X-Ray Studies of Ultraluminous X-Ray Sources
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Durham E-Theses X-ray studies of ultraluminous X-ray sources LUANGTIP, WASUTEP How to cite: LUANGTIP, WASUTEP (2015) X-ray studies of ultraluminous X-ray sources, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/11266/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk X-ray studies of ultraluminous X-ray sources Wasutep Luangtip A Thesis presented for the degree of Doctor of Philosophy Centre for Extragalactic Astronomy Department of Physics University of Durham United Kingdom September 2015 X-ray studies of ultraluminous X-ray sources Wasutep Luangtip Submitted for the degree of Doctor of Philosophy September 2015 Abstract Ultraluminous X-ray sources (ULXs) are extra-galactic, non-nuclear point sources, with X-ray luminosities brighter than 1039 erg s−1, in excess of the Eddington limit for 10 M⊙ black holes. Recent results indicate that the majority of ULXs are stellar remnant black holes accreting material at or above the Eddington rate, rather than sub-Eddington accretion onto intermediate mass black holes. However, precisely how these ULXs accrete material at a super-Eddington rate remains an open question. This thesis focuses on the nature of these system as well as their environments, and attempts to explain physically how the sources operate in this super-critical accretion regime. This work begins with a study of the X-ray spectra of ULXs in very nearby galaxies (D< 5 Mpc). A range of physical models is used to explain the ULX spectra and to interpret the results physically. The outcomes consistently suggest that ULXs are stellar remnant black holes accreting material at or above the Eddington rate. It is demonstrated that the hard spectral component is consistent with emission from the inner radius of an advection-dominated slim accretion disc; the mass of black holes powering ULXs can be constrained from this hard emission, falling in the regime of stellar-mass black hole (∼3–30 M⊙). Assuming that the soft spectral component represents soft thermal emission from an optically-thick outflowing wind, 4 6 the size of the wind is constrained to be between ∼10 – 10 Rg. We further explore the nature of ULXs by studying the X-ray spectral evolution of the individual source Holmberg IX X-1 with observed source luminosity. We find that the spectra tend to evolve from relatively flat or two-component spectra in the iii medium energy band, at lower luminosities, to a spectrum that is distinctly curved and disc-like at the highest luminosities. This spectral variability is consistent with the prediction of super-Eddington accretion models, in which the outflowing wind is expected to be launched from within the photospheric radius; the increase in accretion rate causes the more powerful wind to scatter a higher fraction of hard photons into the line of sight, while those that survive the passage through the wind will be Compton down-scattered to lower energies; these increase and soften the hard spectral component, resulting in a disc-like spectrum peaking at lower energy than the hard component seen at lower luminosity. Furthermore, we find observational evidence that the ULX might precess around its rotational axis, implied by a degree of degeneracy between different spectra observed at the same luminosity. Finally, we study the population of ULXs present in a sample of 17 nearby luminous infrared galaxies (LIRGs). It is found that the LIRGs possess significantly fewer ULXs per unit star formation rate than nearby normal galaxies, by a factor of about 10. We argue that part of the deficit could be due to the high metallicity environment of the host galaxies suppressing the formation of ULXs, and the lag between star formation starting and the appearance of ULXs; however, the majority of the deficit of ULXs is likely to be due to the high amount of gas and dust in the LIRGs obscuring a large fraction of ULXs. Declaration The work in this thesis is based on research carried out at the Centre for Extra- galactic Astronomy, the Department of Physics, University of Durham,UK, under the supervision of Dr. Tim Roberts and Prof. Chris Done. No part of this thesis has been submitted elsewhere for any other degree or qualification and it is all my own work unless referenced to the contrary in the text. Portions of this thesis have appeared in the following papers: • Luangtip, W., Roberts, T. P., Mineo, S., Lehmer, B. D., Alexander, D. M., Jackson, F. E., Goulding, A. D., Fischer, J. L., 2015, MNRAS, 446, 470 Copyright c 2015 by Wasutep Luangtip. “The copyright of this thesis rests with the author. No quotations from it should be published without the author’s prior written consent and information derived