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Optical novae as supersoft X-ray sources in the Andromeda Galaxy [Elektronische Ressource] / Martin Henze

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T U¨ M¨
M-P-I ¨  P
Optical Novae as Supersoft X-ray Sources
in the Andromeda Galaxy
Martin Henze
Vollsta¨ndiger Abdruck der von der Fakulta¨t fu¨r Physik der Technischen Universita¨t Mu¨nchen zur
Erlangung des akademischen Grades eines
Doktors der Naturwissenschaften
genehmigten Dissertation.
Vorsitzender: Univ.-Prof. Dr. A. Ibarra
Pru¨fer der Dissertation:
1. Hon.-Prof. Dr. G. Hasinger
2. Univ.-Prof. Dr. L. Oberauer
Die Dissertation wurde am 15.12.2010 bei der Technischen Universita¨t Mu¨nchen eingereicht und
durch die Fakulta¨t fu¨r Physik am 17.02.2011 angenommen.T U¨ M¨
Optical Novae as Supersoft X-ray Sources
in the Andromeda Galaxy
Dissertation von
Martin Henze
15. Dezember 2010
M-P-I
¨
 PTrick
Optical Novae as Supersoft X-ray Sources in the Andromeda Galaxy
Optical novae are a subclass of variable stars that display a strong, eruptive increase in their optical
luminosity and are sometimes observed as supersoft X-ray sources (SSSs). The physical process
responsible for this behaviour is a thermonuclear runaway in an accreted hydrogen envelope on top
of a white dwarf in a cataclysmic binary system. This dissertation describes the results from the
first dedicated monitoring campaigns for SSS states of optical novae in the central region of the
Andromeda galaxy (M 31) with the X-ray telescopes XMM-Newton and Chandra . I present the first
analysis of the X-ray properties of the nova population in M 31 based on a statistically significant
sample. Peculiar objects from this sample are discussed in detail.
Optische Novae als superweiche Ro¨ntgenquellen in der
Andromedagalaxie
Bei optischen Novae handelt es sich um variable Sterne, deren Helligkeit im Optischen wa¨hrend eines
singula¨ren Ausbruchs stark zunimmt und die teilweise auch als Ro¨ntgenquellen mit extrem weichem
Spektrum beobachtet werden. Die physikalische Ursache fu¨r dieses Verhalten ist eine thermonuk-
leare Explosion auf der Oberfla¨che eines Weißen Zwergs in einem engen Doppelsternsystem. Meine
Dissertation beschreibt Beobachtungen der Andromeda-Galaxie (M 31) mit den Ro¨ntgenteleskopen
XMM-Newton und Chandra , die erstmals speziell auf die Entdeckung von Novae in Ro¨ntgenbereich
ausgelegt waren. Das Ziel meiner Arbeit war die erste Untersuchung der Ro¨ntgeneigenschaften der
Nova-Population in M 31 auf Basis einer statistisch signifikanten Stichprobe. Diese Dissertation stellt
die Ergebnisse der Populationsstudie vor und geht na¨her auf ungewo¨hnliche Einzelquellen ein.Contents
1 Introduction 9
1.1 Classical Novae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.1 Physics of Classical Novae . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.2 Optical properties and classification . . . . . . . . . . . . . . . . . . . . . . 9
1.1.3 X-ray properties - classical novae as supersoft X-ray sources . . . . . . . . . 11
1.1.4 Recurrent novae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.1.5 Novae in M 31 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
1.1.6 Nova population studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
1.1.7 Novae and supersoft X-ray sources in globular star clusters . . . . . . . . . . 14
1.1.8 The naming convention for novae in M 31 . . . . . . . . . . . . . . . . . . . 16
1.2 The XMM-Newton, Chandra and Swift X-ray observatories . . . . . . . . . . . . . . 16
1.2.1 The XMM-Newton observatory . . . . . . . . . . . . . . . . . . . . . . . . 17
1.2.2 The Chandra observatory . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.2.3 The Swift observatory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.3 Dissertation aims and outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
2 Observations and data analysis 23
2.1 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.1.1 The XMM-Newton/Chandra monitoring of the M 31 central region . . . . . 23
