Ultraluminous X-ray Sources (ULXs) are, as suggested by their name, extremely luminous and rare X-ray emitting objects found in galaxies. Because of their luminosity, it has been suggested that they may be powered by accretion onto a black hole (BH) of a few 100 M⊙, more massive than what one would expect to originate from normal stellar evolution. Alternative models include young supernova remnants (SNRs) beamed emission from normal BH X-ray binaries (XRB) with high accretion rates, and relativistically beamed XRB omission. The observational evidence on ULXs suggests that while most of them are likely to be compact accreting objects, there is no clear unique evidence pointing either to the beamed XRB model or to accretion onto a very massive BH. It is possible that what we call ULXs are a heterogeneous family of X-ray sources.

SS433 is the prototype microquasar in the Galaxy and may even be analogous to the ULX sources if the jets kinetic energy is taken into account. However, in spite of 20 years of study, our constraints on the natur of the binary system are extremely limited as a result of the difficulty of locating spectral features that can reveal the nature and motion of the mass donor. Newly acquired, high resolution blue spectra taken when the (precessing) disc is edge-on suggest that the binary is close to a common-envelope phase, and hence providing kinematic constraints is extremely difficult. Nevertheless, we do find evidence for a massive donor, as expected for the inferred very high mass transfer rate, and we compare SS433's properties with those of Cyg X-3.

We present the X-ray luminosity function (XLF) of the Antennae galaxies based on 8 observation performed with Chandra. We detect between 37 and 49 sources in each observation. After combining all observations we detect a total of 120 sources down to a limiting luminosity of ~ 2 x 1037 erg s–1. Comparison between the XLFs of the individual observations shows that they are not affected by source variability. The cumulative XLF of the coadded observations is well represented by a single power-law with a slope of ~ –0.5.

The population of massive X-ray binaries in a galaxy can provide important constraints on its recent history of star formation. However, further knowledge of the Galactic population and formation mechanisms is needed before deeper inferences can be made for other galaxies.

We have studied the peculiar environment around a ULX in NGC 4559 (with Lx ≈ 2 x 1040 erg s–1 and MBH ≳ 50M⊙). The X-ray source is located near the rim of a young (age < 30 Myr), large (diameter ≈ 700 pc) ring-like star forming complex possibly triggered by the impact of a dwarf satellite galaxy through the gas-rich outer disk of NGC 4559. We speculate that galaxy interactions (including the infall of high-velocity clouds and satellites on a galactic disk) and low-metallicity environments offer favourable conditions for the formation of compact remnants more massive than “standard” X-ray binaries, and accreting from a massive Roche-lobe filling companion.

I present, very high resolution hydrodynamical simulations of accretion discs in black hole X-ray binaries accreting near the Eddington limit, including the effects of disc irradiation and mass loss. I show that the extreme variability displayed by sources such as GRS 1915+105 can be explained within the framework of an outburst in a partially irradiated, turbulent, disc.

We present the first results of simultaneous INTEGRAL and RXTE observations of the microquasar GRS 1915+105. We focus on the analysis of the unique highly variable observation and show that we might have observed a new class of variability. We then study the energetic depenelence of a low frequency QPO from our steady observations.

The X-ray source IGR J16318-4848 was the first source discovered by INTEGRAL on 2003, January 29. We carried out optical and near-infrared (NIR) observations at the European Southern Observatory (ESO La Silla) in the course of a Target of Opportunity (ToO) programme. We discovered the optical counterpart and confirmed an already proposed NIR candidate. NIR spectroscopy revealed a large amount of emission lines, including forbidden iron lines and P-Cygni profiles. The spectral energy distribution of the source points towards a high luminosity and a high temperature, with an absorption greater than the interstellar absorption, but two orders of magnitude lower than the X-ray absorption. We show that the source is an High Mass X-ray binary (HMXB) at a distance between ~ 1 and ~ 6 kpc, the mass donor being an early-type star, probably a sgB[e] star, surrounded by a rich and absorbing circumstellar material. This would make the second High Mass X-ray Binary (HMXB) with a sgB[e] star after CI Cam, indicating that a new class of strongly absorbed X-ray binaries is being unveiled by INTEGRAL.

In dense stellar systems the frequent dynamical interactions between stars play a crucial role in the formation and evolution of compact binaries. We study these processes using a novel approach combining a state-of-the- art binary population synthesis code with a simple treatment of dynamical interactions in dense star cluster cores. Here we focus on the dynamical and evolutionary processes leading to the formation of compact binaries containing white dwarfs in dense globular clusters. We demonstrate that dynamics can increase by factors ~ 2 – 100 the production rates of interesting binaries such as cataclysmic variables, “nonflickerers” (He white dwarfs with a heavier dark companion), merging white dwarf binaries with total masses above the Chandrasekhar limit, and white dwarf binaries emitting gravitational waves in the LISA band.

