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Flares on Active M-Type Stars Observed with XMM-Newton and Chandra

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Flares on active M-type stars observed with XMM-Newton and Chandra Urmila Mitra Kraev Mullard Space Science Laboratory Department of Space and Climate Physics University College London A thesis submitted to the University of London for the degree of Doctor of Philosophy I, Urmila Mitra Kraev, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Abstract M-type red dwarfs are among the most active stars. Their light curves display random variability of rapid increase and gradual decrease in emission. It is believed that these large energy events, or flares, are the manifestation of the permanently reforming magnetic field of the stellar atmosphere. Stellar coronal flares are observed in the radio, optical, ultraviolet and X-rays. With the new generation of X-ray telescopes, XMM-Newton and Chandra , it has become possible to study these flares in much greater detail than ever before. This thesis focuses on three core issues about flares: (i) how their X-ray emission is correlated with the ultraviolet, (ii) using an oscillation to determine the loop length and the magnetic field strength of a particular flare, and (iii) investigating the change of density sensitive lines during flares using high-resolution X-ray spectra. (i) It is known that flare emission in different wavebands often correlate in time. However, here is the first time where data is presented which shows a correlation between emission from two different wavebands (soft X-rays and ultraviolet) over various sized flares and from five stars, which supports that the flare process is governed by common physical parameters scaling over a large range. (ii) As it is impossible to spatially resolve any but a very few giant stars, the only information on spatial dimensions as well as the magnetic field strength of stellar coronae has to come from indirect measurements. Using wavelet analysis, I isolated the first stellar X-ray flare oscillation. Interpreting it as a standing coronal flare loop oscillation, I derived a flare loop length as well as the magnetic field strength for this X-ray flare. 3 ABSTRACT 4 (iii) The high-resolution soft X-ray spectra of Chandra and XMM-Newton allow us to determine temperatures, densities and abundances of the stellar coronae. De- spite a low signal-to-noise ratio because of the relatively short duration of a flare, we find that, if adding up the photons of several flares, certain density sensitive spectral lines change significantly between quiescent and flaring states. This project led on to investigate the flaring spectrum further, and it is found that the plasma is no longer in collisional ionisation equilibrium, but that it is dominated by recombinations. Acknowledgments First and foremost, I would like to thank my supervisor Louise Harra for her en- during support and guiding me on my way to become an independent researcher. My grateful thanks also go to my co-supervisors Graziella Branduardi-Raymont and Keith Mason for giving me the opportunity to work with XMM-Newton data and for their valuable contributions, in particularly regarding instrumentation and data analysis. My thanks also go to all my collaborators and colleagues at MSSL: Hilary Kay, for sharing her knowledge on stars and the Sun in general and XMM-Newton data analysis in particular; David Williams, for passing on his knowledge on coronal waves and IDL programming; Lidia van Driel-Gesztelyi, for her everlasting enthusi- asm for solar and stellar physics; Alex Blustin, for introducing me to XMM-Newton data analysis and spectral fitting with SPEX, and many other contemporary col- leagues at MSSL for answering questions, and discussing solar- and astrophysics. I am also gratefully indebted to my collaborators from other institutions, namely: Manuel G¨udel, for introducing me to and passing on his excitement for stellar coro- nal physics, fruitful collaboration and discussions; Marc Audard and Ton Raassen for much valued contributions; Jan-Uwe Ness, for introducing me to Chandra data analysis and enthusiastic discussions about flare spectra; and last but not least my husband Egor Kraev, for his patience, constant support, and sharing his mathemat- ical insight, in particular about wavelet analysis and error statistics. Special thanks also go to my examiners Valery Nakariakov and Mihalis Mathioudakis. And finally I would like to thank everyone who supported me patiently on my journey. 5 Contents Abstract 3 Acknowledgments 5 List of Tables 10 List of Figures 11 1 Introduction 17 1.1 Mdwarfs ................................. 17 1.1.1 Generalproperties . 17 1.1.2 Magnetic activity . 20 1.1.3 Features of the dMe stars treated in this work . 23 1.2 Flares ................................... 