The constant flux of cosmic rays that bombard Earth from within our own galaxy are understood to come from both shell-type supernova remnants and pulsar wind nebulae (PWNe). Multiwavelength study of these objects can help us to understand what types of particles are accelerated, and gamma-ray emission is key to understanding the highest energy cosmic rays. In this thesis, I analyze and interpret observations made with the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a gamma-ray telescope located in Southern Arizona. LS 5039 and HESS J1825-137 occupy the same field of view on the sky and were observed for about 8 hours with VERITAS. LS 5039 is a gamma-ray binary, and the observations supports theories that the compact object hosts a PWN which is continuously interacting with the nearby star. HESS J1825-137 is a very extended PWN with an extent of diameter greater than 1 degree on the sky. Using the VERITAS observations, I am able to measure the radial profile and compare the gamma-ray luminosity to other PWN. DA 495, a "Crab-like" PWN with unusually strong magnetic fields, was observed for about 70 hours with VERITAS. In this study, results are combined with radio and X-ray spectral information to allow for detailed astrophysical modeling of the region. This broadband spectral modeling places constraints on the properties of the particle population in this PWN and allows for both leptonic and hadronic emission scenarios to be evaluated. Hadronic scenarios instil doubt on the pure PWN interpretation and favor a previously undetected shell-type remnant being present.

Pulsar wind nebulae (PWNe), nebulae harbouring a rotation-powered neutron star that was born in a supernova, provide opportunities to study highly relativistic pulsar winds and their interaction with the surrounding medium. Particularly interesting are PWNe that do not show any sign of the expected surrounding SNR shell and were thought to be born in subenergetic explosions or with unusual progenitors. The detection of a shell around one such PWN suggested that shells are indeed produced but may be faint due to unseen shocked ejecta, a low density environment, and/or a young age that has not yet allowed the shell to brighten and become visible. Here, by using observational X-ray data from modern telescopes with excellent spatial and energy resolution (Chandra and XMM-Newton), we target PWNe that do not have prominent SNR shells, and are known to be in varied environments, to further explore the characteristics of this growing, but poorly explored, class of PWNe. By combining imaging and spectroscopic results, we study the morphology of the PWNe, search for thermal emission from shock-heated material, investigate the energetics of the nebulae, and search for candidates for the neutron stars powering the nebulae. We find that while the faint shell surrounding G21.5–0.9 can be explained as a young PWN evolving in a low density medium, CTB 87 (G74.9+1.2) appears to be in an advanced stage of evolution, and G63.7+1.1 appears to be both in an advanced stage of evolution and in a dense environment. By performing spatially resolved spectroscopy, we have shown how the spectral characteristics vary across the PWNe, and note that more data will place better constraints on possible thermal emission in these remnants. The imaging portion of these studies has revealed intriguing large-scale morphologies for CTB 87 and G63.7+1.1, as well as a torus-jet structure in CTB 87 and neutron star candidates in both CTB 87 and G63.7+1.1. We conclude that both CTB 87 and G63.7+1.1 are likely interacting with the supernova remnant reverse shock, and CTB 87 may be additionally influenced by the motion of its neutron star.

Abbildende Cherenkov-Teleskope haben in den letzten Jahren eine groẞe Anzahl neuer Gammastrahlungsquellen im Bereich sehr hoher Energien (VHE, very high energy, E>100 GeV) entdeckt. Diese Studie behandelt Vertreter von drei unterschiedlichen Klassen von galaktischen Gammastrahlungsquellen: die Supernova-Überreste G1.9+0.3 und G330.2+1.0, den Pulsarwind-Nebel HESS J1303-631 und das Binärsystem PSR B1259-63/LS 2883. Für alle Objekte werden die Analyse der H.E.S.S.-Daten und die Modellierung der Emission unter Einbeziehung von Daten aus anderen Wellenlängenbereichen dargestellt. / Recently, imaging atmospheric Cherenkov telescopes have discovered numerous new sources representing various source classes in the very high energy (VHE; E>100 GeV) sky. This work presents studies of representatives of three types of Galactic VHE emitters: the Supernova remnants G1.9+0.3 and G330.2+1.0, the pulsar wind nebula HESS J1303-631 and the binary system PSR B1259-63/LS 2883. The analysis of the H.E.S.S. data and the broadband emission modeling are presented.

