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Observational Studies of X-Ray Binary Systems Observational Studies of X-Ray Binary Systems mmnML mmmm AWD DE» FIOWS INIS-mf—8690 "'•'..'..-. !' ..- -'. .^: C-^;/- V'-^ ^ ^'vV?*f,>4 "' :f ,.?;•• ..f..-XV., . '4*.> • OBSERVATIONAL STUDIES OF X-RAY BINARY SYSTEMS OBSERVATIONAL STUDIES OF X-RAY BINARY SYSTEMS PROPERTIES OF THE NEUTRON STAR AND ITS COMPANION ORBITAL MOTION AND MASS FLOWS ACADEMISCH PROEFSCHRIFT ter verkrijging van de graad van doctor in de Wiskunde en Natuurwetenschappen aan de Universiteit van Amsterdam, op gezag van de Rector Magnificus, Dr. D.W. Bresters, hoogleraar in de Faculteit der Wiskunde en Natuurwetenschappen, in het openbaar te verdedigen in de Aula der Universiteit (tijdelijk in de Lutherse Kerk, ingang Singel 411, hoek Spui) op woensdag 2 februari 1983 te 13:30 uur door Michiel Baidur Maximiliaan van der Klis geboren te 's Gravenhage PROMOTOR : Prof. Dr. E.P.J, van den Heuvel CO-REFERENT : Dr. Ir. J.A.H. Bieeker CONTENTS 0. Introduction and summary 7 1. Mass-Flux Induced Orbital-Period Changes In X-Ray Binaries 17 2. Cygnus X-3 2.1 A Change In Light Curve Asymmetry and the Ephemeris of Cygnus X-3 31 2.2 The X-Ray Modulation of Cygnus X-3 34 2.3 The Cycle-to-Cycle Variability of Cygnus X-3 40 3. Mass Transfer and Accretion in Centaurus X-3 and SMC X-l 3.1 The Accretion Picture of Cen X-3 as Inferred from One Month of Continuous X-Ray Observations 55 3.2 Long Term X-Ray Observations of SMC X-l Including a Turn-On 62 4. Pulsation and Orbital Parameters of Centaurus X-3 and Vela X-l 4.1 Characteristics of the Cen X-3 Neutron Star from Correlated Spin-Up and X-Ray Luminosity Measurements 69 4.2 The Orbital Parameters and the X-Ray Pulsation of Vela X-l (4U 0900-40) 76 5. Ultraviolet and Optical Observations 5.1 Ultraviolet Observations of LMC X-4 and SMC X-l 101 5.2 A Study of Ultraviolet Spectroscopic and Light Variations in the X-Ray Binaries LMC X-4 and SMC X-l 104 5.3 Photometric and Spectroscopic Observations of an Optical Candidate for the X-Ray Source H0544-665 110 6. Low Mass X-Ray Binaries 6.1 The X-Ray Flux Variations of Cygnus X-2 117 6.2 X-Ray Observations of Bright Galactic Bulge Sources in the Vicinity of GX 5-1 122 Nederlandse samenvatting 131 Dankwoord 135 INTRODUCTION AND SUMMARY 0. INTRODUCTION AND SUMMARY All stars in the galaxy probably emit X-rays1. The nearest star, for example, our sun, is the apparently brightest X-ray source in the sky2. In most of these ~ 10*° sources, the X-ray emission comprises only a «rtnute fraction of the total energy output, which is mainly in the optical region. Large X-ray fluxes must be generated in the high-temperature interiors cf the stars, where the nuclear burning takes place. The thick layers of plasma surrounding the stellar core, however, do not allow any of these X-rays to escape from the stellar surface. Only by secondary processes, like the formation of a hot corona, do stars manage to emit X-roys at all. The X-ray sources which I wish to discuss here are many orders of magnitude more luminous than normal stars are in the X-ray band. As we will see, they usually act as signposts indicating small, very energetic objects, which are totally unlike normal stars. 1. Galactic X-ray sources The history of X-ray observations now extends over more than 20 years, from the first observation of an X-ray source outside the solar system3 via the epochal Uhuru survey1* in 1972-'73 to the recent HEAO 2 experiment5. These observations have provided us with a sample of X-ray sources which is approximately complete from the brightest steady sources, typically ~ 103 pjy, down to X-ray intensities of ~ 2 μJy in the 2-10 keV band1*'6. Excluding those which are probably extragalactic objects, there are roughly 200-300 of these sources in the sky. Their sky distribution peaks strongly at the galactic plane, which shows that the great majority of them belongs to our galaxy (about 10 such sources have been detected in the Magellanic Clouds). The X-ray flux we receive from them, together with the fact that these sources form a galactic population, imply that their X-ray luminosities are in the range 10 -103° erg s"1. The observed X-ray spectra suggest in many cases, that the X-rays are produced in a hot plasma, with a temperature of 10 to 10 K. Depending on the precise X-ray emission mechanism, the emitting region can be quite small, of the order of a few kilometers. This implies non- transient emissivities of more than 10 erg s cm, by far the highest we observe in nature9. (By comparison, the most luminous normal stars (giants and supergiants of class I—III) emit as much energy in the'optical region as these sources do in the X-ray band, but their radius is closer to 107 km. Even in their cores, these stars produce at most 106 erg s"1 cm"3.) 