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Search for Variability of the VHE Γ-Ray Source HESS J1745-290 in the Galactic Center

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Search for Variability of the VHE Γ-Ray Source HESS J1745-290 in the Galactic Center Search for variability of the VHE γ-ray source HESS J1745-290 in the Galactic Center Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) im Fach Physik Spezialisierung: Experimentalphysik eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät der Humboldt-Universität zu Berlin von Philipp Wagner Präsidentin der Humboldt-Universität zu Berlin: Prof. Dr.-Ing. Dr. Sabine Kunst Dekan der Mathematisch-Naturwissenschaftlichen Fakultät: Prof. Dr. Elmar Kulke Gutachter: 1. Prof. Dr. Thomas Lohse 2. Prof. Dr. Elisa Bernardini 3. Prof. Dr. Markus Böttcher Tag der mündlichen Prüfung: 2.5.2017 Abstract This work presents a detailed study on the very-high-energy (VHE) γ-ray source HESS J1745-290 in direction of the Galactic Center using 10 years of data from the H.E.S.S. array of Cherenkov telescopes from 2004 to 2014 with the objective to search for variability of the γ-ray fux of this object. The question if HESS J1745-290 shows variability is of special interest, since the source is located at the same direction as the super-massive black hole Sgr A*. From the vicinity of this black hole variable radiation has been reported for diferent wavelength bands. The detection of a variability of the VHE γ-ray fux of HESS J1745-290 would favor the hypothesis of a connection of this object and Sgr A* which could not be confrmed so far. The search for variability was performed for diferent timescales from minutes to years and indeed revealed evidence for variability in diferent statistical tests which will be discussed in detail together with systematic cross-checks. The study focuses on both variability with and without periodic character. While there is evidence for a long- term fux modulation with a period of 110 d at the 4.1σ signifcance level, the χ2 ft of a H.E.S.S. run-wise light curve from 2004–2014 shows variability at the 6.1σ level, which reduces to 4.5σ after adding a 10% systematic error to each fux measurement. Also signs of variable behavior at a timescale of minutes were found at the 3.1σ level. This tentative VHE short-term variability also shows quasi-periodic behavior as it was reported during infrared and X-ray observations of Sgr A*. Such a tentative long-term fux modulation with a period of 110 d has previously also been reported for the radio band. Due to the similarity of time structure of the variability, which is reported for HESS J1745-290 in this thesis to observations of Sgr A* at other wavelength bands, the thesis will close with the discussion if these results can be considered to be frst evidence for a link between HESS J1745-290 and Sgr A*. Kurzzusammenfassung Die folgende Arbeit beschäftigt sich mit der Quelle hochenergetischer Gammastrahlung HESS J1745-290, welche in Richtung des galaktischen Zentrums liegt und präsentiert die Analyse eines Datensatzes, der von den H.E.S.S. Teleskopen zwischen 2004 und 2014 aufgezeichnet wurde. Ziel der Untersuchung war es, eine zeitliche Variabilität des beobachteten Flusses festzustellen. Die Frage ob der Fluss von HESS J1745-290 variables Verhalten zeigt stellt sich, da sich die Quelle in der gleichen Richtung wie das supermassive schwarze Loch Sgr A* befndet, aus dessen Umgebung bereits variable Strahlung in verschiedenen Frequenzbereichen detektiert wurde. Die Beobachtung einer Variabilität des hochenergetischen Gammastrahlenfusses von HESS J1745-290 würde die Hypothese eines Zusammenhangs zwischen diesem Objekt und Sgr A* stützen, welche bis dato nicht bestätigt werden konnte. Die Suche nach Variabilität wurde für verschiedene Zeitskalen von Minuten bis hin zu Jahren durchgeführt, wobei Hinweise für Variabilität in verschiedenen statistischen Tests gefunden wurden. Die Suche konzentriert sich auf Variabilität ohne periodischen Charakter sowie auf Periodizität. Es wurden Hinweise auf eine Periode von 110 Tagen bei einem Signifkanzniveau von 4.1σ gefunden und auch der χ2 Fit einer H.E.S.S. Lichtkurve, die von 2004–2014 reicht, zeigt Variabilität bei einem Signifkanzniveau von 6.1σ, welches sich nach Anwendung eines systematischen Fehlers von 10% auf 4.5σ reduziert. Auch Anzeichen für Variabilität auf einer Zeitskala von Minuten wurden gefunden. Diese Variabilität auf einer Zeitskala von Minuten zeigt quasi-periodischen Charakter, ähnlich derer, welche während Infrarot- und Röntgenbeobachtungen von Sgr A* festgestellt wurde. Die Möglichkeit einer Verbindung zwischen HESS J1745-290 und Sgr A* und ins- besondere auch die Fragestellung, ob die hier präsentierten Ergebnisse als erster Hinweis auf solch einen Zusammenhang gewertet werden können, werden Thema der Diskussion am Ende der Arbeit sein. Contents Contents7 1 Introduction9 1.1 Astroparticle Physics: Studying Cosmic Accelerators . 