Pulsar wind nebulae with H.E.S.S. [Elektronische Ressource] : establishing a connection between high-power pulsars and very-high-energy gamma-ray sources / presented by Svenja Carrigan

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Dissertationsubmitted to theCombined Faculties for the Natural Sciences and forMathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural Sciencespresented byDipl.-Phys. Svenja Carriganborn in Dortmund, GermanyOral examination: 25 July 2007Pulsar Wind Nebulae withH.E.S.S.:Establishing a Connection betweenhigh-power Pulsars and very-high-energygamma-ray SourcesReferees: Prof. Dr. Werner HofmannProf. Dr. Thomas LohseAbstractPulsars energise particles into lighthouse pencil beams and create extendedrelativisticoutflows,pulsarwindnebulae(PWNe). Inthevery-high-energy(VHE)gamma-ray wave band, these PWNe represent to date the most populous class ofGalactic sources. Nevertheless, the details of the energy conversion mechanismsin the vicinity of pulsars are not well understood, nor is it known which pulsarsare able to drive PWNe and emit high-energy radiation. Due to its large field ofview and unprecedented sensitivity, H.E.S.S. is the first instrument to allow fordeep surveys of the Galactic plane in VHE gamma rays. This work presents thefirst ever systematic investigation of the connection of VHE gamma-ray sourcesandPWNe.

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Published 01 January 2007
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Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for
Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Dipl.-Phys. Svenja Carrigan
born in Dortmund, Germany
Oral examination: 25 July 2007Pulsar Wind Nebulae with
H.E.S.S.:
Establishing a Connection between
high-power Pulsars and very-high-energy
gamma-ray Sources
Referees: Prof. Dr. Werner Hofmann
Prof. Dr. Thomas LohseAbstract
Pulsars energise particles into lighthouse pencil beams and create extended
relativisticoutflows,pulsarwindnebulae(PWNe). Inthevery-high-energy(VHE)
gamma-ray wave band, these PWNe represent to date the most populous class of
Galactic sources. Nevertheless, the details of the energy conversion mechanisms
in the vicinity of pulsars are not well understood, nor is it known which pulsars
are able to drive PWNe and emit high-energy radiation. Due to its large field of
view and unprecedented sensitivity, H.E.S.S. is the first instrument to allow for
deep surveys of the Galactic plane in VHE gamma rays. This work presents the
first ever systematic investigation of the connection of VHE gamma-ray sources
andPWNe. Besides presentingtwonewcandidatePWNedetected inthissearch,
it is shown that pulsars with large spin-down energy flux are indeed with high
probability associated with VHE gamma-ray sources, implying the existence of
an efficient mechanism by which a large fraction of pulsar spin-down energy is
converted into kinetic energy of particles. The results presented here make it
very likely that future more sensitive VHE gamma-ray instruments will detect a
rapidly increasing number of lower-luminosity PWNe.
Kurzfassung
Wie kosmische Leuchttu¨rme strahlen Pulsare energiegeladene Teilchen ent-
lang ihrer Magnetfeldachsen ab und erzeugen ausgedehnte relativistische Pulsar-
Wind-Nebel (PWN). Im Wellenl¨angenbereich der sehr hochenergetischen
Gamma-StrahlungsindPWNmomentandiezahlenm¨aßigdominanteKlassegalak-
tischerQuellen. TrotzdemsinddieMechanismen derEnergieumwandlung inPul-
sarenschlecht verstanden undesistnichtbekannt, welche PulsarePWNundsehr
hochenergetische Strahlung erzeugen k¨onnen. Aufgrund des großen Gesichtsfelds
und der beispiellosen Sensitivit¨at ist H.E.S.S. das erste Experiment, fu¨r welches
eineDurchmusterungu¨bergroßeBereichedergalaktischenEbeneimBereichsehr
hochenergetischer Gamma-Strahlung m¨oglich ist. Diese Arbeit berichtet von der
ersten systematischen Erforschung des Zusammenhangs zwischen Quellen sehr