2. The Binary Model7'8 At an early stage in the study of X-ray sources, a model was proposed which involves the transfer of matter in a binary system to a compact star (a white dwarf, a neutron star or a black hole). In this model, matter is removed from the outer layers of the non-compact component of the binary by the tidal forces exerted by the nearby compact star. This process is called Roche-lobe overflow. Because it has considerable angular momentum with respect to the compact star (originating in the binary orbital motion), the matter first settles In a disk around it. Spiralling down towards the compact star, the matter gets rid of its surplus angular momentum by transporting it to the outer edge of the disk by means of viscous forces. Finally, it falls down the deep gravitational potential well of the compact star. In this accretion process, large amounts of gravitational energy are released, a considerable part of which is emitted as thermal X-rays. A similar model, in which the mass transfer is not caused by Roche lobe overflow, but Instead originates in a stellar wind emanating from a hot, massive star, was proposed at a later time. In systems of this type, an accretion disk does not necessarily form. 3. Binaries emitting X-rays The above-mentioned ideas have been confirmed by the observations. Of the 82 point-like X-ray sources in the current sample of bright galactic sources, which have been identified with an optical object, 74 are very probably binaries containing a compact star9. An additional 30 bright sources, which have not been optically identified, are also believed to be binaries. Many of these sources lie in the region of the galactic bulge, within 40* of the center of the galaxy, and their detection by optical means is probably prevented by obscuring gas and dust between us and this region. The low end of the X-ray luminosity distribution of these 74 sources 1s occupied by the so-called cataclysmic variables (CV's), systems containing a white dwarf. Many CV's are known as X-ray sources11, but probably only 6 of them belong to our sample of sources brighter than 2 tfy9. The remaining 68 systems, which contain a neutron star, or, in rare cases, perhaps a black hole, form the jjroup of the X-ray binaries. These systems are the most luminous X-ray sources in the galaxy. 4. X-ray binaries X-ray binaries12'13 divide naturally into two two broad categories: massive X-ray binaries, In which the mass of the non-compact companion to the X-ray source is typically more than 10 Mg, and low-mass X-ray binaries, characterized by a companion with a mass below 1 MG. There 1s only one intermediate case known: Hercules X-l, which contains a 2 Mo companion. I will now discuss the main characteristics of these two classes of X-ray binaries in turn. At the end of this section, I will describe the source Cygnus X-3, which is unlike any other X-ray binary. a) Massive X-ray binaries Massive X-ray binaries contain hot, massive stars of spectral type 0 and B, which show clear evidence in their optical and UV spectra of mass loss in a stellar wind. These early- type stars have short lifetimes (< 10 y)12, so we expect massive X-ray binaries to be young systems. This is confirmed by their strong concentration towards the galactic plane, where regions of star formation are also concentrated. Two groups of massive X-ray binaries can be distinguished: 'close' systems, with orbital periods typically less than 10 d, and the so-called Be transients, which have orbital periods of more than - 15 d. In the close systems, the orbit of the neutron star (which has a radius of ~ 10 km) is not far above the surface of the companion (R = 1 to 2 10 km). The optical flux of the system varies periodically as the tidally deformed optical star rotates. We often observe eclipses of the X-ray source by the companion in these systems. In a Be transient, the X-ray source follows an orbit which is wide relative to the 0.5 to 1 10 km radius of the companion. Strong X-rays are only produced when the Be companion star undergoes an episode of rapid mass loss, which phenomenon Is known to occur In Be stars11».
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