2.1.2 Archival X-ray observations of the M 31 central region . . . . . . . . . . . . 25
2.1.3 Optical monitoring of the M 31 central region . . . . . . . . . . . . . . . . . 29
2.1.4 Ultraviolet observations of novae with Swift . . . . . . . . . . . . . . . . . 31
2.2 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.2.1 Instrument specific X-ray data processing . . . . . . . . . . . . . . . . . . . 32
2.2.2 General X-ray data analysis procedures . . . . . . . . . . . . . . . . . . . . 34
2.2.3 Optical data reduction and analysis . . . . . . . . . . . . . . . . . . . . . . 36
2.2.4 Analysis of Swift ultraviolet data . . . . . . . . . . . . . . . . . . . . . . . . 37
3 Results of the optical and ultraviolet observations 39
3.1 Novae discovered and confirmed in the optical monitoring . . . . . . . . . . . . . . 39
3.2 Swift ultraviolet detections of novae . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4 Results of the X-ray monitoring 43
4.1 Nova counterparts known before this work . . . . . . . . . . . . . . . . . . . . . . . 47
4.2 New nova counterparts discovered in this work . . . . . . . . . . . . . . . . . . . . 53
4.3 The complete sample of M 31 novae with X-ray counterpart . . . . . . . . . . . . . 68
4.4 Upper limits for non-detected novae . . . . . . . . . . . . . . . . . . . . . . . . . . 73
74.5 Non-nova supersoft X-ray sources detected in the monitoring . . . . . . . . . . . . . 80
5 Highlight: First SSSs in M 31 globular clusters 85
5.1 Supersoft source in Bol 111 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
5.2 Supersoft source in Bol 194 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
5.2.1 Search for an optical nova counterpart . . . . . . . . . . . . . . . . . . . . . 92
5.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
5.3.1 Globular cluster nova rate - the optical point of view . . . . . . . . . . . . . 96
5.3.2 Globular cluster nova rate - the X-ray point of view . . . . . . . . . . . . . . 96
5.3.3 Implications on the nova rate in M 31 globular clusters . . . . . . . . . . . . 97
6 Discussion of the sample of M 31 novae with X-ray counterpart 99
6.1 Short-term variable supersoft X-ray light curves of novae . . . . . . . . . . . . . . . 99
6.1.1 M31N 2006-04a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
6.1.2 M31N 2007-12b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
6.2 Novae with long supersoft X-ray source states . . . . . . . . . . . . . . . . . . . . . 100
6.2.1 Two recurrent novae with a very long supersoft X-ray source phase? . . . . . 100
6.2.2 Six novae with long supersoft X-ray source states . . . . . . . . . . . . . . . 101
6.3 Correlations between nova parameters . . . . . . . . . . . . . . . . . . . . . . . . . 102
6.3.1 Supersoft X-ray source turn-on time vs turn-off time . . . . . . . . . . . . . 103
6.3.2 Effective blackbody temperature vs X-ray time scales . . . . . . . . . . . . . 104
6.3.3 Optical decay time vs supersoft X-ray source turn-on time . . . . . . . . . . 105
6.3.4 Optical expansion velocity vs supersoft X-ray source turn-on time . . . . . . 105
6.3.5 Physical interpretations - a note of caution . . . . . . . . . . . . . . . . . . . 106
6.4 Derived nova parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
6.5 Novae with short supersoft X-ray source states and the completeness of the X-ray
monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
6.6 Nova population study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
6.7 Asymmetric distribution of novae in X-rays . . . . . . . . . . . . . . . . . . . . . . 113
7 Summary and conclusions 115
A Discoveries of X-ray transients in M 31 i
B The curious case of the quasar Sharov 21 vii
C Optical light curves from the M 31 centre monitoring xi
Acknowledgements xxixChapter 1
Introduction
1.1 Classical Novae
Classical novae (CNe) originate from thermonuclear explosions on the surface of white dwarfs (WDs)
in cataclysmic binary systems. This section describes the physics of the nova phenomenon, its op-
tical and X-ray properties, and highlights the special role of the Andromeda galaxy (M 31) in nova
research.
