A systematic study of supernova remnants as seen with H.E.S.S. [Elektronische Ressource] / presented by Anne Bochow

Sujets

• Physik

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2011
• Nombre de lectures: 10
• Langue: English
• Poids de l'ouvrage: 6 Mo

Extrait

Dissertation
submitted to the
Combined Faculties of the Natural Sciences and Mathematics
of the
Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Dipl.-Phys. Anne Bochow
born in Eberswalde-Finow
Oral examination: 3rd February 2011A systematic study of
Supernova Remnants
as seen with H.E.S.S.
Referees: Prof. Dr. Werner Hofmann
Prof. Dr. Heinz V olkAbstract
Supernova remnants (SNRs) are the remainders of extremely energetic explosions occurring
at the end of a star’s life. With the energy released during the supernova explosion they
15are believed to accelerate charged particles to energies of up to 10 eV. In the very-high-
11energy (VHE, > 10 eV)-ray band, SNRs represent one of the most populous classes of
Galactic sources. Due to its unprecedented sensitivity, H.E.S.S. was the rst instrument to
allow for a morphological resolution of individual SNRs, proving the existence of particle
acceleration and subsequent VHE -ray emission. However, to date many more SNRs are
known in the radio waveband than in VHE-rays. This work presents a systematic study
of the VHE -ray signal of a sample of around 200 radio SNRs. The VHE -ray-signal
of these SNRs is studied individually. Besides the spatial correlation of radio SNRs and
VHE -ray sources, the measured ux of VHE -rays is compared to theoretical ux pre-
dictions. These predictions are based on assumptions of the total explosion energy, the
particle acceleration e ciency, the density of the surrounding medium and the distance
of the SNRs. The results presented here suggest that these parameters can vary strongly
for individual SNRs. Future observations of SNRs in VHE -rays and other wavebands
will help to constrain the parameter space and will allow to further discuss acceleration
mechanisms in SNRs.
Kurzfassung
Das Leben eines massiven Sterns endet in einer Supernovaexplosion. Dabei werden gro e
Massen von Materie in das interstellare Medium geschleudert und gewaltige Energiemen-
gen freigesetzt. Vom verbleibenden Supernovaub errest wird weithin angenommen, dass
15dort geladene Teilchen auf Energien von bis zu 10 eV beschleunigt werden k onnen. Im
11Wellenl angenbereich der sehr hochenergetischen -Strahlung (> 10 eV) geh oren Super-
novaub erreste zu den zahlenm a ig dominanten Klassen galaktischer Quellen. Dank der
herausragenden Emp ndlichkeit des H.E.S.S.-Detektors konnte die Morphologie mehrerer
Supernovaub erreste aufgel ost werden und somit ein Zusammenhang zwischen Teilchenbe-
schleunigung in Supernovaub erresten und sehr hochenergetischer -Strahlung hergestellt
werden. Dennoch sind im Radiowellenl angenbereich wesentlich mehr Supernovaub erreste
bekannt als bisher im -Bereich detektiert wurden. In der vorliegenden Arbeit wird er-
stmals eine systematische Untersuchung von etwa 200 im Radiobereich bekannten Su-
pernovaub erresten prasentiert. Der gemessene hochenergetische -Fluss wird mit einem
theoretisch ermittelten Wert verglichen, der auf Annahmen der Gesamtenergie der Su-
pernovaexplosion, der Beschleunigungse zienz der Teilchen im Supernovaub errest, der
Umgebungsdichte und der Entfernung basiert. Die Ergebnisse dieser Studie verdeutlichen
die Abweichung dieser Parameter vom angenommen Wert fur viele Uberreste. Zukunftige
Beobachtungen im sehr hochenergetischen-Bereich und anderen Wellenl angen k onnen da-
her einen wichtigen Beitrag liefern, die Parameter einzuschranken und Beschleunigungsmech-
anismen in Supernovaub erresten genauer zu untersuchen.Contents
1 Introduction 1
2 The H.E.S.S. telescope system 5
2.1 The IACT technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 The H.E.S.S. telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1 Mount and dish . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.2 Camera . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.3 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3 The H.E.S.S. central trigger system 11
3.1 Trigger system phase I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.2 T phase II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.3 Tests for the central trigger upgrade . . . . . . . . . . . . . . . . . . . . . . 23
3.3.1 Stability at high frequencies . . . . . . . . . . . . . . . . . . . . . . . 23
3.3.2 Acceptance of mono and stereo events . . . . . . . . . . . . . . . . . 24
3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
