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ROSAT Observations of Superflares on RS Cvn Systems

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ROSAT Observations of Superflares on RS Cvn Systems ROSAT Observations of Superflares on RS CVn Systems Vito Giuseppe GrafFagnino Thesis submitted for the degree of Doctor of Philosophy, in the Faculty of Science of the University of London. UCL Mullard Space Science Laboratory Department of Space & Climate Physics U n i v e r s i t y • C o l l e g e • L o n d o n 2000 ProQuest Number: U642316 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. uest. ProQuest U642316 Published by ProQuest LLC(2015). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. Microform Edition © ProQuest LLC. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ,,. A Mia Moglie E Miei Genitori A bstract The following thesis involves the analysis of a number of X-ray observations of two RS CVn systems, made using the ROSAT satellite. These observa­ tions have revealed a number of long-duration flares lasting several days (much longer than previously observed in the X-ray energy band) and emitting ener­ gies which total a few percent of the available magnetic energy of the stellar system and thus far greater than previously encountered. Calculations based on the spectrally fitted parameters show that simple flare mechanisms and standard two-ribbon flare models cannot explain the observations satisfacto­ rily and continued heating was observed during the outbursts. This is the first time that such departures from two-ribbon flare models have been identified unambiguously for such large flares, however for some of these outbursts the situation is complicated by the fact that other flaring activity may have been superimposed on the observed lightcurves. By analysing the general decay of the flares, loop heights were derived. Although these loop heights are depen- dendent on the amount of heating assumed, the calculations were performed for a wide range of reasonable heating values. For these cases the flare heights obtained for these outbursts were of the order of the inter-binary separation and inter-binary flares are suggested as the cause of the outbursts. C ontents A b stra c t........................................................................................................... 3 Table of Contents 4 List of Tables ................................................................................................. 8 List of Figures..................................................................................................... 10 Acknowledgements ........................................................................................... 16 1 An Introduction to Stellar Extremists — RS CVns 18 1.1 INTRODUCTION ................................................................................ 18 1.2 THE RS CVn BINARY SY S T E M S .....................................................19 1.2.1 RS CVns and evolution.............................................................. 21 1.3 X-RAY OBSERVATIONS OF RS CVn SYSTEMS...........................22 1.3.1 A Brief Historical Introduction ..............................................22 1.3.2 Stellar X-ray Luminosity R elationships .................................23 1.3.3 Development of Spectral Models to Fit Stellar Coronae . 27 1.3.4 A Brief Review Of Characteristics At Other Wavelengths 32 1.4 STELLAR F L A R E S ............................................................................. 39 1.4.1 Types of Stellar Flares .............................................................. 42 1.4.2 Simple Analysis and Inference of Stellar Flare Parameters 43 1.4.3 X-ray flare observations ...........................................................45 1.4.4 In This T hesis .............................................................................. 50 2 HR 5110—A Source Of Inter-Binary Flaring? 53 2.1 INTRODUCTION ................................................................................ 53 2.2 THE D A T A ............................................................................................. 55 2.3 THE A N A LY SIS....................................................................................56 2.3.1 The WFC Survey Tim e-Series ................................................. 56 2.3.2 The Survey PSPC Time-Series .................................................. 57 2.3.3 The Pointed WFC Time-Series ..................................................59 2.3.4 The Pointed PSPC Time-Series ..............................................59 2.3.5 The Spectral Analysis .............................................................. 61 2.4 DISCUSSION.......................................................................................... 69 2.4.1 A Two-Ribbon Flare? .............................................................. 73 2.5 SUM M ARY............................................................................................. 86 3 HR 5110—You Can’t Keep A Good Flare Down 91 3.1 INTRODUCTION ................................................................................ 91 3.2 THE D A T A ............................................................................................. 92 3.3 THE A N A LY SIS....................................................................................92 3.3.1 The Time-Series ............................................................................92 3.3.2 The Spectral Analysis .............................................................. 96 3.3.3 Emission Measure Distribution Va ria tio n s............................97 3.3.4 Analyses of the Observed Fla re s ................................................99 3.3.5 General Decay Phase of the fla re s .......................................... 106 3.4 DISCUSSION........................................................................................ I l l 3.5 SUM M ARY............................................................................................113 4 CF Tucanæ — A Southern Perspective 115 4.1 NOTES ON CF TUCANÆ ...............................................................115 4.2 THE D A T A ............................................................................................117 4.3 THE A N A LY SIS...................................................................................117 4.3.1 The Lightcurves ........................................................................118 4.3.2 Spectral Analysis and Comparison of Spectral Models . 118 4.3.3 Flare A nalysis ...........................................................................129 4.4 SUM M ARY.............................................................................................136 5 CF Tucanæ— The Aftermath 138 5.1 THE POST-FLARE/QUIESCENT STA GE ...................................138 5.1.1 Comparison With The Survey Observations .................... 152 5.2 A DISCUSSION OF KÜRSTER AND SCHMITT’S 1996 PAPER154 5.3 SUMMARY OF RESULTS FOR CF T U C ......................................157 6 A Final Discussion And Conclusions 159 6.1 A SUMMARY OF RESULTS ............................................................ 159 6.1.1 Flare Light curve Characteristics ...........................................160 6.1.2 Flare Analyses R e s u lts ...........................................................164 6.1.3 Flare Spectral C haracteristics .............................................. 168 6.2 SPECTRAL RESULTS FROM CONTEMPORANEOUS OB­ SERVATIONS ......................................................................................... 169 6.3 FUTURE W O R K ...................................................................................170 6.4 A FINAL W O R D ...................................................................................174 7 Bibliography 175 A Spectral Modelling of Stellar Coronae 198 A.l X-RAYS FROM HOT OPTICALLY THIN PLASMAS ...............198 A. 1.1 One-temperature and Two-temperature Thermal Plasma M odels ......................................................................................... 200 A. 1.2 Continuous emission measure m odels................................. 201 A.1.3 Multi-Temperature and Differential emission measure mod­ els ................................................................................................202 A. 1.4 Loop M odelling ......................................................................... 202 A.2 THE NON-UNIQUENESS OF X-RAY SPECTRAL MODEL FITTING ................................................................................................205 B The ROSAT Mission 220 B.l AN OVERVIEW OF THE MISSION ................................................220 B.2 THE ROSAT PSPC .............................................................................222 B.3 THE ROSAT W FC .............................................................................. 224 C Beyond ROSAT... 227 C.l Introduction........................................................................................... 227 List of Tables 1.1 Details of the major X-ray flares observed in RSCVn systems and reported in the literature ...................................................................47 1.2 Major X-ray flares: continued ..................................................................48 1.3 Major X-ray flares: continued
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