27 1.2.1 Stellar flare observations . 27 1.2.2 The standard flare model: Magnetic reconnection and chro- mosphericevaporation . 27 1.2.3 Flaredimensions ......................... 31 1.3 Magneto-hydrodynamicwaves . 31 1.4 X-rayspectroscopy ............................ 32 1.4.1 Temperatures and densities from line ratios . 33 1.4.2 Non-ionisation equilibria . 35 1.5 Openquestions .............................. 36 6 CONTENTS 7 1.6 Overview.................................. 37 2 Instrumentation 38 2.1 XMM-Newton ............................. 39 2.2 Chandra ................................. 42 3 Flares in X-rays and ultraviolet 43 3.1 Summary ................................. 43 3.2 Introduction................................ 43 3.3 Observations................................ 46 3.4 Results................................... 48 3.4.1 Lightcurves............................ 48 3.4.1.1 Determining the luminosity . 48 3.4.1.2 Cross-correlation between the UV and X-ray flux . 51 3.4.1.3 Flare identification . 53 3.4.2 The UV–X-ray relationship in flares . 53 3.5 Discussion................................. 55 3.6 Conclusions ................................ 60 4 X-ray flare oscillation 62 4.1 Summary ................................. 62 4.2 Introduction................................ 63 4.3 TargetandObservation . 65 4.4 Analysis .................................. 67 4.4.1 Datapreparation . 67 4.4.2 Continuous wavelet transform and frequency band decompo- sition................................ 68 4.4.3 CWTerroranalysis. 72 4.5 Results................................... 73 4.5.1 Magneto-acoustic waves . 74 CONTENTS 8 4.5.2 Loop length from radiative cooling times . 76 4.5.3 Pressure balance scaling laws . 76 4.6 Discussionandconclusions . 77 5 High-density flares on EV Lacertae 81 5.1 Summary ................................. 81 5.2 Introduction................................ 82 5.3 Datareduction .............................. 85 5.4 Analysisandresults............................ 86 5.4.1 Lightcurveandspectra . 87 5.4.2 Statisticalassessment. 90 5.4.3 ResultsofotherdMestars . 94 5.5 Discussion................................. 97 5.5.1 Temperature and densities from line ratios . 97 5.5.2 Resonantscattering. 99 5.5.3 Transient recombination . 100 5.5.4 Flaringvolume .. .. .. ... .. .. .. .. .. ... .. ..101 5.6 Conclusions ................................102 6 The recombining X-ray flare spectrum of EV Lac 104 6.1 Summary .................................104 6.2 Introduction................................105 6.3 Resultsanddiscussion . .107 6.3.1 Spectrallines ...........................118 6.3.2 Overalltrends. .118 6.3.3 TheHe-liketransitions . .119 6.3.4 The H-like Lyman-series . 121 6.3.5 Photoionisation? . .122 6.4 Conclusions ................................124 CONTENTS 9 7 Conclusions and outlook 126 7.1 Conclusions ................................126 7.2 Outlook ..................................129 Bibliography 131 List of Tables 1.1 Terminology................................ 18 1.2 Targetsandtheirproperties . 24 1.2 Targets and their properties – continued . 25 1.3 Solarparameters ............................. 25 2.1 Key instrument spectral X-ray features of XMM-Newton and Chan- dra .................................... 40 3.1 Observational Parameters and Results . 47 3.2 Relationship Parameters for log(X-ray) = κ log(UV) + c ....... 55 · 3.3 Neupert Relationship Parameters: = 10c κ ............ 57 Lx · Euv 4.1 Comparison of values derived for the indicated models . ...... 78 5.1 EV Lac line count rates. In bold are significant changes. 97 5.2 EV Lac line ratios and derived densities and temperatures. In bold are ratios which show significant changes. 98 6.1 Chandra HETG fluxes of EV Lac during quiescence and flares . 108 6.2 Collisional ionisation and recombination equilibration timescales τ . 120 10 List of Figures 1.1 Chromospheric evaporation. From the corona, a non-thermal electron beam is accelerated along a magnetic loop down toward the photo- sphere. The relativistic electrons gyrate along the magnetic field lines and emit gyro-synchrotron radiation in the radio frequency. When they reach the chromosphere, they collide with the dense plasma, ex- citing the ions and emitting hard X-ray bremsstrahlung. Thus, the plasma gets heated and flows up into the corona. Eventually, the plasma cools again and emits soft X-rays (SXR) from recombination processes. Subsequently, the material sort of condenses, falls down- ward, gets denser still and thus radiates even more SXR. 29 1.2 The f/i-ratio as a function of density for He-like ions at their respec- tive maximum emission temperature, calculated with CHIANTI. 34 11 LIST OF FIGURES 12 3.1 The light curves (left panels) and their cross-correlations (right pan- els) of EV Lac
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