4 The H.E.S.S. data analysis 27
4.1 Image parametrisation and direction reconstruction . . . . . . . . . . . . . . 27
4.2 Event selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4.3 Background estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.3.1 Ring background method . . . . . . . . . . . . . . . . . . . . . . . . 32
4.3.2 Re ected background method . . . . . . . . . . . . . . . . . . . . . . 32
4.3.3 Signi cance determination . . . . . . . . . . . . . . . . . . . . . . . . 32
4.4 Energy reconstruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.5 Spectrum determination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.5.1 E ective detection area . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.5.2 Flux calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
5 Supernovae and their remnants 35
5.1 Thermonuclear supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.2 Core-collapse supernovae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.3 Stages of a supernova . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.4 Supernova remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5.5 Particle acceleration in shock fronts . . . . . . . . . . . . . . . . . . . . . . 38
5.6 Production of -rays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.6.1 Hadronic interaction . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.6.2 Leptonic in . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
5.7 Theoretical -ray ux prediction . . . . . . . . . . . . . . . . . . . . . . . . 42
6 Supernova remnants in -rays 43
6.1 Green’s Catalog of Radio SNRs . . . . . . . . . . . . . . . . . . . . . . . . . 43
6.1.1 SNR Radius . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
IContents
6.1.2 SNR Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
6.2 Observations with H.E.S.S. . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6.2.1 Analysis parameters for individual target analysis . . . . . . . . . . 50
6.2.2 Results of individual SNR analyses . . . . . . . . . . . . . . . . . . . 50
6.2.3 H.E.S.S. detections associated with SNRs . . . . . . . . . . . . . . . 54
6.2.4 dete without association with SNRs . . . . . . . . . . 66
6.2.5 SNRs far from known H.E.S.S. sources . . . . . . . . . . . . . . . . . 83
6.3 Comparison of experimental and theoretical -ray ux values . . . . . . . . 86
6.3.1 Comparison to HEGRA observations . . . . . . . . . . . . . . . . . . 91
6.3.2 Recommendation for further SNR observations . . . . . . . . . . . . 99
6.4 Spatial correlation of SNRs with H.E.S.S. . . . . . . . . . . . . . . . . . . . 103
6.4.1 Simulation of SNR sample . . . . . . . . . . . . . . . . . . . . . . . . 103
6.4.2 Distance to H.E.S.S. sources . . . . . . . . . . . . . . . . . . . . . . . 103
7 Conclusion and future perspectives 107
A Appendix 109
A.1 List of SNRs with known or calculated distance . . . . . . . . . . . . . . . . 109
A.2 Analysis results for all SNRs observed with H.E.S.S. . . . . . . . . . . . . . 111
List of Figures 115
List of Tables 119
Bibliography 121
II1 Introduction
Supernova explosions belong to the most violent processes in the universe, with energies
51 44released of up to 10 erg (=10 J). Astronomers all over the world have observed super-
novae already hundreds of years ago and studied their behavior. The documentation of
these observations provides detailed information about supernova explosions, predicting
a supernova to occur every few decades within our Galaxy. Most of them are obscured
by dust and so only for few the explosion date is known. These supernovae, observed
by Arabic, Chinese, Japanese and European astronomers in the years 1006, 1054, 1572
(documented by Tycho Brahe), and 1604 (documented by Johannes Kepler) are called
historical supernovae. The last supernova visible to the naked eye occurred in 1987 in the
Large Magellanic Cloud.
Following a supernova explosion a shock front expands into the interstellar medium.
It is widely believed that this shock front is a source of cosmic rays with energies of
15up to 10 eV. Travelling through the galaxy, the cosmic rays are de ected in turbulent
magnetic elds. Therefore their arrival directions are isotropically distributed, making
it impossible to backtrace their source of origin. The only way to visualize the sources
of cosmic rays is through the detection of electromagnetic radiation like -rays and of
neutral secondary particles like neutrinos, which are not de ected by magnetic elds.
The detection of cosmic neutrinos is very di cult and requires large detection volumes.
Electromagnet