@phdthesis{Lypova2021,
  author    = {Lypova, Iryna},
  title     = {The galactic plane in gamma-rays above 10 TeV as seen with H.E.S.S.},
  doi       = {10.25932/publishup-50931},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-509317},
  school      = {Universit{\"a}t Potsdam},
  pages     = {viii, 195},
  year      = {2021},
  abstract  = {The High Energy Stereoscopic System (H.E.S.S.) is an array of five imaging atmospheric Cherenkov telescopes located in the Khomas Highland of Namibia. H.E.S.S. operates in a wide energy range from several tens of GeV to several tens of TeV, reaching the best sensitivity around 1 TeV or at lower energies. However, there are many important topics - such as the search for Galactic PeVatrons, the study of gamma-ray production scenarios for sources (hadronic vs. leptonic), EBL absorption studies - which require good sensitivity at energies above 10 TeV. This work aims at improving the sensitivity of H.E.S.S. and increasing the gamma-ray statistics at high energies. The study investigates an enlargement of the H.E.S.S. effective field of view using events with larger offset angles in the analysis. The greatest challenges in the analysis of large-offset events are a degradation of the reconstruction accuracy and a rise of the background rate as the offset angle increases. The more sophisticated direction reconstruction method (DISP) and improvements to the standard background rejection technique, which by themselves are effective ways to increase the gamma-ray statistics and improve the sensitivity of the analysis, are implemented to overcome the above-mentioned issues. As a result, the angular resolution at the preselection level is improved by 5 - 10\% for events at 0.5◦ offset angle and by 20 - 30\% for events at 2◦ offset angle. The background rate at large offset angles is decreased nearly to a level typical for offset angles below 2.5◦. Thereby, sensitivity improvements of 10 - 20\% are achieved for the proposed analysis compared to the standard analysis at small offset angles. Developed analysis also allows for the usage of events at large offset angles up to approximately 4◦, which was not possible before. This analysis method is applied to the analysis of the Galactic plane data above 10 TeV. As a result, 40 sources out of the 78 presented in the H.E.S.S. Galactic plane survey (HGPS) are detected above 10 TeV. Among them are representatives of all source classes that are present in the HGPS catalogue; namely, binary systems, supernova remnants, pulsar wind nebulae and composite objects. The potential of the improved analysis method is demonstrated by investigating the more than 10 TeV emission for two objects: the region associated with the shell-type SNR HESS J1731-347 and the PWN candidate associated with PSR J0855-4644 that is coincident with Vela Junior (HESS J0852-463).},
  language  = {en}
}
@phdthesis{Haupt2020,
  author    = {Haupt, Maria},
  title     = {The Magellanic Clouds in VHE gamma rays as seen by H.E.S.S.},
  doi       = {10.25932/publishup-47460},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-474601},
  school      = {Universit{\"a}t Potsdam},
  pages     = {iv, 182},
  year      = {2020},
  abstract  = {Das Gebiet der Gammastrahlungsastronomie hat ein neues Fenster in das nicht-thermische Universum geöffnet, welches erlaubt, die Beschleunigungsorte der kosmischen Strahlung und ihrer Rolle in evolutionären Prozessen in Galaxien zu untersuchen. Der Nachweis von fast einhundert sehr hochenergetischen Gammastrahlungsquellen in unserer Milchstraße zeigt, dass Teilchenbeschleunigung bis in den zweistelligen TeV-Energiebereich ein häufiges Phänomen ist. Dar{\"u}ber hinaus hat der Nachweis von sehr hochenergetischer Gammastrahlung von anderen Galaxien bestätigt, dass die kosmische Strahlung nicht ausschließlich in der Milchstraße beschleunigt wird. Die rasante Entwicklung der Gammastrahlungsastronomie in den letzten zwei Jahrzehnten f{\"u}hrte zu einem Übergang von der Detektion und Untersuchung einzelner Quellen hin zu Quellpopulationsstudien. Um die Frage zu beantworten, ob die Quellpopulationen hochenergetischer Gammastrahlung in der Milchstraße einzigartig sind, sind Beobachtungen von anderen Galaxien erforderlich, f{\"u}r die es möglich ist, trotz ihrer Entfernung, einzelne Quellen aufzulösen. Die Magellanschen Wolken, zwei Satellitengalaxien der Milchstraße, sind solche Galaxien, welche im letzten Jahrzehnt durch das H.E.S.S.-Experiment intensiv beobachtet wurden. In dieser Arbeit werden die Daten von insgesamt 450 Stunden H.E.S.S.