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ANNALES UNIVERSITATIS TURKUENSIS UNIVERSITATIS ANNALES AI 637 AI Tuomo Salmi Tuomo MASS AND RADIUS CONSTRAINTS FOR NEUTRON STARS USING X-RAY TIMING, SPECTRAL, AND POLARIZATION OBSERVATIONS Tuomo Salmi Painosalama Oy, Turku, Finland 2020 Finland Turku, Oy, Painosalama ISBN 978-951-29-8277-6 (PRINT) – ISBN 978-951-29-8278-3 (PDF) TURUN YLIOPISTON JULKAISUJA ANNALES UNIVERSITATIS TURKUENSIS ISSN 0082-7002 (Print) SARJA – SER. AI OSA – TOM. 637 | ASTRONOMICA – CHEMICA – PHYSICA – MATHEMATICA | TURKU 2020 ISSN 2343-3175 (Online) MASS AND RADIUS CONSTRAINTS FOR NEUTRON STARS USING X-RAY TIMING, SPECTRAL, AND POLARIZATION OBSERVATIONS Tuomo Salmi TURUN YLIOPISTON JULKAISUJA – ANNALES UNIVERSITATIS TURKUENSIS SARJA – SER. AI OSA – TOM. 637 | ASTRONOMICA – CHEMICA – PHYSICA – MATHEMATICA | TURKU 2020 University of Turku Faculty of Science and Engineering Department of Physics and Astronomy Astronomy Doctoral Programme in Physical and Chemical Sciences Supervised by Prof. Juri Poutanen Docent Sergey Tsygankov Department of Physics and Astronomy Department of Physics and Astronomy University of Turku University of Turku Turku, Finland Turku, Finland PhD Joonas Nättilä Physics Dept. and Astrophysics Lab. Columbia University & Flatiron Institute New York, USA Reviewed by Prof. Denis Leahy Prof. Stephan Rosswog Department of Physics and Astronomy Astronomy and Oskar Klein Centre University of Calgary Stockholm University Calgary, Canada Stockholm, Sweden Opponent Prof. Marat Gilfanov Max Planck Institute for Astrophysics & Space Research Institute Garching, Germany & Moscow, Russia The originality of this publication has been checked in accordance with the University of Turku quality assurance system using the Turnitin OriginalityCheck service. ISBN 978-951-29-8277-6 (PRINT) ISBN 978-951-29-8278-3 (PDF) ISSN 0082-7002 (Print) ISSN 2343-3175 (Online) Painosalama Oy - Turku, Finland 2020 Acknowledgements First, I would like to thank my supervisors Prof. Juri Poutanen, Drs. Joonas N¨attil¨aand Sergey Tsygankov. Their guidance was crucial in con- ducting the research, writing the thesis, and getting practised in the aca- demic life. The journey, started as a master’s project, turned out to be a pleasant and still continuing experience. I would also like to thank Profs. Stephan Rosswog and Denis Leahy for their profound pre-examination of this thesis. In addition, I gratefully acknowledge Prof. Marat Gilfanov for agreeing to become my opponent at the thesis defence. I also thank the University of Turku Graduate School and Magnus Ehrnrooth foundation for theirfi nancial support of my PhD thesis work. Besides my supervisors, many collaborators have strongly influenced the work done in this thesis. First of all, I thank Valery Suleimanov, who contributed substantially to the research and kindly hosted my visits to the University of T¨ubingen. I am also grateful for Joonas and the other people at Nordita for my useful Visiting PhD Fellowship. Furthermore, I thank all the other collaborators, including Vlad Loktev, Karri Korsman, Luca Baldini, Cole Miller, Andrew Steiner, Anna Watts, and Pauli Pihajoki, to name a few. I would also like to give my deep appreciation for the high-energy as- trophysics group of University of Turku. Besides those already mentioned, I thank Alexandra, Andrei, Anna, Armin, Juhani, Ilia, Pavel, Vadim, and Yasir. I would also like to extend my gratitude for all the other (current and previous) members of Tuorla Observatory, who created an inspiring and supportive working atmosphere. Especially, I thank the Tuorla Board Game Club and Kalle for organizing the exciting game events. Finally, I would like to thank my family and friends for all their support. Most importantly, I thank my wife Laura and daughter Ella for their love and encouragement. 3 Contents Acknowledgements 3 Abstract 7 Tiivistelm¨a 9 List of publications 11 List of abbreviations 13 1 Introduction 15 1.1 Neutron stars . 15 1.2 Equation of state . 16 1.3 Millisecond pulsars . 18 1.3.1 Accretion-powered millisecond pulsars . 18 1.3.2 Rotation-powered millisecond pulsars . 19 2 Pulse profile modelling of neutron stars 21 2.1 Geometry of the star . 22 2.2 Light bending and relativity . 25 2.3 Observedfl ux................... 28 2.4 Mass and radius constraints . 29 3 Modelling the atmosphere of a neutron star 33 3.1 Radiative transfer . 33 3.2 Compton scattering . 35 3.3 Atmosphere models . 36 4 X-ray polarimetry constraining EoS of dense matter 41 5 5 Summary of the original publications 45 5.1 Paper I – Bayesian parameter constraints for NS masses and radii using X-ray timing observations of accretion-powered millisecond pulsars . 45 5.2 Paper II – Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars . 