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Eta Carinae Detection and Characterisation of the First η Carinae Detection and Characterisation of the First Colliding-Wind Binary in Very-High-Energy γ-rays with H.E.S.S. Doktorarbeit zur Erlangung des akademischen Grades Dr. rer. nat. in Astroteilchenphysik vorgelegt von Eva Leser (Master of Science) Institut für Physik und Astronomie Universität Potsdam 2018 This work is licensed under a Creative Commons License: Attribution – NonCommercial – NoDerivatives 4.0 International. This does not apply to quoted content from other authors. To view a copy of this license visit https://creativecommons.org/licenses/by-nc-nd/4.0/ 1. Gutachter : Prof. Dr. Christian Stegmann 2. Gutachter : Prof. Dr. Jörn Wilms 3. Gutachter : Prof. Dr. Marek Kowalski Datum des Einreichens der Arbeit: 20. 11. 2018 Published online at the Institutional Repository of the University of Potsdam: https://doi.org/10.25932/publishup-42814 https://nbn-resolving.org/urn:nbn:de:kobv:517-opus4-428141 Selbstständigkeitserklärung Ich versichere, dass ich die vorliegende Arbeit selbständig und nur mit den angegeben Quellen und Hilfsmitteln angefertigt habe. Alle Stellen der Arbeit, die ich aus diesen Quellen und Hilfsmitteln dem Wortlaut oder dem Sinne nach entnommen habe, sind kenntlich gemacht und im Literaturverzeichnis aufgeführt. Weiterhin versichere ich, dass ich die vorliegende Arbeit, weder in der vorliegenden, noch in einer mehr oder weniger abgewandelten Form, als Dissertation an einer anderen Hochschule eingereicht habe. Potsdam, Eva Leser −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− III Kurzfassung Das außergewöhnliche Doppelsternsystem η Carinae fasziniert WissenschaftlerInnen und BeobachterInnen auf der südlichen Erdhalbkugel seit hunderten Jahren. Nach einem Supernova-ähnlichem Ausbruch war η Carinae zeitweise der hellste Stern am Nachthimmel. Heute sind durch zahlreiche Beobachtungen, von Radiowellen bis zu Röntgenstrahlung, der Aufbau des Sternsystems und die Eigenschaften seiner Strahlung bis zu Energien von ∼ 50 keV gut erforscht. Das Doppelsternsystem besteht aus zwei massiven Sternen (∼ 30 und ∼ 100 Sonnenmassen Gewicht) mit starken Sternwinden, über die sie kontinuierlich einen Teil ihrer Masse verlieren. Wenn diese Sternwinde kollidieren, entsteht auf beiden Seiten ein Kompressionsschock der das Plasma in der Kollisionszone aufheizt, was sich in Röntgenstrahlung beobachten lässt. Bei Energien oberhalb von ∼ 50 keV ist der Ursprung der Strahlung nicht mehr thermisch: um ein Plasma auf die entsprechende Temperatur zu bringen, wird mehr mechanische Energie benötigt, als in den Sternwinden vorhanden. In hoch-energetischer γ-Strahlung ist η Carinae das einzige eindeutig detektierte Sternsystem seiner Art und sein Energiespek- trum reicht bis zu ∼ hundert GeV. Bodengebundene γ-Strahlungsexperimente haben in diesem Energiebereich den Vorteil von großen Detektorflächen. H.E.S.S. ist das einzige bodengebundene γ-Strahlungsexperiment auf der Südhalbkugel und somit in der Lage, η Carinae in diesen Energien zu beobachten. H.E.S.S. misst γ-Strahlung mit Hilfe der elektromagnetischen Teilchenschauer, die sehr hoch-energetische Photonen in der At- mosphäre auslösen. Die größte Herausforderung der Messung von η Carinae’s Strahlung mit H.E.S.S. ist die ultraviolette Strahlung des Carina Nebels, die zu einem Hintergrund führt, der bis zu zehn mal stärker ist als der Durchschnitt in H.E.S.S. In dieser Arbeit wird die erste Detektion eines Doppelsternsystems mit kollidierenden Sternwinden in sehr hoch-energetischer γ-Strahlung präsentiert und die Studien, die diese ermöglicht haben. Das differentielle γ-Strahlungsspektrum bis 700 GeV wird untersucht. Hadro- nische und leptonische Szenarios für den Ursprung der γ-Strahlung werden diskutiert und das hadronische Szenario wird aufgrund eines Vergleichs der Kühlzeiten bevorzugt. V Abstract The exceptional binary star η Carinae has been fascinating scientists and the people in the Southern hemisphere alike for hundreds of years. It survived an enormous outbreak, comparable to a supernova energy-wise, and for a short period became the brightest star of the night sky. From observations from the radio regime to X-rays the system’s characteristics and its emission in photon energies up to ∼ 50 keV are well studied today. The binary is composed of two massive stars of ∼ 30 and ∼ 100 solar masses. Either star drives a strong stellar wind that continuously carries away a fraction of its mass. The collision of these winds leads to a shock on each side of the encounter. In the wind- wind-collision region plasma gets heated when it is overrun by the shocks. Part of the emission seen in X-rays can be attributed to this plasma. Above ∼ 50 keV the emission is no longer of thermal origin: the required plasma temperature exceeds the available mechanical energy input of the stellar winds. In contrast to its