Emitters of VHE {γ-radiation [gamma-radiation] as revealed by the H.E.S.S. galactic plane survey [Elektronische Ressource] / put forward by Stefan Hoppe

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Dissertationsubmitted to theCombined Faculties for the Natural Sciences and for Mathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural SciencesPut forward byDipl.-Phys.: Stefan HoppeBorn in: Cologne, GermanyOral examination: 3.12.2008Emitters of VHE γ-radiation as revealed by the H.E.S.S.Galactic plane surveyReferees: Prof. Dr. Werner HofmannProf. Dr. Heinz V¨olkAbstractThe High Energy Stereoscopic System (H.E.S.S.) – an array of four Imaging AtmosphericCherenkov Telescopes located in the Khomas highlands of Namibia – explores the non-thermal universe by means of very-high-energy (VHE;>100GeV)γ-ray emission. Its highsensitivity and large field-of-view make it the ideal instrument for a survey of the MilkyWay in VHE γ-rays. The H.E.S.S. Galactic Plane Scan resulted in the most completeview of our Galaxy achieved so far in this energy regime. The results of this extensivesurvey will be presented in this work. From the 45 VHE γ-ray sources detected so farwithin the Galactic Plane, a short summary is given for the 19 sources detected withinthe framework of this thesis and three representative sources were analyzed in even moredetail: HESS J1912+101 was detected in the vicinity of an energetic pulsar, suggestinga pulsar wind nebula origin of the γ-ray emission. A clear signal of VHE γ-ray emissionwas also detected from RCW 86, a well-known shell-type supernova remnant.

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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
Put forward by
Dipl.-Phys.: Stefan Hoppe
Born in: Cologne, Germany
Oral examination: 3.12.2008Emitters of VHE γ-radiation as revealed by the H.E.S.S.
Galactic plane survey
Referees: Prof. Dr. Werner Hofmann
Prof. Dr. Heinz V¨olkAbstract
The High Energy Stereoscopic System (H.E.S.S.) – an array of four Imaging Atmospheric
Cherenkov Telescopes located in the Khomas highlands of Namibia – explores the non-
thermal universe by means of very-high-energy (VHE;>100GeV)γ-ray emission. Its high
sensitivity and large field-of-view make it the ideal instrument for a survey of the Milky
Way in VHE γ-rays. The H.E.S.S. Galactic Plane Scan resulted in the most complete
view of our Galaxy achieved so far in this energy regime. The results of this extensive
survey will be presented in this work. From the 45 VHE γ-ray sources detected so far
within the Galactic Plane, a short summary is given for the 19 sources detected within
the framework of this thesis and three representative sources were analyzed in even more
detail: HESS J1912+101 was detected in the vicinity of an energetic pulsar, suggesting
a pulsar wind nebula origin of the γ-ray emission. A clear signal of VHE γ-ray emission
was also detected from RCW 86, a well-known shell-type supernova remnant. And finally,
HESS J1708-441 was discovered in the vicinity of the well known pulsar PSR B1706-44,
but cannot be clearly associated with this strong counterpart. As the energy spectra of
thesesourcesyieldmostvaluableinformationabouttheprocessesatworkintheunderlying
astrophysical objects, extensive studies of the spectral extraction method were performed
to ensure their correct determination.
Kurzfassung
H.E.S.S. ist ein Experiment zur Erforschung des nicht-thermischen Universums duch die
Detektion sehr hochenergetischer Gamma-Strahlung. H.E.S.S. besteht aus vier abbilden-
den Cherenkov Teleskopen im Khomas Hochland von Namibia. Die hohe Sensitivita¨t und
das grosses Gesichtsfeld bef¨ahigen H.E.S.S. zu einer grossfl¨achigen Durchmusterung der
Galaktischen Ebene. Dieses Durchmusterung resultierte in der bisher vollsta¨ndigsten Kar-
tographierung unserer Galaxie in dem Energiebereich zwischen 100GeV und 100TeV. Die
Ergebnisse werden in dieser Abhandlung vorgestellt. Von der grossen Anzahl an Quellen
– 45 an der Zahl – werden die 19 Quellen, welche im Rahmen dieser Arbeit entdeckt wur-
den, naeher beschrieben und drei representative Quellen gesondert analysiert: Zum einen
HESS J1912+101, eine Quelle, die in der N¨ahe eines hochenergetischen Pulsares entdeckt
wurde. Die positionelle Koinzidenz legt eine Assoziation mit einem Pulsar-Wind-Nebel
(PWN) nahe. Ebenfalls na¨her untersucht wird die Emission hochenergetischer Gamma-
Strahlung von dem bekannten Supernovau¨berrest RCW 86. Als letztes vorgestellt wird
HESS J1708-44, eine Quelle in der Umgebung des bekannten Pulsaren PSR B1706-44, die
trotz des suggestiven Gegenstu¨cks keine klare Zuordnung erlaubt. Da die Energiespektren
der obengenannten Quellen wertvolle Informationen u¨ber die zugrundeliegenden Prozesse
innerhalb der assoziierten Objekte liefern, werden zusa¨tzlich die Methoden der Spektrums-
bestimmung im Detail beschrieben und ausfu¨hrlichen Tests unterzogen.Contents
1 Astrophysics Overview 5
1.1 Shock Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.1 Diffusive Shock Acceleration . . . . . . . . . . . . . . . . . . . . . . 7
1.1.2 Relativistic Shock Acceleration . . . . . . . . . . . . . . . . . . . . . 10
1.1.3 Maximum Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
