In the presented thesis, the most advanced photon reconstruction technique of ground-based γ-ray astronomy is adapted to the H.E.S.S. 28 m telescope. The method is based on a semi-analytical model of electromagnetic particle showers in the atmosphere. The properties of cosmic γ-rays are reconstructed by comparing the camera image of the telescope with the Cherenkov emission that is expected from the shower model. To suppress the dominant background from charged cosmic rays, events are selected based on several criteria. The performance of the analysis is evaluated with simulated events. The method is then applied to two sources that are known to emit γ-rays. The first of these is the Crab Nebula, the standard candle of ground-based γ-ray astronomy. The results of this source confirm the expected performance of the reconstruction method, where the much lower energy threshold compared to H.E.S.S. I is of particular importance. A second analysis is performed on the region around the Galactic Centre. The analysis results emphasise the capabilities of the new telescope to measure γ-rays in an energy range that is interesting for both theoretical and experimental astrophysics. The presented analysis features the lowest energy threshold that has ever been reached in ground-based γ-ray astronomy, opening a new window to the precise measurement of the physical properties of time-variable sources at energies of several tens of GeV. / In der vorliegenden Dissertation wird die zur Zeit sensitivste Methode zur Photonrekonstruktion in der bodengebundenen Gammastrahlungsastronomie an das 28 m H.E.S.S. Teleskop angepasst. Die Analyse basiert auf einem semi-analytischen Modell von elektromagnetischen Teilchenschauern in der Erdatmosphäre. Die Rekonstruktion erfolgt durch den Vergleich des Bildes der Teleskopkamera mit der Tscherenkow-Emission, die mittels des Schauermodells berechnet wurde. Zur Verringerung des dominanten Untergrundes, der hauptsächlich durch Teilchen der geladenen kosmischen Strahlung hervorgerufen wird, werden Ereignisse anhand bestimmter Kriterien ausgewählt. Die Leistungsfähigkeit der Analyse wird unter Verwendung simulierter Ereignisse evaluiert. Die Methode wird anschließend auf zwei Gammastrahlungsquellen angewendet. Zuerst wird der Krebsnebel analysiert, die Standardkerze der bodengebundenen Gammastrahlungsastronomie. Die Resultate der Analyse des Krebsnebels bestätigen die bereits zuvor erwartete Leistungsfähigkeit der Rekonstruktionsmethode, wobei hier insbesondere die im Vergleich zu H.E.S.S. I stark verringerte Energieschwelle hervorzuheben ist. Als Zweites werden Beobachtungen der Region um das galaktische Zentrum ausgewertet. Die Analyseergebnisse dieser Daten unterstreichen die Fähigkeiten des neuen Teleskops zur Messung kosmischer Gammastrahlung in einem für die theoretische und experimentelle Astrophysik interessanten Energiebereich. Die vorgestellte Analyse besitzt die niedrigste Energieschwelle, die in der bodengebundenen Gammastrahlungsastronomie je erreicht wurde. Sie ermöglicht damit präzise Messungen der physikalischen Eigenschaften von zeitabhängigen Quellen im Energiebereich von 10 bis 100 GeV.