from it should be acknowledged.” iv Acknowledgements First of all, I would like to thank my supervisors, Dr. Timothy Roberts and Prof. Christine Done, for their support, enthusiastic guidance and encouragement throughout the course of my PhD studies at Durham. I am also grateful to Dr. An- drew Sutton and Dr. Fabio Pintore, who took the time to answer a lot of questions about ULXs as well as X-ray astronomy, and taught me the basics of X-ray data analysis; Dr. Stefano Mineo, who spent a lot of time discussing in X-ray population and helping me to understand statistical methods for data fitting; and to the rest of the research group, who made this period of study so enjoyable. Thanks must also be given to Ministry of Science and Technology of The Royal Thai Government for the financial support throughout my academic career. Thank to my friends both in UK and Thailand who encouraged me and helped to create a fun when I was stressful. Finally, I am very grateful to my family; without their encouragement and support, I could not have got this far. v Contents Abstract ii Declaration iv Acknowledgements v 1 Introduction 1 1.1 AbriefhistoryofX-rayastronomy . 1 1.2 CurrentX-rayobservatories . 9 1.2.1 Principle of X-ray telescopes . 9 1.2.2 Chandra .............................. 13 1.2.3 XMM-Newton ........................... 18 1.2.4 Suzaku ............................... 20 1.2.5 Swift ................................ 22 1.2.6 NuSTAR .............................. 24 1.3 Radiationprocesses inX-rayastronomy. ... 26 1.3.1 Blackbodyradiation . 26 1.3.2 Bremsstrahlung . .. .. .. 28 1.3.3 Synchrotron ............................ 28 1.3.4 Comptonisation . .. .. .. 30 1.3.5 Photoelectriceffect . 31 1.4 Blackholes ................................ 33 1.4.1 Blackholetypesandmasses . 35 1.4.2 Blackholeaccretion . 38 1.4.3 X-rayspectra ........................... 43 vi Contents vii 1.5 UltraluminousX-raysources . 48 1.5.1 Accretion in the supercritical regime . 53 1.5.2 X-rayspectra ........................... 55 1.5.3 ULXpopulations ......................... 60 1.6 Aimsofthiswork............................. 63 2 Suzaku observations of ultraluminous X-ray sources in very nearby galaxies 65 2.1 Introduction................................ 65 2.2 Source selection and data reduction . .. 66 2.2.1 Source selection and observations . 66 2.2.2 Datareduction .......................... 70 2.3 Spectralanalysisandresults . 72 2.3.1 Singlecomponentmodels. 73 2.3.2 Discpluspower-lawmodel . 77 2.3.3 Disc plus Comptonisation model . 82 2.3.4 Blackbody plus relativistic disc around spinning black hole model ............................... 87 2.3.5 Blackbody plus slim disc model . 91 2.3.6 Blackbody plus cutoff power-law model . 95 2.4 Discussion ................................. 95 2.4.1 The effect of telescope observational bandpass on spectral analyses .............................. 95 2.4.2 The existence of a cool and optically thick corona? . ... 98 2.4.3 The mass of black holes powering ULXs . 100 2.4.4 Super-Eddington accretion models with an outflowing wind . 102 2.5 Conclusion.................................104 3 The X-ray spectral evolution of the ultraluminous X-ray source Holmberg IX X-1 108 3.1 Introduction................................108 3.2 Observationsanddatareduction. 109 Contents viii 3.2.1 Swift data.............................109 3.2.2 XMM-Newton data........................111 3.2.3 NuSTAR data...........................113 3.3 Spectralanalysisandresults . 114 3.3.1 Swift spectralanalysis . .115 3.3.2 XMM-Newton spectralanalysis . .127 3.3.3 Broadbandspectralanalysis . .130 3.4 Discussion .................................136 3.4.1 Slimaccretiondiscmodel . .139 3.4.2 The effect of a massive outflowing wind . 140 3.4.3 Sourceprecession . .142 3.5 Conclusion.................................143 4 A deficit of ultraluminous X-ray sources in luminous infrared galax- ies 146 4.1 Introduction................................146 4.2 Observationsanddatareduction. 148 4.2.1 Observations and initial analyses . 148 4.2.2 Thesourcecatalogue . .150 4.3 Dataanalysis&results. .159 4.3.1 Spectralanalyses . .159 4.3.2 The correlation between ULXs and star formation in LIRGs . 167 4.3.3 X-ray luminosity functions . 172 4.4 Discussion .................................179 4.4.1 X-rayspectralresults. .179 4.4.2 Why is there a deficit of ULXs in LIRGs? . 182 4.5 Conclusion.................................198 5 Conclusion 201 5.1 TheX-rayspectralpropertiesofULXs . 201 5.2 ThepopulationofULXsinLIRGs. .204 5.3 Futurework ................................205 Contents ix 5.3.1 Probing deeper into the nature of ULXs and their outflowing wind................................205 5.3.2 The obscuration of ULXs by gas and dust in LIRGs? .