We have devised a new technique for identifying stellar mass black holes in low mass X-ray binaries, and have applied it to XMM-Newton observations of two X-ray sources in M31. In particular we search for low accretion rate power density spectra; these are very similar for all LMXB, whether the primary is a black hole or a neutron star. Galactic neutron star LMXB exhibit these distinctive PDS at very low luminosities (~ 1036 erg s–1) while black hole LMXB can exhibit them at luminosities > 1038 erg s–1! Following the work of van der Klis (1994), we assume a maximum accretion rate (as a fraction of the Eddington limit) for low accretion rate PDS that is constant for all LMXB, and obtain an empirical value of ~10% Eddington. We have so far discovered two candidate black hole binaries in M31, exhibiting low accretion rate PDS at up to 3 x 1038 and 5 x 1037 erg s–1. If we assume that they are at <10% Eddington, they have minimum masses of 20 and 4 M⊙ respectively. Furthermore, any LMXB exhibiting a low accretion rate power density spectrum at a luminosity > 5 x 1037 erg s–1 is likely to have a black hole primary.

We summarize the key observations made in recent observations of X-ray sources in early-type galaxies. Typically about half of the X-ray binaries in early-type galaxies are in globular clusters, they are preferentially found in metal rich globular clusters, and there is no indication that cluster-age is an important parameter. Theoretical challenges are presented by these results.

Globular clusters (GCs) harbour a large number of close binaries which are hard to identify optically due to high stellar densities. Observing these GCs in X-rays, in which the compact binaries are bright, diminishes the over-crowding problem. Using the new generation of X-ray observatories, it is possible to identify populations of neutron star low mass X-ray binaries, cataclysmic variables and millisecond pulsars as well as other types of binaries. We present the spectra of a variety of binaries that we have identified in four GCs observed by XMM-Newton. We show that through population studies we can begin to understand the formation of individual classes of binaries in GCs and hence start to unfold the complex evolutionary paths of these systems.

Far-ultraviolet (FUV) observations with the Hubble Space Telescope are an excellent way to find and study the hot, blue stellar populations in the cores of globular clusters. These populations include dynamically formed blue stragglers and interacting binaries (such as cataclysmic variables, i.e. the products of stellar collisions and near misses. Using the cluster 47 Tuc as an example, we show how the combination of FUV imaging and slitless spectroscopy can be used to uncover and study these populations.

Over the last couple of years we carried out long-term CCD monitoring of open clusters and associations with as principal goal the detection and study of pulsating stars. A by-product of this project is the serendipitous collection of multicolour photometry of most interesting compact binaries which show signs of activity, viz. variable periods and strongly variable light curves. This presentation illustrates the project, and concentrates on intriguing species of interacting binaries in NGC 6231.

The stellar evolutionary processes responsible for the formation of compact objects in interacting binary systems and their evolution are described. The common envelope phase plays a crucial role in their formation and angular momentum losses associated with magnetic braking and/or mass loss are important for their evolution. An application of these processes provides the evolutionary link between classes of interacting binary systems.

We give an overview of the work that has been done in Brussels since 1997 on various aspects of population studies and the effects of binaries: stellar populations and supernova rates in regions where star formation is continuous and in starbursts, the expected spectral evolution of starburst regions, the chemical evolution of galaxies.

Year to decade-long cyclic period changes have been observed in many classes of close binaries. The Algol binary WW Cygni shows a cyclic change in its orbital period with an amplitude of slightly more than 0.02 days and a period of 56 years. A hypothetical third or fourth body does not satisfactorily explain the observed variation in the orbital period. The change in luminosity and color of the system at primary eclipse minimum are in agreement with the model proposed by Applegate for a magnetic cycle induced period change in WW Cygni. We have commenced monitoring 9 close binaries for evidence of the luminosity and color changes consistent with the magnetic cycle hypothesis. δ Librae is suggested as a case suitable for observation with an optical interferometer to test the third body proposed for this Algol system.

The present-day formation of cataclysmic variables (CVs) with brown dwarf (BD) secondaries is investigated using a population synthesis technique. Results from the latest, detailed models for BDs have been incorporated into the population synthesis code. We find that zero-age CVs (ZACVs) with BD secondaries have orbital periods in the range 46 min to 2.5 hrs, and that they comprise 18% of the total, present-day ZACV population. Consequently, we find that 15% of the present-day ZACV population should have orbital periods shorter than the observed orbital period minimum for CVs. We also investigate the dependence of the present-day formation rate of CVs with BD secondaries on the assumed value of the common envelope efficiency parameter, αCE, for three different assumed mass ratio distributions in ZAMS binaries. We find that the common envelope process must be extremely inefficient (αCE < 0.1) in order for CVs with BD secondaries not to be formed. Finally, we find that the progenitor binaries of ZACVs with BD secondaries have ZAMS orbital separations < 3 AU and ZAMS primary masses between ~1-10 M⊙ , with ~75% of the primary masses less than ~ 1.6 M⊙. Interestingly, these ranges in orbital separation and primary mass place the majority of the progenitor binaries within the so-called “brown dwarf desert.”

The Mira AB interacting binary provides an ideal laboratory for studying mass loss and accretion processes in systems containing an AGB star and a compact accretor, because its components have been resolved and can be studied individually. We present here results from our long term study of accretion processes in this system. Those include recent HST observations suggesting that the accretion rate onto Mira B is much lower than before, indicating possible disruption of the accretion disk.

We present the first realistic set of arbitrarily degenerate stellar models appropriate for donors in ultracompact binaries. The mass-radius M-R relations for these objects are parameterized by the donor's entropy and composition. We highlight how this fact can affect our understanding on the current configuration of ultracompact binaries today and their potential evolutionary tracks.