1.2 Radiation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1.2.1 Hadronic Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3 Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.3.1 Supernova Remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.3.2 Pulsars and Pulsar Wind Nebulae . . . . . . . . . . . . . . . . . . . 26
2 H.E.S.S. and its Data Analysis Scheme 37
2.1 Air Showers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.2 Imaging Atmospheric Cherenkov Technique . . . . . . . . . . . . . . . . . . 41
2.3 The H.E.S.S. Experiment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
2.4 The Standard Data Analysis Scheme . . . . . . . . . . . . . . . . . . . . . . 45
2.5 Basic Performance of H.E.S.S. . . . . . . . . . . . . . . . . . . . . . . . . . 50
3 Spectral Analysis 53
3.1 Effective Gamma-Ray Collection Area . . . . . . . . . . . . . . . . . . . . . 54
3.2 Probability Density Function . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.3 Energy Threshold. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.4 Differential Gamma-Ray Spectrum: Method A . . . . . . . . . . . . . . . . 60
3.5 Differential Gamma-Ray Spectrum: Method B . . . . . . . . . . . . . . . . 63
3.6 Systematic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6.1 Toy Model MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
3.6.2 Real Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
4 The H.E.S.S. Galactic Plane Survey 77
4.1 The General Principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2 The Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2.1 Observation Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
4.2.2 Zenith Angle of Observations . . . . . . . . . . . . . . . . . . . . . . 79
4.3 The Survey Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.3.1 Processing the Individual Observations. . . . . . . . . . . . . . . . . 83
4.3.2 Combination of Run-wise Information . . . . . . . . . . . . . . . . . 83
iCONTENTS
4.3.3 Detection of Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
4.4 Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.4.1 Source Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
4.4.2 Flux Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
4.4.3 Source Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . 92
4.5 New Galactic VHE γ-ray Sources . . . . . . . . . . . . . . . . . . . . . . . . 97
4.5.1 Pulsar Wind Nebulae and Plerions . . . . . . . . . . . . . . . . . . . 97
4.5.2 Supernova Remnants . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
4.5.3 Unidentified Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.5.4 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5 Individual Source Analysis 113
5.1 HESS J1912+101 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
5.1.1 Observations and Results . . . . . . . . . . . . . . . . . . . . . . . . 114
5.1.2 Possible Associations . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.1.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.2 RCW 86 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.2.2 Analysis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.2.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
5.2.4 The Morphology Analysis of RCW 86 . . . . . . . . . . . . . . . . . 127
5.2.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
5.2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
5.3 HESS J1708-442 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
5.3.2 H.E.S.S. observations and analysis methods . . . . . . . . . . . . . . 135
5.3.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
5.3.4 Possible Counterparts . . . . . . . . . . . . . . . . . . . . . . . . . . 138
5.3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
6 Conclusions and Outlook 147
A The H.E.S.S. Galactic Plane Survey: More Details 151
A.1 Additional Significance and Flux Maps . . . . . . . . . . . . . . . . . . . . . 152
A.2 Sources and their References . . . . . . . . . . . . . . . . . . . . . . . . . . 156
iiList of Figures
1.1 Structure of a strong, modified shock . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Klein-Nishina scattering cross section . . . . . . . . . . . . . . . . . . . . . . 13
1.3 Synchrotron and invere Compton spectrum in SED represention . . . . . . 14
1.4 Leptonic model SEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
1.5 Hadronic model SEDs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
1.6 Radius and velocity of the SNR forward shock front . . . . . . . . . . . . . 23
1.7 Maximum momentum of accelerated protons . . . . . . . . . . . . . . . . . 25
1.8 CR energy in and TeV γ-ray flux from a SNR . . . . . . . . . . . . . . . . . 26
1.9 Model of the structure of the Crab PWN . . . . . . . . . . . . . . . . . . . 28
1.10 Energy composite image of the Crab nebula . . . . . . . . . . . . . . . . . . 29
1.11 Post-shock flux velocity and magnetic field within the Crab nebula . . . . . 30
1.12 Characteristic of the γ-ray emission processes within a PWN . . . . . . . . 32
1.13 Smoothed VHE γ-ray excess map of HESS J1825-137 . . . . . . . . . . . . . 34
2.1 Cherenkov emission of simulated air showers . . . . . . . . . . . . . . . . . . 40
2.2 Sketch of the IACT’s imaging geometry . . . . . . . . . . . . . . . . . . . . 42