-Beobachtungen der Großen Magellanschen Wolke und der Kleinen Magellanschen Wolke vorgestellt. Während der Analyse der Datensätze wird besonderer Wert auf die Evaluierung der systematischen Unsicherheiten des Experiments gelegt, um eine unverfälschte Flussabschätzung der potentiellen hochenergetischen Gammastrahlungsquellen der Magellanschen Wolken zu gewährleisten. Die detaillierte Analyse der Beobachtungen f{\"u}hrte zur Detektion hochenergetischer Gammastrahlung des Binärsystems LMC P3 in der Großen Magellanschen Wolke und erhöht somit die Anzahl der detektierten Gammastrahlungsquellen in dieser Galaxie auf vier. Dieses neuentdeckte Binärsystem ist das bisher leuchtstärkste in der Quellklasse der Gammastrahlungsbinärsysteme. F{\"u}r keine andere Quelle in den Magellanschen Wolken wird hochenergetische Gammastrahlung nachgewiesen und es werden Obergrenzen auf den integralen Fluss ermittelt. Diese Flussobergrenzen werden verwendet, um Populationsstudien auf der Grundlage bekannter hochenergetischer Quellklassen sowie bestehender Quellkataloge anderer Wellenlängen durchzuf{\"u}hren. Ein systematischer Vergleich zwischen den Quellpopulationen der Magellanschen Wolken und der Milchstraße ergab, dass keine andere Quelle der Magellanschen Wolken so leuchtstark ist wie die leuchtstärkste hochenergetische Gammastrahlungsquelle in der LMC: der Pulsarwindnebel N157B. Des Weiteren ist ein Drittel der untersuchten Quellpopulation der Magellanschen Wolken weniger leuchtstark als die vier bekannten Gammastrahlungsquellen in der Großen Magellanschen Wolke. F{\"u}r einige wenige Quellen kann gezeigt werden, dass sie weniger leuchtstark sind als die leuchtstärksten Objekte in der Milchstraße, deren Leuchtkraft um mehr als eine Größenordnung schwächer ist als die der detektierten Quellen in der Großen Magellanschen Wolke. Basierend auf den Flussobergrenzen werden Unterschiede in den Quellpopulationen der Magellanschen Wolken und der Milchstraße sowie die Bedeutung der Quellumgebungen diskutiert.},
  language  = {en}
}
@phdthesis{Hoischen2018,
  author    = {Hoischen, Clemens},
  title     = {Multi-Messenger Astronomy with H.E.S.S: the Starburst Galaxy NGC253 and the Search for Short Time-Scale Transients},
  doi       = {10.25932/publishup-42452},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-424521},
  school      = {Universit{\"a}t Potsdam},
  pages     = {135},
  year      = {2018},
  abstract  = {Gamma-ray astronomy has proven to provide unique insights into cosmic-ray accelerators in the past few decades. By combining information at the highest photon energies with the entire electromagnetic spectrum in multi-wavelength studies, detailed knowledge of non-thermal particle populations in astronomical objects and systems has been gained: Many individual classes of gamma-ray sources could be identified inside our galaxy and outside of it. Different sources were found to exhibit a wide range of temporal evolution, ranging from seconds to stable behaviours over many years of observations. With the dawn of both neutrino- and gravitational wave astronomy, additional messengers have come into play over the last years. This development presents the advent of multi-messenger astronomy: a novel approach not only to search for sources of cosmic rays, but for astronomy in general. In this thesis, both traditional multi-wavelength studies and multi-messenger studies will be presented. They were carried out with the H.E.S.S. experiment, an imaging air Cherenkov telescope array located in the Khomas Highland of Namibia. H.E.S.S. has entered its second phase in 2012 with the addition of a large, fifth telescope. While the initial array was limited to the study of gamma-rays with energies above 100 GeV, the new instrument allows to access gamma-rays with energies down to a few tens of GeV. Strengths of the multi-wavelength approach will be demonstrated at the example of the galaxy NGC253, which is undergoing an episode of enhanced star-formation. The gamma-ray emission will be discussed in light of all the information on this system available from radio, infrared and X-rays. These wavelengths reveal detailed information on the population of supernova remnants, which are suspected cosmic-ray accelerators. A broad-band gamma-ray spectrum is derived from H.E.S.S. and Fermi-LAT data. The improved analysis of H.E.S.S. data provides a measurement which is no longer dominated by systematic uncertainties. The long-term behaviour of cosmic rays in the starburst galaxy NGC253 is finally characterised. In contrast to the long time-scale evolution of a starburst galaxy, multi-messenger studies are especially intriguing when shorter time-scales are being probed. A prime example of a short time-scale transient are Gamma Ray Bursts. The efforts to understand this phenomenon effectively founded the branch of gamma-ray astronomy. The multi-messenger approach allows for the study of illusive phenomena such as Gamma Ray Bursts and other transients using electromagnetic radiation, neutrinos, cosmic rays and gravitational waves contemporaneously. With contemporaneous observations getting more important just recently, the execution of such observation campaigns still presents a big challenge due to the different limitations and strengths of the infrastructures. An alert system for transient phenomena has been