observational history in thermal energies observational evidence of η Carinae’s non-thermal emission has only recently built up. In high-energy γ-rays η Carinae is the only binary of its kind that has been detected unambiguously. Its energy spectrum reaches up to ∼ hundred GeV, a regime where satellite-based γ-ray experiments run out of statistics. Ground-based γ-ray experiments have the advantage of large photon collection areas. H.E.S.S. is the only γ-ray experiment located in the Southern hemisphere and thus able to observe η Carinae in this energy range. H.E.S.S. measures γ-rays via electromagnetic showers of particles that very-high-energy γ-rays initiate in the atmosphere. The main challenge in observations of η Carinae with H.E.S.S. is the UV emission of the Carina nebula that leads to a background that is up to 10 times stronger than usual for H.E.S.S. This thesis presents the first detection of a colliding-wind binary in very-high-energy γ-rays and documents the studies that led to it. The differential γ-ray energy spectrum of η Carinae is measured up to 700 GeV. A hadronic and leptonic origin of the γ-ray emission is discussed and based on the comparison of cooling times a hadronic scenario is favoured. VII Contents 1 Introduction 1 2 The colliding-wind binary η Carinae 4 2.1 Observational history . 4 2.2 Colliding-wind binaries: Thermal and non-thermal emission . 7 2.3 The neighbourhood - the Carina Nebula . 9 2.4 η Carinae across the electromagnetic spectrum . 10 2.4.1 Radio, microwave and mm emission . 11 2.4.2 Infrared, optical and ultraviolet emission . 12 2.4.3 X-rays . 14 2.5 Gamma-ray observations . 16 2.6 Origin of γ-ray emission . 18 2.6.1 Particle acceleration in colliding-wind binaries . 18 2.6.2 Non-thermal emission processes . 20 2.6.3 Modelling the γ-ray emission in η Carinae . 22 3 Measuring VHE γ-rays from η Carinae with H.E.S.S. 24 3.1 Principle of γ-ray detection with IACTs . 25 3.2 The High Energy Stereoscopic System . 26 3.3 Calibration of raw data . 29 3.4 Air shower reconstruction and treatment of NSB . 33 3.4.1 Reconstruction with Hillas parametrization . 34 3.4.2 Reconstruction with Model templates . 36 3.5 Source detection and sky image generation . 43 3.6 Spectrum determination . 47 IX CONTENTS 4 H.E.S.S. observations of η Carinae 52 4.1 Properties of the data set . 53 4.2 Hillas analysis of η Carinae . 54 4.2.1 Analysis configurations . 54 4.2.2 Low-level checks . 56 4.2.3 Results . 58 4.3 Semi-analytical shower model analysis of η Carinae . 64 4.3.1 Analysis configurations . 65 4.3.2 Low-level checks . 66 4.3.3 Detection of η Carinae . 70 4.3.4 Studies of the PSF broadening under high NSB . 71 4.3.5 HOTS J1044-5957: A hint for a new source . 76 4.3.6 NSB investigations . 80 4.3.7 The γ-ray spectrum of η Carinae and its light curve . 82 4.3.8 Discussion and interpretation . 84 5 Conclusion and Outlook 90 List of Figures 93 List of Tables 101 Acknowledgements 102 Bibliography 104 X Chapter 1 Introduction Die Wissenschaft stellt sowohl diese bildende Bewegung in ihrer Ausführlichkeit und Notwendigkeit als [auch] das, was schon zum Momente und Eigentum des Geistes herabgesunken ist, in seiner Gestaltung dar. Das Ziel ist die Einsicht des Geistes in das, was das Wissen ist. [1] Science lays before us the morphogenetic process of this cultural development in all its detailed full- ness and necessity, and at the same time shows it to be something that has already sunk into the mind as a moment of its being and become a possession of mind. The goal to be reached is the mind’s insight into what knowing is. [2] As a young branch of modern physics, γ-ray astronomy seeks to further our knowl- edge about the flux of highly energetic photons reaching the Earth in GeV to TeV energies. In general, these photons are produced in interactions of accelerated cosmic rays with ambient radiation fields and matter of the interstellar medium. The search for acceleration sites of cosmic rays is thus linked to the search for γ-ray sources. As neutral particles, γ-rays have the advantage of tracing back to their sources while the charged cosmic rays are deflected by interstellar magnetic fields. The composition of cosmic rays is dominated by protons and α-particles (∼ 98 %). Heavier nuclei, positrons and electrons also contribute on a smaller scale (∼ 1:8 %). The last two decades have brought substantial progress in our understanding of sources of very-high energy γ-rays. In this energy regime, ground-based γ-ray telescopes outperform space-based satellite experiments because the decreased flux of γ-rays requires larger collection areas. Imag- ing Atmospheric Cherenkov Telescopes, like H.E.S.S., have successfully mapped the very-high energy γ-ray sky. 1 Chapter 1. Introduction Up to the "knee" of the cosmic-ray energy spectrum at ∼ 4 × 1015 eV, it can be described by a power law with an index of -2.7.
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