hochenergetischer Gamma-Strahlung und PWN. Neben der Pr¨asentation zweier
neuer PWN Kandidaten, die im Rahmen dieser Suche gefunden wurden, wird
gezeigt, daß Pulsare mit hohem Spin-Down-Energiefluß tatsa¨chlich mit hoher
Wahrscheinlichkeit mit Quellen sehr hochenergetischer Gamma-Strahlung ver-
bunden sind. Das setzt die Existenz eines effizienten Mechanismus voraus, der
einen großen Teil der Spin-Down-Energie des Pulsars in kinetische Energie von
Teilchen umwandelt. Diese Ergebnisse suggerieren, daß zuku¨nftige, sensitivere
Instrumente zur Detektion sehr hochenergetischer Gamma-Strahlung eine rasch
anwachsende Zahl PWN niedrigerer Luminosit¨at entdecken werden.Contents
1 Introduction 1
2 Pulsars and Pulsar Wind Nebulae 5
2.1 Neutron Stars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3 Pulsar Wind Nebulae . . . . . . . . . . . . . . . . . . . . . . . . . 13
2.3.1 Formation of PWNe, Acceleration and Radiation Processes 13
2.3.2 PWN in an evolving SNR . . . . . . . . . . . . . . . . . . 15
2.4 H.E.S.S. Observations of PWNe . . . . . . . . . . . . . . . . . . . 21
3 H.E.S.S. and the Imaging Atmospheric Cherenkov Technique 27
3.1 Air Showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
3.2 The Imaging Atmospheric Cherenkov Technique . . . . . . . . . . 31
3.3 The H.E.S.S. Experiment . . . . . . . . . . . . . . . . . . . . . . . 33
3.4 The H.E.S.S. Analysis . . . . . . . . . . . . . . . . . . . . . . . . 36
3.4.1 Data Quality and systematic Uncertainties . . . . . . . . . 36
3.4.2 The standard H.E.S.S. Analysis . . . . . . . . . . . . . . . 39
3.4.3 Background Estimation. . . . . . . . . . . . . . . . . . . . 41
3.4.4 Energy Reconstruction, Flux Measurements and Spectrum 43
4 Data Set, Pulsar Sample and First Results 45
4.1 H.E.S.S. Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
4.2 Pulsar Data Set: ATNF & PMPS . . . . . . . . . . . . . . . . . . 48
4.3 First Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
5 HESS J1718 – 385 57
5.1 Observations and Analysis . . . . . . . . . . . . . . . . . . . . . . 57
5.2 Possible Associations . . . . . . . . . . . . . . . . . . . . . . . . . 64
6 HESS J1809 – 193 67
6.1 Observations and Analysis . . . . . . . . . . . . . . . . . . . . . . 67
6.2 Possible Associations . . . . . . . . . . . . . . . . . . . . . . . . . 70
iCONTENTS
7 Systematic Search for the VHE γ-ray Emission of Pulsars 73
7.1 PMPS Pulsars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
7.2 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
7.3 Significance of PMPS Pulsar Associations with VHE γ-ray Sources 78
7.4 Check of the Simulation: additional Selection of Pulsars . . . . . . 84
Conclusion 87
A The Smart Pixel Camera 89
A.1 Layout of the Smart Pixel Camera . . . . . . . . . . . . . . . . . 90
A.2 Testing the Prototype . . . . . . . . . . . . . . . . . . . . . . . . 94
A.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
Danksagung 109
iiList of Figures
2.1 Optical image of the Crab Nebula . . . . . . . . . . . . . . . . . . 5
2.2 Schematic of a Neutron Star . . . . . . . . . . . . . . . . . . . . . 8
2.3 Schematic of a Pulsar . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.4 Models for high-energy emission from pulsars. . . . . . . . . . . . 12
2.5 Schematics of a PWN in an SNR . . . . . . . . . . . . . . . . . . 14
2.6 Spectral energy distribution for a PWN . . . . . . . . . . . . . . . 16
2.7 Simulation PWN in SNR - supersonic expansion . . . . . . . . . . 18
2.8 Simulation PWN in SNR - reverse shock interaction . . . . . . . . 19
2.9 Simulation PWN in SNR - subsonic expansion . . . . . . . . . . . 19
2.10 Simulation PWN in SNR - bow shock . . . . . . . . . . . . . . . . 20
2.11 PWNe detected by H.E.S.S. . . . . . . . . . . . . . . . . . . . . . 21
2.12 H.E.S.S. and X-ray observations of MSH 15−52 . . . . . . . . . . 22
2.13 H.E.S.S. and X-ray observations of HESS J1825−137 . . . . . . . 23
2.14 H.E.S.S. and X-ray observations of Vela X . . . . . . . . . . . . . 24
