PDF hosted at the Radboud Repository of the Radboud University Nijmegen The following full text is a publisher's version. For additional information about this publication click this link. http://hdl.handle.net/2066/101517 Please be advised that this information was generated on 2021-09-26 and may be subject to change. ULTRACOMPACT X-RAY BINARY STARS LENNART VAN HAAFTEN Copyright c 2013 by Lennart van Haaften Ultracompact X-ray binary stars PhD thesis, Radboud University Nijmegen Illustrated; with bibliographic information and Dutch summary ISBN: 978-90-9027332-7 Cover: Milky Way panorama, with the Galactic Bulge on the front. Credit: ESO/S. Brunier Typeset using LATEX 2e Printed in the Netherlands ULTRACOMPACT X-RAY BINARY STARS PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Radboud Universiteit Nijmegen op gezag van de rector magnificus prof. mr. S. C. J. J. Kortmann, volgens besluit van het College van Decanen in het openbaar te verdedigen op dinsdag 12 maart 2013 om 10.30 uur precies door LENNART MARIUS VAN HAAFTEN geboren op 22 mei 1980 te Sleeuwijk PROMOTORES: Prof. dr. G. Nelemans Prof. dr. P. J. Groot COPROMOTOR: Dr. R. Voss MANUSCRIPTCOMMISSIE: Prof. dr. W. J. van der Zande Dr. E. G. Körding Prof. dr. S. F. Portegies Zwart Universiteit Leiden Dr. T. M. Tauris Universität Bonn Prof. dr. F. W. M. Verbunt This research was supported by the Netherlands Organisation for Scientific Research (NWO). To my ancestors, especially my parents CONTENTS 1 Introduction1 1.1 The Universe.............................1 1.2 Galaxies................................2 1.3 Stars..................................3 1.3.1 Massive stars.........................3 1.3.2 Intermediate-mass and low-mass stars............5 1.4 Binary stars..............................6 1.4.1 Roche lobes..........................6 1.4.2 Roche-lobe overflow.....................7 1.4.3 Accretion...........................9 1.5 X-ray binaries.............................9 1.6 Ultracompact X-ray binaries..................... 11 1.6.1 Known population...................... 13 1.6.2 Importance.......................... 14 1.7 Millisecond pulsars.......................... 15 1.8 Population synthesis......................... 17 1.9 This thesis............................... 17 1.9.1 Unsolved problems...................... 17 1.9.2 Approach........................... 18 1.9.3 Overview of scales...................... 20 i CONTENTS 2 The evolution of ultracompact X-ray binaries 23 2.1 Introduction.............................. 24 2.1.1 Evolutionary history..................... 25 2.1.2 Present research....................... 26 2.2 Overview of UCXB orbital mechanics................ 27 2.2.1 Angular momentum flows.................. 27 2.2.2 Stable mass transfer..................... 28 2.2.3 Dynamical stability...................... 30 2.2.4 Isotropic re-emission..................... 31 2.2.5 The deciding stability criterion at the onset of mass transfer 32 2.2.6 Complications........................ 32 2.3 Method................................ 36 2.3.1 Isotropic re-emission..................... 36 2.3.2 Response of Roche-lobe radius to mass transfer....... 37 2.3.3 White dwarf radius...................... 41 2.3.4 Propeller effect........................ 42 2.3.5 Disk Instability Model.................... 44 2.4 Results................................. 45 2.4.1 Feedback of angular momentum............... 45 2.4.2 The evolution of ultracompact X-ray binaries........ 47 2.4.3 System evolution during the propeller phase......... 56 2.5 Discussion and conclusions...................... 64 2.A Fitted tracks.............................. 66 2.B Approximate analytic solution.................... 68 2.B.1 Luminosity.......................... 70 2.C First Lagrangian point location approximations........... 70 2.D Roche-lobe potential approximations................. 71 3 Formation of the planet around the millisecond pulsar J1719–1438 73 3.1 Introduction.............................. 74 3.2 Problems with the standard UCXB scenario............. 75 3.3 Possible resolutions.......................... 79 3.3.1 Bloated donor scenario.................... 80 3.3.2 Additional angular momentum loss scenario......... 80 3.3.3 Detachment at very low donor mass............. 81 3.3.4 Detachment due to thermal-viscous disk instability..... 82 3.4 Discussion and conclusions...................... 83 ii CONTENTS 4 Long-term luminosity behavior of 14 ultracompact X-ray binaries 85 4.1 Introduction.............................. 86 4.1.1 Present research....................... 87 4.2 Method................................ 87 4.2.1 Known UCXB population.................. 87 4.2.2 Observations......................... 90 4.2.3 Data analysis......................... 90 4.2.4 Theoretical evolution..................... 