Les sursauts gamma sont des événements fascinants de par leur origine longtemps restée mystérieuse, leur apparition imprévisible dans le ciel, et la formidable énergie qu'ils libèrent sous forme de bouffées de rayonnement gamma. Découverts fortuitement au début des années 1970, ils se traduisent par un intense flash de rayons gamma de brève durée (de quelques ms à quelques min), appelé émission prompte, suivi d'une émission longue, appelée rémanence, qui décroît rapidement, en émettant depuis les rayons X jusqu'au domaine radio. L'origine des sursauts gamma est encore largement discutée mais ces phénomènes extrêmes sont très vraisemblablement associés à la formation de nouveaux trous noirs stellaires. Mon sujet de thèse se situe au coeur de la mission sino-française SVOM dont le lancement du satellite est prévu en 2021, qui scrutera le ciel pour observer les sursauts avec une précision inégalée, associant observations spatiales et terrestres. Mon travail concerne l'instrument principal de la mission, le télescope spatial ECLAIRs. Il s'agit d'une caméra à masque codé sensible aux rayons X et gamma de basse énergie, en charge de la détection et de la localisation de l'émission prompte des sursauts. Durant mon travail de thèse, j'ai travaillé sur les performances scientifiques de l'instrument ECLAIRs et j'ai en particulier estimé le nombre de sursauts qui seront détectés et leurs caractéristiques. Pour cela, j'ai mis en place des simulations de performances utilisant les prototypes des algorithmes de détection embarqués combinés au modèle de l'instrument ECLAIRs. Les données en entrée des simulations comportent un bruit de fond simulé, et une population synthétique de sursauts gamma générée à partir de catalogues existants issus des observations des missions antérieures (CGRO, HETE-2, Swift et Fermi). Mon étude a permis d'estimer finement l'efficacité de détection d'ECLAIRs et prédit un taux de sursauts attendu par ECLAIRs entre 40 et 70 sursauts par an. Par ailleurs, mon travail a montré qu'ECLAIRs sera particulièrement sensible à une population de sursauts très riches en rayons X, population encore mal connue. Ma thèse présente plusieurs autres études complémentaires portant sur la performance de localisation, le taux de fausses alertes et les caractéristiques des déclenchements des algorithmes. Enfin, j'ai proposé deux nouvelles méthodes originales de détection de sursauts dont les résultats préliminaires présentés dans ma thèse sont très encourageants. Ils montrent que la sensibilité d'ECLAIRs aux sursauts courts (population d'intérêt particulier en raison de son lien attendu avec les ondes gravitationnelles) peut être encore améliorée. / Discovered in the early 1970s, gamma-ray bursts (GRBs) are amazing cosmic phenomena appearing randomly on the sky and releasing large amounts of energy mainly through gamma-ray emission. Although their origin is still under debate, they are believed to be produced by some of the most violent explosions in the Universe leading to the formation of stellar black-holes. GRBs are detected by their prompt emission, an intense short burst of gamma-rays (from a few millisecondes to few minutes), and are followed by a lived-afterglow emission observed on longer timescales from the X-ray to the radio domain. My thesis participates to the developement of the SVOM mission, which a Chinese-French mission to be launched in 2021, devoted to the study of GRBs and involving space and ground instruments. My work is focussed on the main instrument ECLAIRs, a hard X-ray coded mask imaging camera, in charge of the near real-time detection and localization of the prompt emission of GRBs. During my thesis, I studied the scientific performances of ECLAIRs and in particular the number of GRBs expected to be detected by ECLAIRs and their characteristics. For this purpose, I performed simulations using the prototypes of the embedded trigger algorithms combined with the model of the ECLAIRs instrument. The input data of the simulations include a background model and a synthetic population of gamma-ray bursts generated from existing catalogs (CGRO, HETE-2, Fermi and Swift). As a result, I estimated precisely the ECLAIRs detection efficiency of the algorithms and I predicted the number of GRBs to be detected by ECLAIRs : 40 to 70 GRBs per year. Moreover, the study highlighted that ECLAIRs will be particularly sensitive to the X-ray rich GRB population. My thesis provided additional studies about the localization performance, the rate of false alarm and the characteristics of the triggers of the algorithms. Finally, I also proposed two new methods for the detection of GRBs.The preliminary results were very promising and demonstrate that the sensitivity of ECLAIRs to the short GRBs (an interesting population due to the predicted association with gravitational waves) could be improved further.

