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Study of the Accretion Flow in Eclipsing Polars STUDY OF THE ACCRETION FLOW IN ECLIPSING POLARS C h r is t ia n M ic h a e l B r id g e Milliard Space Science Laboratory Department of Space and Climate Physics University College London August 2004 A thesis submitted to the University of London for the degree of Doctor of Philosophy UMI Number: U602501 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI U602501 Published by ProQuest LLC 2014. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 i ACKNOWLEDGMENTS There are many people who have helped and supported me while I have worked on this thesis. Specifically I wish to thank my supervisor Mark Cropper, from whom I have learnt a great deal. I consider myself fortunate to have had the opportunity to work with him. At MSSL I would like to thank Gavin Ramsay for his help, patience, support and friendship and Mat Page for reading and commenting on this thesis. For help with the theory side of this thesis, and especially Chapter 7, I thank Kinwah Wu. Further afield, I would like to thank Pasi Hakala, without whom I would not have had the opportunity to use the fire-fly model and the eclipse mapping model. At ESTEC, I thank Michael Perryman for allowing me access to the S-Cam 2 instrument and Jos de Bruijne and A1 Reynolds for their subsequent support and help. Finally, to all those people whom I have met, shared flats with and drank beer with, you all remain my very good friends. 2 ABSTRACT This thesis is concerned with the accretion flow in the high magnetic field cata­ clysmic variables known as polars. I first provide a brief introduction to the subjects discussed in this thesis, followed by an introduction to the instruments used to make the observations: the optical observations were made using the S-Cam 2 supercon­ ducting tunnel junction camera and X-ray observations using XMM-Newton. The chapters that follow this are concerned with two polars: HU Aqr and EP Dra. In Chapter 3, I apply an eclipse mapping technique to the optical light curves of HU Aqr to reconstruct the brightness distribution along the accretion flow. In Chapter 4 I discuss the light curves of EP Dra, a system which until now has not been well studied. The light curves of EP Dra all show variability and a unique light curve shape. Based on the observations and the eclipse mapping I make insights into the nature of the accretion flow in these two systems. Chapter 5 presents modelling of the optical light curves of HU Aqr and EP Dra using a technique based on ’genetically modified fire-flies’. Unlike previous tech­ niques to reconstruct the brightness distribution of tha^accretion flow, this places as few constraints on the location of the material as possible. This is the first time this technique has been applied to observed light curves. In Chapter 6 I present XMM-Newton observations of EP Dra. I use X-ray and simultaneous optical/UV observations to provide a comparison with the observations of Chapter 4, and provide further insights into the accretion flow in this system. Finally, I present model accretion flows irradiated by X-ray emission from the accretion shock. This allows the construction of accretion flow temperature pro­ files, the location of different ionized species along the flow, and the construction of theoretical Doppler maps. Contents Acknowledgments 1 Abstract 2 1 Introduction 19 1.1 Cataclysmic variables .................................................................................... 19 1.2 P o l a r s ..................................................................................................................20 1.3 The accretion flow ........................................................................................... 25 1.3.1 The Roche lobe and mass transfer ...................................................... 25 1.3.2 Stream trajectory and threading by the fie ld ................................. 27 1.3.3 Inhomogeneous accretion flows ........................................................... 30 1.3.4 Heating and cooling ...........................................................................34 1.4 Techniques to study the accretion stream .....................................................36 1.5 Individual objects .............................................................................................. 37 1.6 Aim of this th e s is ...............................................................................................38 2 Instrumental overview 40 2.1 The STJ instrument S-Cam 2 ........................................................................40 2.1.1 Principles of an S T J ...........................................................................42 2.1.2 A Tantalum STJ a r r a y ....................................................................... 44 2.1.3 Performance of S-Cam 2 .................................................................... 45 2.2 XM M -N ew ton .....................................................................................................50 3 CONTENTS 4 2.2.1 XMM D etectors .......................................................................................50 3 STJ observations of HU Aqr 53 3.1 Observations .........................................................................................................53 3.2 D ata re d u c tio n .................................................................................................. 55 3.3 The Light C u rv e s ......................................... 59 3.3.1 Pre-eclipse dip .......................................................................................62 3.3.2 Accretion region eclipse ...................................................................... 62 3.3.3 Stream eclipse variations ...................................................................... 65 3.3.4 Colour variations ....................................................................................66 3.4 Indirect imaging .................................................................................................. 68 3.4.1 Methodology ..........................................................................................69 3.4.2 Stream map artifacts............................................................................. 72 3.4.3 Fixed parameters and R^....................................................................76 3.4.4 Model results ..........................................................................................80 3.5 D iscussion ............................................................................................................82 3.5.1 Comparison with earlier results ..........................................................82 3.5.2 Heating in the magnetically confined stream ...................................84 3.5.3 Temporal variations in the stream profile ...........................................85 3.5.4 Mass transfer rate ....................................................................................86 3.6 Summary and co n clu sio n s...............................................................................87 4 Variability in the accretion stream of EP Dra 89 4.1 Observations and red u ctio n s ........................................................................... 89 4.2 Light-curve features ........................................................................................... 98 4.2.1 White dwarf and accretion region eclipse ...........................................98 4.2.2 Stream eclipse..........................................................................................99 4.2.3 Out of eclipse ........................................................................................ 102 4.3 Discussion ..........................................................................................................102 4.3.1 Accretion region location .................................................................... 102 CONTENTS 5 4.3.2 Stream variations ..................................................................................103 4.3.3 Trough feature .....................................................................................105 4.3.4 The accretion flo w .............................................................................. 108 4.4 Conclusions ........................................................................................................ 110 5 Eclipse mapping with ‘fire-flies’ 112 5.1 M otivation ........................................................................................................... 112 5.2 D ata sets ........................................................................................................... 113 5.3 The M odel ........................................................................................................... 115 5.3.1 Overview ...............................................................................................115 5.3.2 Evaluation of the fit ............................................................................... 116 5.3.3 Fixed parameters ..................................................................................119
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