developed over the course of this thesis for H.E.S.S. It aims to address many follow-up challenges in order to maximise the science return of the new large telescope, which is able to repoint much faster than the initial four telescopes. The system allows for fully automated observations based on scientific alerts from any wavelength or messenger and allows H.E.S.S. to participate in multi-messenger campaigns. Utilising this new system, many interesting multi-messenger observation campaigns have been performed. Several highlight observations with H.E.S.S. are analysed, presented and discussed in this work. Among them are observations of Gamma Ray Bursts with low latency and low energy threshold, the follow-up of a neutrino candidate in spatial coincidence with a flaring active galactic nucleus and of the merger of two neutron stars, which was revealed by the coincidence of gravitational waves and a Gamma-Ray Burst.},
  language  = {en}
}
@phdthesis{Wechakama2013,
  author    = {Wechakama, Maneenate},
  title     = {Multi-messenger constraints and pressure from dark matter annihilation into electron-positron pairs},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-67401},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {Despite striking evidence for the existence of dark matter from astrophysical observations, dark matter has still escaped any direct or indirect detection until today. Therefore a proof for its existence and the revelation of its nature belongs to one of the most intriguing challenges of nowadays cosmology and particle physics. The present work tries to investigate the nature of dark matter through indirect signatures from dark matter annihilation into electron-positron pairs in two different ways, pressure from dark matter annihilation and multi-messenger constraints on the dark matter annihilation cross-section. We focus on dark matter annihilation into electron-positron pairs and adopt a model-independent approach, where all the electrons and positrons are injected with the same initial energy E_0 ~ m_dm*c^2. The propagation of these particles is determined by solving the diffusion-loss equation, considering inverse Compton scattering, synchrotron radiation, Coulomb collisions, bremsstrahlung, and ionization. The first part of this work, focusing on pressure from dark matter annihilation, demonstrates that dark matter annihilation into electron-positron pairs may affect the observed rotation curve by a significant amount. The injection rate of this calculation is constrained by INTEGRAL, Fermi, and H.E.S.S. data. The pressure of the relativistic electron-positron gas is computed from the energy spectrum predicted by the diffusion-loss equation. For values of the gas density and magnetic field that are representative of the Milky Way, it is estimated that the pressure gradients are strong enough to balance gravity in the central parts if E_0 < 1 GeV. The exact value depends somewhat on the astrophysical parameters, and it changes dramatically with the slope of the dark matter density profile. For very steep slopes, as those expected from adiabatic contraction, the rotation curves of spiral galaxies would be affected on kiloparsec scales for most values of E_0. By comparing the predicted rotation curves with observations of dwarf and low surface brightness galaxies, we show that the pressure from dark matter annihilation may improve the agreement between theory and observations in some cases, but it also imposes severe constraints on the model parameters (most notably, the inner slope of the halo density profile, as well as the mass and the annihilation cross-section of dark matter particles into electron-positron pairs). In the second part, upper limits on the dark matter annihilation cross-section into electron-positron pairs are obtained by combining observed data at different wavelengths (from Haslam, WMAP, and Fermi all-sky intensity maps) with recent measurements of the electron and positron spectra in the solar neighbourhood by PAMELA, Fermi, and H.E.S.S.. We consider synchrotron emission in the radio and microwave bands, as well as inverse Compton scattering and final-state radiation at gamma-ray energies. For most values of the model parameters, the tightest constraints are imposed by the local positron spectrum and synchrotron emission from the central regions of the Galaxy. According to our results, the annihilation cross-section should not be higher than the canonical value for a thermal relic if the mass of the dark matter candidate is smaller than a few GeV. In addition, we also derive a stringent upper limit on the inner logarithmic slope α of the density profile of the Milky Way dark matter halo (α < 1 if m_dm < 5 GeV, α < 1.3 if m_dm < 100 GeV and α < 1.5 if m_dm < 2 TeV) assuming a dark matter annihilation cross-section into electron-positron pairs (σv) = 3*10^-26 cm^3 s^-1, as predicted for thermal relics from the big bang.},
  language  = {en}
}