In this thesis, we present our two studies focused on the detection of cosmic γ-rays and the analysis of data from γ-ray observations. One study deals with the method of the Cherenkov transparency coefficient. This method is suitable for the detector calibration in experiments employing imaging atmo- spheric Cherenkov telescopes for the indirect detection of cosmic γ-rays. Us- ing rates of recorded air showers initiated by charged cosmic rays, the method aims at the monitoring of the atmospheric transparency to Cherenkov light and the calibration of the responses of Cherenkov telescopes. We present an extension of this method for the purposes of the Cherenkov Telescope Array observatory and demonstrate its feasibility using Monte Carlo simu- lations. Our other analysis utilizes more than 7 years of data from direct γ-ray observations by the Fermi Large Area Telescope. We describe in detail signal observed from the parts of the sky around the active galactic nuclei 1ES 0229+200 and Centaurus A. We report on the findings of new astro- physical sources of high energy photons and document spectral and temporal properties of their γ-ray fluxes. 1

Le Large Area Telescope (LAT) à bord du satellite Fermi, lancé le 11 Juin 2008, est sensible au rayonnement gamma de20 MeV à plus de 300 GeV. 148 pulsars gamma à ce jour ont été détectés avec le Fermi-LAT, dont 117 sont détaillés dansle Deuxième Catalogue de Pulsars gamma de Fermi (2PC). Les pulsars forment la plus grande classe de sources au GeVdans la Voie Lactée. Les études de pulsars gamma apportent des contraintes importantes sur les modèles d’émission gamma etpermettent des percées dans notre compréhension des pulsars.Cette thèse présente des détections de sept pulsars gamma faibles déjà connus en radio, dont quatre jeunes ou d’âgesmoyens et trois millisecondes (MSP). L’analyse spectrale et la caractérisation des courbes de lumiére sont décriteset comparées à la population de 2PC. Ces détections ont l’intérêt particulier de sonder et d’étendre l’espace desparamètres de la population actuelle. Elles illustrent la distorsion inhérente dans 2PC et marquent la transition de lapremière phase (pulsars brillants) à la seconde phase (pulsars faibles) d’opération de Fermi. Le développement desmodèles théoriques et un recensement complet de la population galactique des étoiles à neutrons bénéficieront de plusde détections de pulsars faibles dans les années à venir.Certains pulsars gamma dans 2PC manifestant de grande efficacité gamma sont étudiés dans cette thèse. L’examen desdifférents paramètres qui influencent l’efficacité conduit à quelques candidats pour lesquels un plus grand momentd’inertie est nécessaire afin de résoudre le paradoxe de grande efficacité. Les observations de Fermi en rayons gamma ontl’air à apporter des contraintes indépendantes sur le moment d’inertie et les équations d’état des étoiles à neutrons. / The Large Area Telescope (LAT) on board the Fermi satellite, launched on June 11 2008, is sensitive to g raysfrom 20 MeV to over 300 GeV. gamma-ray pulsations from 148 pulsars to date have been detected with the Fermi-LAT, 117are detailed in the Second Fermi-LAT Gamma-ray Pulsar Catalog (2PC). Pulsars are by far the largest GeV sourceclassin the Milky Way. Studies of the increasing gamma-ray pulsar sample bring important constraints on theoretical gamma-rayemission models and enable breakthroughs in our understanding of pulsars.This thesis presents detections of seven faint gamma-ray pulsars that are already known at radio wavelengths, includingfour young or middle aged pulsars and three millisecond pulsars (MSPs). Spectral analysis and light curve characterizationsare detailed and compared to the 2PC sample. These detections have the special interest of