Suzaku Studies of White Dwarf Stars and the Galactic X-ray Background Emission

162 Pages·2013·7.368 MB·English

by  Takayuki Yuasa (auth.)

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Takayuki Yuasa
Suzaku Studies of
White Dwarf Stars
and the Galactic X-ray
Background Emission
Doctoral Thesis accepted by
the University of Tokyo, Japan
123
Author Supervisor
Dr. TakayukiYuasa Prof.KazuoMakishima
Department of Physics Department of Physics
Graduate School ofScience Graduate School ofScience
The Universityof Tokyo The Universityof Tokyo
Tokyo Tokyo
Japan Japan
ISSN 2190-5053 ISSN 2190-5061 (electronic)
ISBN 978-4-431-54218-6 ISBN978-4-431-54219-3 (eBook)
DOI 10.1007/978-4-431-54219-3
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Parts of this thesis have been published in the following articles:
Takayuki Yuasa, Kazuo Makishima, Kazuhiro Nakazawa, ‘‘Broad-band spectral
analysis of the Galactic Ridge X-ray Emission’’, Astrophysical Journal, vol. 753,
129, 2012
Takayuki Yuasa, Kazuhiro Nakazawa, Kazuo Makishima, Kei Saitou, Manabu
Ishida,KenEbisawa,HideyukiMori,andShin’yaYamada,‘‘Whitedwarfmasses
in intermediate polars observed with the Suzaku satellite’’, Astronomy & Astro-
physics, vol. 520, A25, 2010
Supervisor’s Foreword
Half a century of cosmic X-ray observations has been filled with enchanting
mysteries.ThePh.D.thesisofDr.TakayukiYuasadealswithoneofthem:thatthe
Milky Way is shining diffusely in X-rays as a narrow belt in the sky. Since its
discovery in the 1970s, this phenomenon, called the Galactic Ridge X-ray Emis-
sion (hereafter GRXE), has attracted many investigators. In the 1980s, prominent
X-ray emission lines from highly ionized iron atoms were detected in the GRXE
spectrawiththeJapaneseTenmasatellite.Therefore,theGRXEcanbeunderstood
as arising from hot plasmas with a temperature of (cid:2)108 K. This has led to two
alternativeinterpretations.OneofthemassumesthattheGRXEisemittedbytruly
diffuse hot plasmas filling the interstellar space. The other considers the same
phenomenonasacollectionofalargenumberofdiscreteGalacticsourcesthatare
too faint and numerous to be individually resolved. However, the former idea
presentedadifficulty,becausesuchplasmaswouldbetoohottobegravitationally
bound to the Galaxy. The latter also involved a difficulty, namely, that such a
population of hot plasma sources was not known. Thus, the interpretations have
remained controversial.
Utilizing the unprecedented angular resolution of the Chandra X-ray Obser-
vatory of the United States, a considerable fraction of the GRXE surface bright-
ness in a particular sky direction was recently resolved into discrete sources, for
which mass-accreting magnetic white-dwarf (WD), binaries are suggested as the
candidate population. In such a binary system, gas from the companion star falls
onto two magnetic poles of the WD, forming therein X-ray emitting hot plasmas.
On this basis, Dr. Yuasa decided to use the Japanese Suzaku mission to examine
whetherWDbinariescanactuallyaccountforthespectrumandsurfacebrightness
of the GRXE. Suzaku is particularly suited for this purpose, because the X-ray
Imaging Spectrometer onboard provides sensitive diagnostics of the iron line
complex, while the co-aligned Hard X-ray Detector can measure the hard X-ray
continuum of GRXE much better than ever.