2.15 H.E.S.S. and X-ray observations of the Kookaburra . . . . . . . . 25
3.1 Shower model according to Heitler. . . . . . . . . . . . . . . . . . 29
3.2 Simulated Cherenkov light distributions: γ-ray / proton . . . . . . 31
3.3 The Imaging Atmospheric Cherenkov Technique . . . . . . . . . . 32
3.4 Schematic and real H.E.S.S. camera image . . . . . . . . . . . . . 33
3.5 The H.E.S.S. Telescope Array . . . . . . . . . . . . . . . . . . . . 33
3.6 The Milky Way in infrared, optical and VHE γ-rays . . . . . . . . 36
3.7 System trigger rate vs time. . . . . . . . . . . . . . . . . . . . . . 37
3.8 Example for optical efficiency correction on spectrum . . . . . . . 39
3.9 Hillas parameters for a γ-ray image . . . . . . . . . . . . . . . . . 40
3.10 Background regions for different background estimation methods . 42
4.1 Search: H.E.S.S. significance map and exclusion regions . . . . . . 46
4.2 Search: H.E.S.S. significance distributions . . . . . . . . . . . . . 47
4.3 Search: H.E.S.S. exposure map . . . . . . . . . . . . . . . . . . . 48
4.4 ATNF & PMPS search sample pulsars in the Milky Way . . . . . 49
4.5 PMPS: All & search sample: distribution in Gal. coordinates . . . 50
2 2˙ ˙4.6 PMPS search sample: E/d distribution & E/d vs Galactic lat. . 51
iiiLIST OF FIGURES
2˙4.7 PMPS search sample: Distribution of E/d vs Age . . . . . . . . 51
4.8 PMPS search sample: Different distance estimates . . . . . . . . . 52
4.9 Search: H.E.S.S. significance & exposure map with PMPS pulsars 53
5.1 HESS J1718−385: Other sources in the field of view . . . . . . . . 58
5.2 HESS J1718−385: Excess map . . . . . . . . . . . . . . . . . . . . 59
5.3 HESS J1718−385: Slice HESS J1718−385 – RX J1713.7−3946 . . 60
5.4 HESS J1718−385: Background regions, reflected background . . . 61
5.5 HESS J1718−385: Zenith dist. for reflected and on/off background 62
5.6 HESS J1718−385: Offset dist. for reflected and on/off background 62
5.7 HESS J1718−385: Spectrum . . . . . . . . . . . . . . . . . . . . . 63
5.8 HESS J1718−385: Multiwavelength data . . . . . . . . . . . . . . 66
6.1 HESS J1809−193: Excess map . . . . . . . . . . . . . . . . . . . . 68
6.2 HESS J1809−193: Slice through HESS J1809−193. . . . . . . . . 69
6.3 HESS J1809−193: Spectrum . . . . . . . . . . . . . . . . . . . . . 70
6.4 HESS J1809−193: Multiwavelength data . . . . . . . . . . . . . . 72
2˙7.1 PMPS search sample: E/d distribution of all & detected . . . . . 74
2˙7.2 PMPS search sample: Distribution Gal. coord. & E/d vs Gal. lat. 76
7.3 PMPS search sample: Modelling of parent population . . . . . . . 77
2˙7.4 E/d dist. of all & detected PMPS and detected random “pulsars” 78
7.5 Fraction of detected PMPS pulsars & chance probability . . . . . 79
2˙7.6 VHE γ-ray flux vs E/d . . . . . . . . . . . . . . . . . . . . . . . 81
7.7 Size VHE γ-ray emission vs pulsar age . . . . . . . . . . . . . . . 82
7.8 Offset pulsar – VHE γ-ray emission vs pulsar age . . . . . . . . . 83
7.9 Offset pulsar – VHE γ-ray emission vs Size VHE γ-ray emission . 83
2˙7.10 E/d dist.: All & det. PMPS and det. random “pulsars”, with cuts 85
7.11 Fraction of detected PMPS pulsars & chance prob., with cuts . . 86
A.1 Smart Pixel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
A.2 SPC: Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . 91
A.3 SPC: Signal integration . . . . . . . . . . . . . . . . . . . . . . . . 92
A.4 SPC: Prototype . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
A.5 SPC: Single-p.e. spectrum . . . . . . . . . . . . . . . . . . . . . . 96
A.6 SPC: Amplitude resolution . . . . . . . . . . . . . . . . . . . . . . 97
A.7 SPC: Noise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
A.8 SPC: Resolution of time measurement . . . . . . . . . . . . . . . 99
A.9 SPC: Trigger threshold . . . . . . . . . . . . . . . . . . . . . . . . 100
A.10SPC: Dead time calculation . . . . . . . . . . . . . . . . . . . . . 101
iv