allowing us toprobe and extend the parameter space of the current population. They illustrate the inherent bias in 2PC and mark thetransition from the early part (bright pulsars) to the later part (faint pulsars) of the Fermi mission. Theoretical modeldevelopments and a complete census of Galactic neutron star populations will benefit from more detections of weakpulsars in the years to come.Some high gamma-ray efficiency pulsars in 2PC are studied in this thesis. Investigations on different parameters whichinfluence the efficiency result in a few candidates for which a large moment of inertia is necessary to cure the highefficiency paradox. Fermi gamma-ray observations seem to bring independent constraints on the moment of inertia andequations of state of neutron stars

Le satellite Fermi a été lancé le 11 juin 2008, avec à son bord le Large Area Telescope (LAT). Le LAT est un télescope sensible au rayonnement ? de 20 MeV à plus de 300 GeV. Avant la mise en service du LAT, six pulsars jeunes et énergétiques étaient connus dans le domaine ? . Le nombre de détections de pulsars par le LAT prédit avant lancement était de plusieurs dizaines au moins. Le LAT permettait également l’étude des pulsars milliseconde (MSPs), jamais détectés avec certitude à très haute énergie jusqu’alors. Cette thèse aborde dans un premier temps la campagne de chronométrie des pulsars émetteurs radio et/ou X, candidats à la détection par le LAT, en collaboration avec les grands radiotélescopes et télescopes X. Cette campagne a permis la recherche de signaux ? pulsés avec une grande sensibilité. En outre, la plupart des MSPs galactiques ont été suivis dans le cadre de cette campagne, sans biais de sélection a priori sur cette population d’étoiles. Pour la première fois, des pulsations ont été détectées pour huit MSPs galactiques au-dessus de 100 MeV. Quelques bons candidats à une détection prochaine apparaissent. Une recherche similaire a été conduite pour des MSPs d’amas globulaires, sans succès à présent. L’analyse des courbes de lumière et des propriétés spectrales des huit MSPs détectés révèle que leur rayonnement ? est relativement similaire à celui des pulsars ordinaires, et est vraisemblablement produit dans la magnétosphère externe. Cette découverte suggère que certaines sources non-identi?ées sont des MSPs, pour l’instant inconnus. / The Fermi observatory was launched on June 11, 2008. It hosts the Large Area Telescope (LAT), sensitive to ? -ray photons from 20 MeV to over 300 GeV. Before the LAT began its activity, six young and energetic pulsars were known in ? rays. At least several tens of pulsar detections by the LAT were predicted before launch. The LAT also allowed the study of millisecond pulsars (MSPs), never ?rmly detected in ? rays before Fermi. This thesis ?rst presents the pulsar timing campaign for the LAT, in collaboration with large radiotelescopes and X-ray telescopes, allowing for high sensitivity pulsed searches. Furthermore, it lead to quasi-homogeneous coverage of the galactic MSPs, so that the search for pulsations in LAT data for this population of stars was not affected by an a priori bias. We present a search for pulsations from these objects in LAT data. For the ?rst time, eight galactic MSPs have been detected as sources of pulsed ? -ray emission over 100 MeV. In addition, a couple of good candidates for future detection are seen. A similar search for globular cluster MSPs has not succeeded so far. Comparison of the phase-aligned ? -ray and radio light curves, as well as the spectral shapes, leads to the conclusion that their ? -ray emission is similar to that of normal pulsars, and is probably produced in the outer-magnetosphere. This discovery suggests that many unresolved ?-ray sources are unknown MSPs.