Dr.Yuasa’sthesisconsistsoftwomajorparts.Inthefirstpart,heconstructeda
numerical model, which predicts X-ray spectra of mass-accreting magnetic WDs
vii
viii Supervisor’sForeword
when the WD mass and the plasma metallicity are specified. The model assumes
thathotplasmas,shock-createdatacertainheightabovetheWDpoles,flowdown
along the magnetic fields onto the WD surface while cooling by emitting thermal
bremsstrahlung continua and atomic emission lines. This model was successfully
applied to 2–40 keV Suzaku spectra of about 20 magnetic WDs, which was
observedmainlybasedonDr.Yuasa’sownproposals.Thelatterpartofthethesis,
indeed its highlight, is an attempt to apply the validated WD emission model to
high-qualitySuzakuspectra ofthe GRXE,accumulatedovermanyskyfields near
theGalacticcenterbycarefullyavoidingbrightpointX-raysources.Asaresult,he
has successfully demonstrated that the GRXE spectra can be reproduced by his
own WD emission model with plausible mass and abundance parameters, on
condition that another softer plasma emission (presumably from active stars) is
added.
In this way, Dr. Yuasa has demonstrated spectroscopically that the GRXE, in
particular in the (cid:2)5 to 40 keV range, can be understood principally as a super-
positionofalargenumberofmass-accretingWDbinaries.Hehasalsoshownthat
these binaries are numerous enough to constitute the GRXE surface brightness
whenintegratedalongthelineofsight.Thisisindeedamonumentalresultthathas
settled the long-lasting controversy of the GRXE. In short, the X-ray view of the
Milky Way repeated its much older visible-light recognition; what appeared as a
diffuse optical belt to the naked eye was later understood as an assembly of
uncountablefaintstars.Basedontheseachievements,Dr.Yuasawasawardedthe
Student Research Prize in March 2011 by the Graduate School of Science, The
University of Tokyo.
January 2012 Kazuo Makishima
Acknowledgments
I would like to thank Prof. Kazuo Makishima for guiding and encouraging me
throughout my five years at the graduate school of The University of Tokyo.
DiscussionswithhimledmetostudytheGalacticX-raybackgroundemissionand
deepened my understanding of its origin. I greatly enjoyed being in his group,
improving my skills in experimental physics and celestial data analyses.
IalsothankLecturerKazuhiroNakazawafordiscussionsontechnicaldetailsof
theSuzakuinstruments.Dr.Shin’yaYamadahasbeenawonderfulcolleaguewith
hisknowledgeofblackholes,andmoderncomputingparadigms, andhisspiritof
collaboration. His continuous encouragement helped me to pursue my research
activities in experiments and X-ray astrophysics. I have been very happy to work
with him in the same office for the past five years.
The significant efforts of past members of the High Energy Astrophysics
Laboratory of The University of Tokyo, which were offered for developing the
Hard X-ray Detector (ASTRO-E) and the Hard X-ray Detector II (ASTRO-E2/
Suzaku), made possible the present wide-band spectral study of the Galactic
background emission with the highest quality ever. I acknowledge the satellite
operation staff working in ISAS/JAXA and JAXA’s Uchinoura Space Center in
Kagoshima,Japan,forprovidingscientificdatathatwereessentialinmyanalysis.
Finally,Iamgratefultomyfamilyfortheirconstantsupportinmanywaysover
the years.
