole.

A new approach to binary population synthesis

Marchant, Pablo

In the last decade binary evolution has been recognized as a fundamental aspect in the evolution of massive stars, with more than half of them expected to undergo interaction with a companion. Current developments in the observations of transient events, X-ray binaries and gravitational wave detections require efficient and accurate methods to model stellar populations in order to understand their origin. Previous approaches to population synthesis have mostly relied on calculations of single star evolution and simple algorithms to account for binary interaction. Despite their flexibility and computational efficiency, these 'rapid' methods cannot precisely model important phases of evolution, such as thermal timescale mass-transfer, delayed dynamical instability, and partial stripping of stellar envelopes. In this talk I will present a new population synthesis code that relies on full calculations of binary evolution and interaction using the MESA stellar evolution code, together with a new implementation of common envelope evolution. This tool significantly enhances the accuracy of population synthesis models, while remaining computationally efficient to explore uncertainties such as birth kicks on compact objects. I will discuss the applications of this new tool to model different types of supernovae, X-ray binaries and merging compact objects.

High-mass X-ray binaries as progenitors of gravitational wave sources

Klencki, Jakub

The recent discovery of the first double neutron star (NS-NS) merger, if representative for the underlying population, indicates rates of such events as high as 320-4700 Gpc-3 yr-1.This challenges our understanding of the binary evolution, as models that predict high enough rates of NS-NS mergers overpredict those of double black holes (BH-BH).One of the major open questions in the formation scenario of compact binary mergers from isolated binaries is what are the stability criteria for the mass transfer during a high-mass X-ray binary (HMXB) phase. Recent simulations suggest that, in the case of black hole accretors, the mass transfer from massive donors may remain stable in a wider range of parameters than previously thought, thus avoiding a common envelope evolution, and giving rise to a population of Roche-lobe overflowing (RLOF) HMXBs with supercritical mass transfer rates. Those of such HMXBs that fall within the optimal range of periods will eventually become close BH-BH binaries that are going to spiral in and merge.We present a method for reconstructing the evolution of massive binaries based on the information obtained from the gravitational wave detections. In particular, we infer about the population of HMXBs that form BH-BH mergers through a phase of stable mass transfer. We also discuss the connection between the mass transfer stability criteria and the high-luminosity end of the observed X-ray luminosity function.

Populations of HMXBs in the Milky Way and the Magellanic Clouds

Haberl, Frank

The study of X-ray source populations in nearby galaxies is of major importance in understanding the X-ray output of more distant galaxies as well as learning about processes that occur on interstellar scales within our own Galaxy. In comparison with the Milky Way, the Small Magellanic Cloud harbours an extraordinary large and different population of high mass X-ray binaries (HMXBs). A most recent compilation identified about 120 HMXBs with Be-type companion star, while only one supergiant system is known. Correlations with the star formation history suggest that this is caused by a star formation event about 40 My ago. The large sample allows to investigate various statistical properties and to find interesting individual cases. In this talk I'll focus on the X-ray properties of the HMXB populations in the Magellanic Clouds in comparison to the Milky Way.

The Very Young Be-Star X-ray Binary Circinus X-1

Schulz, Norbert

Accretion conditions and morphologies of X-ray transients containing neutron starsare still poorly understood. Circinus X-1 is a specifically enigmatic case where we observe X-ray flux changes covering four orders of magnitude. We observed Circinus X-1 many times since the launch of the Chandra X-ray Observatoryusing the high energy transmission grating spectrometer andeach time the source gave us a vastly different look. Most recently we caught the source at its very lowest X-ray flux at a flux of 1.8×10-11 erg cm-2 s-1.Its spectrum, a single 1.7 keV blackbody spectrum, showed a low emission radius of 0.4 kmwhich implies a high magnetic field between 1.7 and 4.8×1011 G depending on neutron star radius. Photoionized line emissions suggesta large emission volume and low plasma densities. The observed bluehifts of ~400 km s-1and emission volume is consistent with the ionized but distorted wind of a B5Ia supergaint companion confirming a previous identification. We argue that the companion of Cir X-1 isfast rotating Be-star and its stellar disk provides much of the                   observed excess column densities. We paint a scenario in which a precessing oblateBe-star rotator may explain the vast X-ray flux variations in the past.

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