Die bodengebundene Gammaastronomie beschäftigt sich mit der Detektion von Photonen mit Energien >10GeV mittels der abbildenden Cherenkov-Technik. Dabei wird die Atmosphäre als Detektor verwendet: Photonen in diesem Energiebereich produzieren in ihr Teilchenschauer, die Cherenkov-Licht aussenden. Das Licht wird von Teleskopen gesammelt um Abbildungen der Schauer zu erhalten, aus denen Eigenschaften der Gammaquanten abgeleitet werden können. Da die Schauer statistischen Prozessen unterliegen, ist die Rekonstruktion der Richtung des Gammaquants durch Fluktuationen begrenzt. Die Qualität der Rekonstruktion hängt von der Energie des Quants, Beobachtungsbedingungen, Teleskop-Eigenschaften und der Rekonstruktionsmethode ab. Die Präzision der Richtungsrekonstruktion einzelner Gammaquanten wird als Winkelauflösung bezeichnet. Diese Arbeit beschäftigt sich mit der Winkelauflösung der vier Teleskope von HESS, die für bodengebundene Gammaastronomie im Energiebereich >100GeV verwendet werden. Es werden systematische Fehler der Winkelauflösungsfunktion und ihre Abhängigkeiten von Beobachtungsparametern mit Beobachtungsdaten und Simulationen abgeschätzt. Abweichungen der simulierten Auflösung von der mit HESS an Hand von Punktquellen gemessenen werden in dieser Arbeit identifiziert und quantifiziert, um mit einer korrigierten Funktion zwei Phänomene zu studieren: Erstens wird die Ausdehnung des Krebsnebels im TeV-Bereich untersucht, für die Werte bis ca. 0.03º vorausgesagt werden. Es werden die Beobachtungen des Nebels selektiert, die eine gute Auflösung und Kontrolle des systematischen Fehlers versprechen. Auf einem Konfidenzniveau von 95% wird die Obergrenze der Ausdehnung des Nebels damit zu 0.034º bestimmt. Zweitens wird nach ausgedehnter Emission um Aktive Galaxienkerne (AGN) gesucht. Verschiedene Modell-Vorhersagen werden mit den HESS-Daten dreier AGN verglichen - die so erzielten Obergrenzen auf den Fluss sind die niedrigsten bisher veröffentlichten im TeV-Bereich. / Very-high-energy (VHE) gamma-ray astronomy deals with the ground-based detection of photons with energies of tens of GeV to few 100 TeV by employing the Imaging Air Cherenkov Technique (IACT). This method uses the atmosphere as a detector for VHE gamma-rays, exploiting that photons in that energy range produce particle showers in it. The showers emit Cherenkov light, which is collected by telescopes to image single showers. Properties of the gamma-rays can be deduced from the shower images. However, the interactions in the atmosphere are statistical processes, imposing a limit on the direction reconstruction. The quality of the reconstruction depends on the energy of the primary particle, telescope properties, observational conditions and reconstruction algorithm. The precision of the direction reconstruction of single photons is called the angular resolution. In this work, the angular resolution function of the IACT experiment HESS is studied in detail. HESS consists of five telescopes, four of which were built for the energy range >100 GeV. For this sub-array, the systematic errors on the angular resolution and their dependence on observation parameters are estimated from known point sources in HESS data and Monte-Carlo simulations. A mismatch between HESS data and simulations is quantified and corrected to assess two phenomena: First, the size of the the Crab Nebula at VHE is investigated. Some models predict a size of the emission region of more than 0.03º. Including a detailed accounting of the systematic errors, an upper limit on the size of emission region of the nebula is given by 0.034º at a 95% confidence level. Second, extended emission around Active Galactiv Nuclei is searched for. Various models are probed with HESS data and the most constraining upper limits on the so-called pair halo scenario are found. Assuming a different model, extra-galactic magnetic field strengths in the range of (0.1 - 10)* 10^{-15} G are excluded at a 99% confidence level.