ix
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Outline. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.3 The X-ray/Gamma-ray Instruments . . . . . . . . . . . . . . . . . . . . . 3
1.4 Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4.1 Notations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.4.2 The Energy Flux Unit ‘‘Crab’’ . . . . . . . . . . . . . . . . . . . 3
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2 Review of the Galactic X-ray Background Emission
and White Dwarf Binaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1 The Galactic Ridge X-ray Emission. . . . . . . . . . . . . . . . . . . . . 7
2.1.1 Discovery and Observational Characteristics . . . . . . . . . 7
2.1.2 Proposed GRXE Origins . . . . . . . . . . . . . . . . . . . . . . . 9
2.1.3 The ‘‘Point Source’’ Scenario. . . . . . . . . . . . . . . . . . . . 10
2.1.4 The ‘‘Diffuse’’ Scenarios . . . . . . . . . . . . . . . . . . . . . . . 16
2.1.5 The GRXE in the Galactic Center Region . . . . . . . . . . . 18
2.1.6 Our Approach Towards Understanding the GRXE . . . . . 18
2.2 Accreting White Dwarf Stars (Cataclysmic Variables). . . . . . . . 19
2.2.1 White Dwarf Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 White Dwarf Binaries . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.3 Cataclysmic Variables. . . . . . . . . . . . . . . . . . . . . . . . . 25
2.2.4 Magnetic CVs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.2.5 Measuring the White Dwarf Mass. . . . . . . . . . . . . . . . . 30
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3 Constructing an X-ray Model of Accreting
White Dwarf Binaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.1 Improvements in This Work. . . . . . . . . . . . . . . . . . . . . . . . . . 35
3.2 Assumed Accretion Geometry and Boundary Conditions . . . . . . 35
xi
xii Contents
3.3 Formulation of Conservation Equations . . . . . . . . . . . . . . . . . . 37
3.4 Numerical Implementation . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.5 Composing an Emerging X-ray Spectrum. . . . . . . . . . . . . . . . . 42
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4 Instrumentation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1 The Suzaku X-ray Observatory . . . . . . . . . . . . . . . . . . . . . . . . 47
4.1.1 The X-ray Imaging Spectrometer
and the X-ray Telescope . . . . . . . . . . . . . . . . . . . . . . . 49
4.1.2 The Hard X-ray Detector. . . . . . . . . . . . . . . . . . . . . . . 52
4.2 Data Transmission and Data Processing on Ground. . . . . . . . . . 57
4.3 Model Fit to an Observed Spectrum . . . . . . . . . . . . . . . . . . . . 58
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
5 Estimating the Masses of White Dwarfs in Magnetic
Cataclysmic Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.1 Observation and Preparation of Data Set . . . . . . . . . . . . . . . . . 61
5.1.1 Target Selection and Involved Bias. . . . . . . . . . . . . . . . 61
5.1.2 Data Processing and Extraction of Spectra. . . . . . . . . . . 63
5.2 Deriving WD Masses by Fitting X-ray Spectra. . . . . . . . . . . . . 65
5.2.1 Construction of a Fit Model. . . . . . . . . . . . . . . . . . . . . 65
5.2.2 WD Masses Estimated from Wide-Band
Spectral Fitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
5.2.3 Comparison of Fittings Between the XIS
and HXD/PIN Energy Bands . . . . . . . . . . . . . . . . . . . . 73
5.3 Systematic Uncertainties Involved in Our Analysis . . . . . . . . . . 74
5.3.1 Effects of Systematic Fluctuations
of the HXD/PIN Background . . . . . . . . . . . . . . . . . . . . 76
5.3.2 Dynamics of Accreting Gas and System Geometries. . . . 77
5.3.3 Accretion Rate Dependence. . . . . . . . . . . . . . . . . . . . . 77
5.3.4 X-ray Reflection on the WD Surface. . . . . . . . . . . . . . . 78
5.4 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.4.1 WD Mass Spectrum and its Average. . . . . . . . . . . . . . . 79
5.4.2 Comparison with Measurements of WD Masses. . . . . . . 80
5.4.3 Comparison of Fe Abundances. . . . . . . . . . . . . . . . . . . 84
5.5 Summary of the WD Mass Estimation. . . . . . . . . . . . . . . . . . . 86
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
6 Decomposing the Galactic Ridge X-ray Emission . . . . . . . . . . . . . 89
6.1 Observation and Preparation of Data Sets. . . . . . . . . . . . . . . . . 89
6.1.1 Data Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
6.1.2 Contaminating Fluxes from Apparent Point Sources . . . . 91
6.1.3 Time Variations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
6.1.4 Individual Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . 99