10 1.2 The non-thermal Milky Way and the Galactic Center Region . 11 2 The High Energy Stereoscopic System 13 2.1 Air Showers and Cherenkov Light Cones . 14 2.1.1 The Cherenkov Efect . 14 2.1.2 Diferent Types of Air Showers . 15 2.2 The H.E.S.S. Experiment: Science and Detector . 17 2.2.1 The H.E.S.S. Site . 17 2.2.2 H.E.S.S. Phase I . 19 2.2.3 H.E.S.S. Phase II . 19 2.2.4 The diferent Subsystems of the H.E.S.S. Detector . 20 2.2.5 Calibration and Detector Simulations . 24 2.2.6 Detector Simulations . 25 2.3 Reconstruction and Particle Identifcation . 25 2.3.1 The Standard Hillas Reconstruction . 26 2.3.2 Particle Identifcation: Moving towards Pattern Recognition . 28 3 The Galactic Center Source HESS J1745-290 35 3.1 Variability of Sgr A*: multi-wavelength Results . 36 3.1.1 A Periodicity at about 110 Days . 36 3.1.2 Short and weak Flares during Infrared and X-Ray Observations . 38 3.1.3 The G2 Gas Cloud . 43 3.1.4 Giant X-Ray Flares . 44 3.1.5 Estimates of the Frequency of Galactic Center Flares . 44 8 Contents 3.2 Summary of H.E.S.S. I Results from 2006 . 45 3.3 Summary . 47 4 Search for Variability of HESS J1745-290 49 4.1 Methods: Variability and Periodicity Tests . 49 4.1.1 Light Curves: A Defnition . 49 4.1.2 Periodicity Tests . 50 4.2 Data Analysis . 64 4.2.1 The Dataset, Cuts and Background Method . 65 4.2.2 Skymap of the Galactic Center Region . 67 4.2.3 The Spectrum of HESS J1745-290 with Loose Cuts . 69 4.2.4 Search for a long-term Variability . 73 4.2.5 Search for a Variability at a Timescale of Minutes . 98 4.2.6 Run-wise χ2 Test of the post-cut Event Rates . 117 5 Summary and Outlook 127 5.1 Summary . 127 5.2 Comparison to MWL Observations . 128 5.3 Cross-Checks and External Data . 131 5.4 Candidate Mechanisms explaining the observed Variability . 133 5.5 Conclusion and Outlook . 134 Bibliography 139 A Simulated Monte Carlo Event Displays for gamma-like Events 153 B Studies with respect to Broken Pixels 155 Chapter 1 Introduction The cosmic rays (CRs) were discovered by Victor H.E.S.S. in August 1912 during a balloon fight from Aussig to Pieskow which was preceded by several other measurements. His important discovery, which was awarded with the Nobel Prize in physics in 1936, can be considered to be a mile stone on the way towards modern particle physics and also the starting point of a new discipline: The astroparticle physics which needed about 60 years after the discovery of cosmic radiation to emerge as independent discipline next to the classical accelerator based particle physics. Today the astroparticle physics studies the most violent phenomena in the universe and particles reaching energies that no accelerator on earth has reached so far. While the Large Hadron Collider (LHC) in Geneva reaches a center of mass energy of 13 TeV since 2015, modern γ-ray detectors measure particles at several 10 TeV. Even so-called PeVatrons, accelerators reaching energies in the petaelectronvolt (PeV) range are in sight now. While the majority of CRs reaching the atmosphere of the earth consists of protons and heavier nuclei, there are also highly energetic electrons (positrons) and photons. There should be a correlation between regions of CR production and the direction of high energy photons which are emitted due to the interaction of CRs with magnetic felds and matter in their environment. An important hadronic production mechanism of high energy photons is via the production of secondary π0 mesons which decay into photon pairs afterwards. Photons with an origin in the π0 decay usually have larger energies than those which are produced via synchrotron emission. Their minimum energy is half the mass of the π0 in the rest frame which implies that photons which are produced via the π0 decay have energies of at least ∼ 70 MeV. In general photons with energies ≥ 1 MeV are called γ-rays. Next to these hadronic production mechanisms γ-rays can also be produced by leptons. While at MeV energies the synchrotron emission is the dominant process, for the γ-ray production in the GeV and TeV range Inverse Compton (IC) 10 Introduction scattering where γ-rays (which could be produced as synchrotron radiation before) gain energy from an interaction with relativistic electrons is the most important mechanism. The photons produced by these diferent mechanisms cover a large energy range up to 100 TeV. Due to the fact that γ-rays are not bent by magnetic felds like electrons and protons they form an excellent tool to observe the universe at high and very high energies (VHE, E > 0.1 TeV). From the 70ies of the last century onwards systematic eforts were undertaken to study γ-rays and the γ-ray astronomy was born as a new discipline.
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