Das Energiespektrum der kosmischen Strahlung bietet wichtige Hinweise auf ihren Ursprung und ihre Ausbreitung. Verschiedene Messtechniken müssen kombiniert werden, um den ganzen Energiebereich abdecken zu können: Direkte Messungen mit Teilchendetektoren bei niedrigen Energien sowie indirekte Messungen von Luftschauern bei hohen Energien. Dazu kommt die Messung von Photonen, hauptsächlich im GeV- und TeV-Bereich, die bei der Wechselwirkung von kosmischer Strahlung mit Materie oder elektromagnetischen Feldern entstehen. Im Folgenden werde ich zwei Studien dazu vorstellen, die beide auf Daten des abbildenden Tscherenkow-Teleskopes VERITAS beruhen. Erstens stelle ich eine Messung das Energiespektrums von Eisenkernen in der kosmischen Strahlung vor. Für die Bestimmung der Energie und Ankunftsrichtung der Primärteilchen benutze ich eine neuartige Template-Likelihood-Methode, die hier erstmals auf Eisenschauer angewendet wird. Zur Identifizierung der Eisenschauer benutze ich unter anderem das sogenannte direkte Tscherenkow-Licht, welches von geladenen Teilchen vor der ersten Wechselwirkung ausgestrahlt wird. Dazu kommt eine multivariate Klassifizierungsmethode, um den Verbleibenden Untergrund zu charakterisieren. Das so gemessene Energiespektrum von Eisen wird im Bereich von 20 TeV bis 500 TeV gut durch ein Potenzgesetz beschrieben. Zweitens beschreibe ich eine Suche nach Gammastrahlung oberhalb von 100 GeV von den drei Galaxien Arp 220, IRAS 17208-0014 und IC 342. Diese drei Galaxien haben hohe Sternentstehungsraten und daher viele Supernova-Überreste, welche kosmische Strahlung erzeugen. Diese wechselwirkt erwartungshalber mit den dichten Staubwolken in den Sternentstehungsgebieten und erzeugt Gammastrahlung. VERITAS konnte keine solche Gammastrahlung messen. Die daraus abgeleitete Höchstgrenze für die Luminosität schränkt theoretische Modelle der Erzeugung und Propagation von kosmischer Strahlung in der Galaxie Arp 220 ein. / The energy spectrum of cosmic rays can provide important clues as to their origin and propagation. Different experimental techniques have to be combined to cover the full energy range: Direct detection experiments at lower energies and indirect detection via air showers at higher energies. In addition to detecting cosmic rays at Earth, we can also study them via the electromagnetic radiation, in particular gamma rays, that they emit in interactions with gas, dust, and electromagnetic fields near the acceleration regions or in interstellar space. In the following I will present two studies, both using data taken by the imaging air Cherenkov telescope (IACT) VERITAS. First, I present a measurement of the cosmic ray iron energy spectrum. I use a novel template likelihood method to reconstruct the primary energy and arrival direction, which is for the first time adapted for the use with iron-induced showers. I further use the presence of direct Cherenkov light emitted by charged primary particles before the first interaction to identify iron-induced showers, and a multi-variate classifier to measure the remaining background contribution. The energy spectrum of iron nuclei is well described by a power law in the energy range of 20 to 500 TeV. Second, I present a search for gamma-ray emission above 100 GeV from the three star-forming galaxies Arp 220, IRAS 17208-0014, and IC342. Galaxies with high star formation rates contain many young and middle-aged supernova remnants, which accelerate cosmic rays. These cosmic rays are expected to interact with the dense interstellar medium in the star-forming regions to emit gamma-ray photons up to very high energies. No gamma-ray emission is detected from the studied objects and the resulting limits begin to constrain theoretical models of the cosmic ray acceleration and propagation in Arp 220.