1.1.1 Physics of Classical Novae
The host systems of CNe constitute a subclass of cataclysmic variables (CVs), which are close inter-
acting binary star systems (Bode & Evans 2008). The primary component in a CV is a WD. WDs are
the final evolutionary stages of stars with initial masses of. 8..10 M (see e.g. Truran & Livio 1986,⊙
and references therein). The secondary component is typically a main-sequence star or a red giant
star, which fills its gravitational Roche-lobe (Bode & Evans 2008). This leads to the transfer of matter
from the secondary to the WD. If the magnetic field of the WD is not strong, in contrast to AM Her
or polar systems, this matter forms an accretion disk around the WD due to conservation of angular
momentum. Friction induced energy loss in the accretion disk then causes the matter to settle on the
WD’s surface. These processes cause the luminosity of the CV to vary significantly. The variability
manifests itself in different observational characteristics for different subtypes of CVs, dependent on
the masses and separation of the binary stars and the magnetic field of the WD (see Warner 1995, for
an extensive review on CVs).
In the case of CNe, the transferred hydrogen-rich matter eventually accumulates on the surface of
the WD until hydrogen ignition starts a thermonuclear runaway in the degenerate matter of the WD
envelope. The resulting expansion of the hot envelope (see Fig. 1.1) causes the optical brightness of
the WD to rise by∼ 12 magnitudes within a few days, and mass to be ejected at high velocities (see
Hernanz 2005; Warner 1995, and references therein).
1.1.2 Optical properties and classification
Novae stand out among the plethora of variable star types due to distinctive observational character-
istics, although it should be noted that no two novae show exactly the same properties (Bode & Evans
2008).
910 Chapter 1. Introduction
Figure 1.1: Artist’s impression of a nova occurring on a WD (left) in a close binary system (Credits:
Mark A. Garlick, http://www.space-art.co.uk).
The light curve of a star describes the evolution of its luminosity over time. Optical light curves
of CNe reflect the singular nature of the explosive outburst by showing a rapid rise of the star’s
luminosity to the maximum within days. This is followed by a slower decline on time scales of
weeks to years back to the pre-outburst luminosity. A schematic nova light curve is shown in Fig. 1.2.
Observed nova light curves from the optical monitoring of M 31 are given in Appendix C.
Based on the speed of the optical decline, light curves of CNe can be categorised into speed
classes, according to the scheme of Payne-Gaposchkin (1964). More precisely, measuring the time
the nova luminosity needs to decline by two magnitudes from maximum light, the so-called t time,2
allows the classification the nova as very fast (t < 10 days), fast (11− 25), moderately fast (26− 80),2
slow (81− 150), or very slow (151− 250) (Bode & Evans 2008).
Recently, Strope et al. (2010) introduced a new classification system for nova light curves based
on a catalogue of 93 light curves of Galactic novae. This system utilises the shape of the light curve
to distinguish seven basic classes, the prototypes of which are shown in Fig. 1.3 (figure 2 from Strope
et al. 2010). In the context of this classification scheme a further specification of the light curve can
be given using its t time, which is defined in a similar way as the t time described above.3 2
While an optical light curve with outburst-like features already gives hints of the underlying nova
phenomenon, an optical spectrum allows to classify a nova candidate as a CN. The strongest emission
lines in nova spectra are Balmer lines of hydrogen (Williams 1992). Indeed, surveys for novae have
made use of the fact that novae are bright sources in observations taken with Hα filter, where they are