94 4.3 Results................................. 95 4.3.1 Signal-to-noise threshold................... 95 4.3.2 Average luminosity...................... 97 4.3.3 Faint-end power-law function................ 99 4.4 Implications for population studies.................. 100 4.5 Discussion and conclusions...................... 101 5 Population synthesis of ultracompact X-ray binaries in the Galactic Bulge105 5.1 Introduction.............................. 106 5.2 Method................................ 107 5.2.1 Population synthesis..................... 108 5.2.2 Star formation history and initial binary parameters..... 108 5.2.3 Formation scenarios..................... 110 5.2.4 Behavior of old UCXBs................... 118 5.2.5 Present-day population.................... 118 5.3 Results................................. 123 5.3.1 Birth rates and total number of systems........... 123 5.3.2 Present-day population.................... 124 5.3.3 Observable population.................... 127 5.3.4 Donor surface composition.................. 132 5.3.5 Collective emission as function of orbital period...... 133 5.4 Discussion............................... 134 5.4.1 Comparison with previous studies.............. 137 5.4.2 Overprediction of UCXBs with long orbital period..... 138 5.4.3 Overprediction of UCXBs with short orbital period..... 140 5.4.4 Consequences for the population of millisecond radio pulsars 141 5.5 Summary and conclusions...................... 142 5.A Binary initial mass function and normalization of the simulation.. 144 iii CONTENTS 6 Population synthesis of classical low-mass X-ray binaries in the Galactic Bulge 147 6.1 Introduction.............................. 148 6.2 Method................................ 149 6.3 Results................................. 152 6.3.1 The total population of LMXBs............... 152 6.3.2 The observable population of LMXBs............ 155 6.4 Discussion............................... 156 6.4.1 Comparison with the population of UCXBs......... 160 6.5 Conclusions.............................. 163 7 Summary and conclusions 167 7.1 Prospects............................... 172 8 Samenvatting in het Nederlands 175 8.1 De levensloop van sterren....................... 175 8.2 Dubbelsterren en massa-overdracht.................. 178 8.3 Ultracompacte röntgendubbelsterren................. 179 8.4 Mijn proefschrift........................... 181 Bibliography 185 List of publications 205 Acknowledgments 207 Curriculum vitae 209 iv CHAPTER 1 INTRODUCTION This thesis aims to increase our knowledge of ultracompact X-ray binaries, a class of close binary star systems that are bright sources of X-ray radiation. This introduction provides background information about astronomy in general, and the topics dis- cussed in this thesis. A discussion of the unsolved problems related to ultracompact X-ray binaries is given, followed by an outline of the thesis. 1.1 THE UNIVERSE By terrestrial standards, the Universe is an extremely empty and dark place. From almost any location in the Universe there are no celestial objects such as stars or even galaxies to be seen (see e.g. Einasto et al. 1994). Even though the Universe started in a very homogeneous state, after 13:75 billion years of expansion and gravitational in- teraction, matter is now distributed highly inhomogeneously. Astronomers, by virtue of being material themselves, are located in one of the small, bright, matter-rich parts of the Universe. Even with the naked eye one can see several thousands of stars, as well as a handful of planets and galaxies. These are the objects traditionally studied by astronomers, but with the arrival of the optical telescope, and later telescopes and detectors sensitive to other electromagnetic radiation and particles, more and more objects and phenomena could be observed and subsequently explained theoretically. Precise observations have revealed that the type of matter we are familiar with (and of which the Sun, the Earth, and humans consist) occupies only about five per- cent of the content of the Universe (Hinshaw et al. 2012). The remaining part consists of the enigmatic ‘dark energy’, which is distributed homogeneously throughout the 1 CHAPTER 1 : INTRODUCTION Universe, and the also mysterious and probably unrelated ‘dark matter’, which is clustered in roughly the same way as normal (baryonic) matter. Another unknown is whether the Universe has a finite or infinite volume. Astronomers can only observe the part of the Universe from which light reaches us today, the ‘observable Universe’, and the Universe as a whole may be much larger. Either way, the Universe is expected to have no spatial boundaries. As for boundaries in time, even though the Universe had a beginning, the Big Bang,