Teilchenschauer können sowohl durch Photonen als auch durch geladene Teilchen erzeugt werden. Letztere kommen etwa 1000-mal häufiger vor als die durch Photonen erzeugten Teilchenschauer. Dies beeinflusst die Sensitivität des VERITAS-Experiments erheblich. Um diese gegenüber Gammastrahlung zu steigern, ist es notwendig, die Gamma-Hadron Separation zu verbessern. In dieser Dissertation wurde eine Analysemethode, basierend auf Boosted Decision Trees (BDTs), entwickelt und für die Analyse der Daten des VERITAS-Observatoriums optimiert. Das große Potential zur Unterscheidung von Teilchenschauern der Photonen und der Hadronen wird anhand ausführlicher Tests und systematischer Studien mit Simulationen von Gammastrahlung und Beobachtungsdaten der kosmischen Strahlung verdeutlicht. Im Vergleich zur Standardanalyse kann die Sensitivität mit Hilfe der BDT Methode deutlich erhöht werden. Die entwickelte und optimierte BDT Methode wird auf Beobachtungsdaten der Cygnus-Region angewandt. Diese ist eine der aktivsten sternbildenden Regionen unserer Galaxie und beherbergt eine Vielzahl von potentiellen kosmischen Teilchenbeschleunigern. Aufgrund der enormen Dichte an potentiellen Quellen sowie der hohen Wahrscheinlichkeit, neue Quellen zu detektieren und zu identifizieren, wurde die Cygnus-Region von April 2007 bis Juni 2012 mit VERITAS beobachtet. Die Beobachtungsdaten wurden mit einer für diese Himmelsregion optimierten Analysetechnik aufbereitet und ausgewertet. Vier Quellen hochenergetischer Gammastrahlung wurden detektiert: VER J2031+415, VER J2019+407, VER J2019+368 und VER J2016+371. Detaillierte spektrale Untersuchungen werden vorgestellt, gefolgt von einer Diskussion möglicher assoziierter Objekte in anderen Wellenlängenbereichen. Schließlich konnten mit Hilfe der verbesserten Sensitivität von VERITAS durch die BDT Methode niedrigere Obergrenzen für den Fluss der hochenergetischen Gammastrahlung von 50 potentiellen Gammastrahlungsquellen abgeleitet werden. / Particle showers can be generated by photons or charged cosmic rays. Before applying any selection requirements, showers initiated by cosmic rays are about 1000 times more common than those initiated by photons. This constitutes a vast amount of background events measured by VERITAS, limiting the sensitivity to gamma rays. To improve the separation power between gamma-ray and cosmic-ray showers, an analysis technique based on Boosted Decision Trees (BDTs) is developed. Extensive tests are performed to study the discrimination capabilities of the BDT method using cosmic-ray data and Monte-Carlo simulations of gamma rays. Compared to the VERITAS standard analysis, the BDT method improves the sensitivity of detecting gamma rays. The BDT method is applied to data obtained from observations of the Cygnus region, one of the most active star-forming regions of our Galaxy. It hosts numerous astrophysical objects capable of accelerating particles to extremely high energies, such as supernova remnants, pulsar wind nebula, binary systems, and associations of massive OB stars. The high density of potential sources and the information from multiwavelength observations led VERITAS to perform observations of the Cygnus region between April 2007 and June 2012. Four sources were detected in very-high-energy gamma rays: VER J2031+415, VER J2019+407, VER J2019+368, and VER J2016+371. They were analysed in detail and compared to possible counterparts measured at other wavelengths. The spectra of the three of the sources were fit to a power law. Two out of three spectra are consistent with those obtained in previous measurements, where the third one shows a softer spectral index than the published result. Finally, the greater sensitivity reached with the BDT method allowed the derivation of the most stringent upper limits to date on 50 potential gamma-ray sources.

The history of recent galactic nucleosynthesis activity can be studied by <br />measurements of the 1.809 MeV gamma-ray line arising from the decay of <br />radioactive 26Al.<br />The COMPTEL telescope aboard the Compton Gamma-Ray Observatory, launched on <br />April 5, 1991, permits for the first time an extensive investigation of the <br />1.8 MeV radiation throughout the entire sky.<br />The aim of this thesis is to infer the galactic distribution of 26Al from <br />these measurements and to identify the dominant sources of this <br />radioactive isotope.<br /><br />The first part of the thesis is dedicated to the reconstruction of <br />the 1.8 MeV intensity distribution from the measured data.<br />It is demonstrated that the use of conventional deconvolution <br />algorithms, like maximum likelihood or maximum entropy inversion, <br />leads to lumpy, noise-dominated intensity distributions.<br />Nevertheless, simulations can help to assess the uncertainties in the <br />reconstructed images, which permits the scientific exploitation of the <br />recovered skymaps.