HESS J1741−302 ist eine nicht identifizierte Quelle sehr hochenergetischer Gammastrahlen, welche circa 1,7 Grad vom Zentrum der Milchstraße entfernt liegt. Diese Quelle ist eines der schwächsten Objekte im TeV-Bereich mit einem Photonfluss von Φ(>1 TeV) = (1.65 ± 0.28stat ± 0.33sys) × 10^−13 cm^−2 s^−1, was ~1% des Krebsnebelflusses im gleichen Energiebereich entspricht. Die Analyse des aktuellen H.E.S.S. Datensatzes von 145 Stunden Beobachtungen mit hoher Qualität gibt Einblicke in die Morphologie von HESS J1741−302. Das Energiespektrum von HESS J1741−302 geht über 10 TeV hinaus, ohne dabei ein klares Anzeichen für einen spektralen Abbruch zu zeigen. Das Spektrum kann durch ein Potenzgesetz mit einem spektralen Index von Γ = 2.28 ± 0.16stat ± 0.20sys und einer Normierung bei 1 TeV von Φ0 = (2.12 ± 0.42stat ± 0.42sys) × 10^−13 cm^−2 s^−1 TeV^−1 beschrieben werden. In der vorliegenden Arbeit werden verschiedene Szenarien für die beobachtete Gammastrahlung und deren Entstehung in Betracht gezogen. Diese beinhalten die Wechselwirkung von Protonen der kosmischen Strahlung mit Molekülwolken entlang der Sichtlinie, IC Streuung an Infrarot-Photonen eines nahe gelegenen OH/IR Sterns und die Präsenz eines Pulsarwindnebels, welcher möglicherweise zu PSR B1737−30 gehört. / HESS J1741−302 is an unidentified very-high-energy (VHE) γ-ray source located in the Galactic Plane at about 1.7° away from the Galactic Center. It is one of the faintest TeV objects detected so far, with a flux Φ(>1 TeV) = (1.65 ± 0.28stat ± 0.33sys) × 10^−13 cm^−2 s^−1 corresponding to ~ 1% of the Crab Nebula flux at the same energies. The data analysis of an updated high-quality dataset of ~145 hours of VHE H.E.S.S. data taken between 2004 and 2013 has revealed the morphology of HESS J1741−302. The γ-ray spectrum of HESS J1741−302 extends beyond 10 TeV without showing any clear evidence of a cut-off. The source spectrum is well described by a power-law model with a spectral index of Γ = 2.28 ± 0.16stat ± 0.20sys and a normalization at 1 TeV of Φ0 = (2.12 ± 0.42stat ± 0.42sys) × 10^−13 cm^−2 s^−1 TeV^−1. Different scenarios will be considered in this thesis, including the interaction of cosmic-ray protons with molecular clouds found along the line of sight, inverse Compton scattering of infra-red photons provided by a nearby OH/IR star and the presence of a nearby pulsar wind nebula possibly related to PSR B1737−30, in order to explain the observed VHE gamma-ray emission.

Diese Arbeit beschreibt die Identifikation der bisher unidentifizierten TeV Gammastrahlungsquelle, HESS J1303-631 als Pulsarwind-Nebel, angetrieben von dem Pulsar PSR J1301-6305. Dieses Ergebnis wird durch den Nachweis von energieabhängiger Morphologie in den vom High Energy Stereoscopic System (H.E.S.S.) genommenen Daten und durch die Detektion eines neuen Röntgen-Pulsarwind-Nebels in XMM-Newton Daten erreicht. Zudem wird eine obere Schranke auf den Fluss von Radiostrahlung aus Beobachtungen mit dem Parkes Radioteleskop bei 4.48 GHz abgeleitet. Diese Ergebnisse können in einem leptonischen Modell des Pulsarwind-Nebels verstanden werden, wo Elektronen und Positronen in der Nähe des Termination Shocks des Pulsarwindes auf ultrarelativistische Energien beschleunigt werden. Diese Leptonen bilden einen ausgedehnten Pulsarwind-Nebel, der auf Grund des inversen Compton-Effekts und Synchrotronstrahlung TeV Gammastrahlung beziehungsweise Röntgen- und Radiostrahlung erzeugt. Da nur eine obere Grenze auf den Radiofluss abgeleitet wurde, erfolgte die Modellierung im Rahmen eines einfachen ``one zone models'''', wo angenommen wird, dass die Radio-, Röntgen- und Gammastrahlung alle von derselben Leptonenpopulation erzeugt werden. Das Modell wird aber trotzdem von den Daten schon eingeschränkt und liefert ein schwaches Magnetfeld von ungef 0.9 Microgauss. Diese Magnetfeldstärke ist überraschend niedrig, da in ähnlichen Systemen die Magnetfeldstärken eher bei 10 Microgauss liegen. Andererseits passt das Ergebnis gut zu dem sehr niedrigen Synchrotronstrahlungsfluss. Ein derart schwaches Magnetfeld wird im theoretischen Szenario eines ausgedehnten, beziehungsweise entwickelten Pulsarwind-Nebels erklärt. / This work represents the identification of the very high energy, E > 100 GeV (VHE), Gamma-ray source HESS J1303-631 as a pulsar wind nebula (PWN) powered by the pulsar PSR J1301-6305. This is achieved through the detection of energy dependent morphology in the High Energy Stereoscopic System (H.E.S.S.) data, the detection of a new X-ray PWN in archival XMM-Newton X-ray observations, as well as multi-wavelength modeling of the source and its energetics. An upper limit on the radio synchrotron flux is obtained from observations made by the Parkes telescope at 4.48 GHz. The combined Gamma-ray, X-ray and radio measurements are used to constrain a leptonic emission model, where strong winds of relativistic electrons and positrons from the pulsar power the acceleration of particles to ultrarelativistic energies at the wind termination shock region, and these shock accelerated leptons then form a nebula which emits in the X-ray and radio bands via synchrotron emission in the ambient magnetic field and Gamma-rays through the inverse Compton mechanism. One surprising result of this analysis is the anomalously low magnetic field derived for the PWN. Typical values for PWNe are on the order of 10 microgauss. For this source, however, the low synchrotron levels predict an average magnetic field of approximately 0.9 microgauss. The low magnetic field is explained in the scenario of an expanded/evolved PWN.