46 5.3 Paper III – Magnetospheric return-current-heated atmospheres of rotation-powered millisecond pulsars . 46 5.4 Paper IV – Oblate Schwarzschild approximation for polar- ized radiation from rapidly rotating neutron stars . 47 5.5 Paper V – Neutron star parameter constraints for accretion- powered millisecond pulsars from the simulated IXPE data 47 5.6 The author’s contribution to the publications . 48 6 Future studies 51 Bibliography 53 6 Abstract Neutron stars (NSs) are the most dense objects in the Universe that can be directly observed. The nature of the cold ultra-dense matter inside them is still unresolved, and determining the equation of state (EoS) of that matter is a fundamental problem in nuclear physics. Measurements of sizes and masses of NSs can be used to constrain the EoS, and thus NSs can be described as astrophysical laboratories for nuclear physics. The size (or radius) and mass measurements can be done, for example, using the X-ray timing observations of millisecond pulsars (MSPs), which are very rapidly rotating NSs. In thefi rst part of this thesis, I have pre- sented a framework that can be used to model the observed X-ray pulse profiles from MSPs and to obtain constraints for the model parameters, in- cluding NS mass and radius. I have also estimated how the upcoming X-ray polarization measurements will improve the constraints. In addition, I have shown that there also exist problems in the current models in explaining all the features of the X-ray data. Especially, the emission pattern from an atmosphere of an accretion-powered millisecond pulsar (AMP) should be accurately solved for more robust estimates. Moreover, modelling NS atmospheres is not only important for AMPs, but also essential for pulse profile modelling of rotation-powered millisec- ond pulsars (RMPs), and that is considered in the second part of the the- sis. I have studied the importance of the exact formulation of Compton scattering in the RMP atmospheres and created a novel model for RMP atmospheres heated by magnetospheric return-currents. This model differs from the preceding ones in that it does not assume that all the heat is released in the deepest layers of the atmosphere. The results imply that the emission pattern also from RMP surface may significantly deviate from that predicted by previous models, which could affect also the recent ra- dius constraints obtained from the observations of Neutron star Interior Composition ExploreR (NICER) instrument. In thefi nal part of the thesis, I have studied, in more detail, how the 7 upcoming X-ray polarization observations of rapidly rotating NSs can be accurately modelled accounting for thefl attened shape of the star and used to obtain further constraints on the EoS of ultra-dense matter. The results show that the unknown physics of NS interiors can be probed by combining X-ray timing, spectral, and polarization measurements of MSPs. 8 Tiivistelm¨a Neutronit¨ahdetovat maailmankaikkauden tiheimpi¨asuoraan havaittavia kohteita. Niiden sis¨all¨aolevan eritt¨aintihe¨ankylm¨anaineen olemus on yh¨aselvitt¨am¨att¨a,ja kyseist¨aainetta kuvaavan tilanyht¨al¨onratkaiseminen on perustavanlaatuinen ongelma ydinfysiikassa. Neutronit¨ahtien massoja ja s¨ateit¨amittaamalla voidaan rajata tilanyht¨al¨o¨a,ja t¨atenneutronit¨ahtien voidaankin sanoa olevan ydinfysiikan astrofysikaalisia laboratiorioita. Neutronit¨ahdenkokoa (tai s¨adett¨a)ja massaa voidaan mitata hy¨odyn- t¨am¨all¨aesimerkiksi r¨ontgen-ajoitus havaintoja millisekuntipulsareista, jotka ovat hyvin nopeasti py¨orivi¨aneutronit¨ahti¨a.V¨ait¨oskirjan ensimm¨aisess¨aos- assa olen esitt¨anyt menetelm¨an,jota voidaan k¨aytt¨a¨ahavaittujen r¨ontgen- pulssien mallintamiseen ja t¨ahdenparametrien, kuten massan ja s¨ateen, rajaamiseeen. Olen my¨osarvioinut kuinka tulevat r¨ontgen-polarisaatio- mittaukset auttavat massan ja s¨ateenrajaamisessa. Lis¨aksiolen osoittanut, ett¨anykyisill¨amalleilla on vaikea selitt¨a¨akaikkia r¨ontgen-havaintojen yksi- tyiskohtia. Erityisesti neutronit¨ahdenilmakeh¨ans¨ateilymalli aineen ker- tymisest¨avoimansa saavissa millisekuntipulsareissa (accretion-powered mil- lisecond pulsars, AMP) pit¨aisiratkaista tarkemmin luotettavampia arvioita varten. Neutronit¨ahtien ilmakehien mallintaminen ei ole t¨arke¨a¨avain AMP:ille, vaan se on keskeist¨amy¨ospy¨orimisest¨avoimansa saaville millisekuntipul- sareille (rotation-powered millisecond pulsars, RMP), ja sit¨ak¨asitell¨a¨ankin v¨ait¨oskirjani toisessa osassa. Olen tutkinut Compton sironnan tarkan esi- tyksen t¨arkeytt¨aRMP:iden ilmakeh¨amalleissa ja luonut my¨osuuden mallin kuvaamaan magnetosf¨a¨arisenpaluuvirran l¨ammitt¨ami¨aRMP:iden ilmake- hi¨a.T¨am¨amalli eroaa aiemmmista siin¨a,ett¨ase ei oleta kaiken l¨amm¨onva- pautuvan ilmakeh¨ansyvimmiss¨akerroksissa.