<br />Alternatively, a multiresolution approach is proposed, which largely <br />reduces the uncertainties in the reconstructed 1.8 MeV intensity <br />distribution.<br />In summary, 1.8 MeV emission is mainly concentrated towards the <br />galactic plane, which clearly demonstrates that the bulk of 26Al is of <br />galactic rather than local origin.<br />However, distinct emission features towards Cygnus, Carina, and the <br />Auriga-Camelopardalis-Perseus region are inconsistent with a smooth <br />galactic 1.8 MeV emission profile, pointing towards a massive star <br />origin of 26Al.<br /><br />The second part of the thesis consists of a multi-wavelength <br />comparison of COMPTEL 1.8 MeV data which aims in the <br />identification of the origin of galactic 26Al.<br />For the comparison, a rigorous Bayesian analysis is applied, which <br />is the only consistent framework that allows inference based on the <br />comparison.<br />It turned out that the 1.8 MeV distribution follows very closely <br />the distribution of free electrons in the Galaxy which is traced by <br />thermal bremsstrahlung, observable in the microwave domain.<br />The similarity of the 1.8 MeV intensity distribution to the<br />thermal bremsstrahlung distribution implies a direct proportionality <br />between the 26Al and the massive star column densities, which strongly <br />supports that massive stars are the origin of galactic 26Al.<br />In particular, ONeMg-novae and AGB stars can be excluded as dominant <br />26Al sources since their galactic distribution is not expected to <br />correlate with the distribution of free electrons.<br /><br />The correlation between 26Al and free electrons established, the <br />analysis of 1.8 MeV gamma-ray line emission can complement our knowledge <br />about star formation and the distribution of ionized gas throughout <br />the entire Galaxy. <br />While COMPTEL made the first step in providing the first all-sky map <br />in the light of the 1.809 MeV line, INTEGRAL, the next generation gamma-ray spectrometer, will allow a detailed study of current star formation <br />in the Galaxy.

In this thesis I describe the development of a compact camera for ground-based multi TeV gamma-ray astronomy, using the Imaging Atmospheric Cherenkov Telescope (IACT) technique. The camera is based on multi-anode photomultipliers (MAPM) and is designed for use on the Gamma Cherenkov Telescope (GCT), which is proposed to be part of the Small Size Telescope (SST) array of the Cherenkov Telescope Array (CTA). GCT achieves high performance with a compact and cost efficient design via a Schwarzschild-Couder (SC) dual-mirror optical system. The GCT optical design allows the use of a compact camera of diameter roughly 0.5 m. The curved focal plane is equipped with 32 tiles of 64-pixels MAPM for a total of 2048 pixels of ~0.2° angular size, resulting in a field of view of ~9°. The GCT camera is designed to record the flashes of Cherenkov light from electromagnetic cascades, which last only a few tens of nanoseconds. I give a detailed description of the design, the challenges encountered during testing in the lab, and the performance of the most critical components. I give details on the custom front-end electronics modules that provide the required fast electronics, facilitating sampling and digitization, as well as first level of triggering. The camera-level triggering system is a custom backplane, developed to reject spurious triggers on the night sky background, which typically is of the order of few tens of millions of photons per pixel per second. This is to be compared with the rate of the astrophysical signal, which is of the order of few hundreds of events per second at the relevant energies. Additionally I provide a detailed description of all the software needed for the data acquisition and control of the camera, from the very low level drivers to high level and user friendly processes. I follow the commissioning of the camera, from the individual core components to the integration of the system. I then describe the integration of the camera on the GCT prototype telescope structure, and the achievement of "first light", validating for the first time the full proof-of-concept of an IACT with SC optics. I also report a study I performed on expectations for an extragalactic survey for blazars with CTA. The cumulative source count distribution of blazars is presented, including implications from two different phenomena: axion-like particle (ALP) to gamma-ray oscillations in the intergalactic magnetic field, and secondary gamma rays from hadronic origins. I conclude that a shallow and wide survey will provide the best science return for CTA, that the impact of ALP is modest and that the secondary mechanism of gamma-ray production would allow detection of blazars up to redshift of 1 in the multi-TeV energy band.