2.3 The H.E.S.S. telescope array . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.4 One of the H.E.S.S. telescopes . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.5 Hillas parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.6 MRSW and MRSL distributions . . . . . . . . . . . . . . . . . . . . . . . . 48
2.7 Geometry on the on- and off-regions used for background estimation . . . . 49
2.8 Basic performance parameters of the H.E.S.S. experiment . . . . . . . . . . 52
3.1 Effective areas for true and reconstructed energy, for different zenith angles 56
3.2 Effective areas for different offsets and azimuth angles . . . . . . . . . . . . 57
3.3 Effective areas for different selection cuts; Energy resolution function . . . . 58
3.4 Differential γ-ray rate; Energy reconstruction bias . . . . . . . . . . . . . . 59
3.5 Analysis theshold for different zenith angles and selection cuts. . . . . . . . 60
3.6 Illustration of the various steps in spectral reconstruction Method A . . . . 62
3.7 Illustration of the various steps in spectral reconstruction Method B . . . . 66
3.8 Spectral reconstruction performance: Set 1 and 2 . . . . . . . . . . . . . . . 69
3.9 Spectral reconstruction performance: Set 3 . . . . . . . . . . . . . . . . . . 70
3.10 Spectral reconstruction performance: Set 3, pull-distribtions . . . . . . . . . 71
3.11 Spectral reconstruction performance: Set 4 and 5 . . . . . . . . . . . . . . . 72
3.12 Spectral reconstruction performance: Set 6 . . . . . . . . . . . . . . . . . . 73
3.13 Differential energy spectra of HESS J1745-290 and HESS J1846-029 . . . . 74
iiiLIST OF FIGURES
4.1 Illustration of the observation pointings within the survey region . . . . . . 78
4.2 Observability of the Galactic plane; Zenith angle distribution . . . . . . . . 80
4.3 Sensitivity map of the survey region . . . . . . . . . . . . . . . . . . . . . . 81
4.4 Exposure, energy threshold and sensitivity within the Galactic plane . . . . 82
4.5 Significance map of the H.E.S.S. Galactic plane survey region . . . . . . . . 86
4.6 Flux map and spectral analysis of the Crab nebula . . . . . . . . . . . . . . 90
4.7 Flux map of the H.E.S.S. Galactic plane survey region . . . . . . . . . . . . 91
4.8 Position parameters of all sources within the survey region . . . . . . . . . . 92
4.9 Spectral parameters of all sources within the survey region . . . . . . . . . . 93
4.10 Spectral and size parameters of all sources within the survey region . . . . . 94
4.11 Image of HESS J1356-645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
4.12 Image of HESS J1718-385 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.13 Image of HESS J1809-193 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.14 Image of HESS J1833-105 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
4.15 Image of HESS J1857+026 . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.16 Image of HESS J1800-240 / HESS J1801-233 . . . . . . . . . . . . . . . . . 103
4.17 Image of HESS J1714-385 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
4.18 Image of HESS J1731-347 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
4.19 Images of the “dark” sources . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.20 Image of HESS J1908+063 . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.21 Image of HESS J1023-575 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4.22 Image of HESS J1848-018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
5.1 Image of the VHE γ-ray excess from HESSJ1912+101 . . . . . . . . . . . . 115
5.2 Spitzer image of the vicinity of HESS J1912+101 . . . . . . . . . . . . . . . 116
5.3 GRS image of the vicinity of HESS J1912+101 . . . . . . . . . . . . . . . . 117
135.4 CO(J=1→0) velocity profile of the vicinity of HESS J1912+101 . . . . . . 118
5.5 Differential energy spectrum of HESSJ1912+1011 . . . . . . . . . . . . . . 119
5.6 H.E.S.S.γ-ray image of RCW 86 . . . . . . . . . . . . . . . . . . . . . . . . 123
5.7 H.E.S.S. and XMM radial profiles of RCW 86 . . . . . . . . . . . . . . . . . 124
5.8 H.E.S.S. and XMM azimuth profiles of RCW 86 . . . . . . . . . . . . . . . 125
5.9 XMM X-ray image of RCW 86 . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.10 Differential energy spectrum of RCW 86 . . . . . . . . . . . . . . . . . . . . 127
5.11 Sketch of a radial profile of a shell . . . . . . . . . . . . . . . . . . . . . . . 128
5.12 RCW 86 shell fit: Fitted map, fit results and residuals . . . . . . . . . . . . 130
5.13 Radio data G343.1−2.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
5.14 Sketch of G343.1−2.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.15 Smoothed VHE γ-ray excess from HESS J1708−443 . . . . . . . . . . . . . 136
5.16 Uncorrelated VHE γ-ray excess from HESS J1708−443 . . . . . . . . . . . . 137
5.17 Differential energy spectrum of HESS J1708−443 . . . . . . . . . . . . . . . 139
5.18 Overview map of the region containing HESS J1708−443 . . . . . . . . . . 140
5.19 HI line emission intensity (SGPS) of the vicinity of HESS J1708−443 . . . . 143
5.20 Velocity profile of HI line emission intensity . . . . . . . . . . . . . . . . . . 144
5.21 Spectral energy distribution (HESS J1708−443) . . . . . . . . . . . . . . . . 145
6.1 Simulated view of the Galactic plane with CTA . . . . . . . . . . . . . . . . 148
iv