Les expériences actuelles en astronomie gamma sont le satellite Fermi-LAT et les expériences au sol tel que H.E.S.S., VERITAS et MAGIC. La surveillance des sources d’énergie très élevées indique une physique diversifiée. Afin d’étudier la forme la plus énergétique de radiation et les phénomènes les plus violents qui se déroulent dans l’Univers, l’analyse des sources individuelles est importante. Les BL Lac, un type de galaxie active, constituent la classe de source extragalactique la plus abondante détecté dans les énergies du GeV au TeV, tandis que le nébuleuses de vent de Pulsar sont la classe la plus peuplée dans le plan galactique. Ces deux types  de sources ont des émissions variables de rayons gamma.Dans cette thèse, la variabilité de l'objet BL Lac B2 1215 + 30 est étudiée avec les données du satellite Fermi-LAT. Une grande variation de flux, détectée par Fermi-LAT en février 2014, est simultanée avec un éruption très lumineux observé au TeV par l'expérience VERITAS. En collaboration avec la collaboration VERITAS, la variabilité du flux de rayons gamma a été utilisée pour établir des contraintes sur la taille de la région d'émission et sur le facteur Doppler. La variabilité à long terme, en utilisant près de neuf ans de données de Fermi-LAT de 100 MeV jusqu'à 500 GeV, a permis de détecter plusieurs flares. L'étude de la variabilité du flux indique un comportement quasi périodique avec une période de jours.Ensuite, la variabilité du flux de l’un des objets les plus étudié, la Nébuleuse du Crabe, au TeV est étudiée avec dix ans d'observation de l'expérience H.E.S.S. Le spectre de la nébuleuse du crabe est mesuré de 280 GeV jusqu'à 62 TeV. Ceci est la première mesure qui s'étend à ces très hautes énergies. Considérée comme une “chandelle standard” en astronomie gamma, la nébuleuse du crabe est une source utilisée pour l'étalonnage et l'étude des instruments. L’observation de variations du flux au GeV par le satellite Fermi-LAT a par conséquent été une découverte inattendue. Ces variations de flux au GeV ont motivé la recherche de variations de flux au TeV en utilisant les données de l'expérience H.E.S.S. La position de la nébuleuse de crabe dans l'hémisphère nord et la localisation de H.E.S.S. en Namibie rendent cette enquête complexe en raison des importantes erreurs systématiques introduites par des conditions d'observation non optimales. Le travail sur la nébuleuse du crabe montre que la prise en compte de la transparence atmosphérique pour l'étude de l'évolution du flux avec le temps résulte en une réduction des effets systématiques. Aucune variation de flux n'a été observée à des énergies supérieures à 1 TeV dans les données de H.E.S.S. I. Une autre variation de flux au GeV signalée par le Fermi-LAT en octobre 2016 par télégramme astronomique, a été étudiée avec H.E.S.S. II. Cette analyse a montré que le GeV éruption a duré pendant un mois, et le flux avec H.E.S.S. a une variance excessive de 15 %. Cela devrait être comparé à l'incertitude systématique de 20 % fréquemment citée par H.E.S.S. / The current state-of-the-art experiments in gamma-ray astronomy are the Fermi-LAT in space and the ground-based H.E.S.S., VERITAS and MAGIC experiments. The monitoring of the very-high-energy gamma-ray emitting sources indicates the diverse physics taking place in astrophysical environments. To study the most energetic form of radiation and the most violent phenomena taking place in the Universe, individual source analyses are important. BL Lac objects, a subcategory of active galaxies, are the most abundant source class detected both in the GeV andTeV energies, while pulsar wind nebulae represent the most numerous identified source class in the galactic plane. Both source classes exhibit gamma-ray flux variations.In this thesis, the gamma-ray variability of the BL Lac object B2 1215+30 is presented with Fermi-LAT data. A bright flare, with 16 times the average quiescent flux, was detected in February 2014.In collaboration with the VERITAS experiment, the gamma-ray variability was investigated over five decades in energy. This work resulted in the detection of a luminous flare, seen simultaneously in GeV and TeV energies by both instruments. These results were used to set constraints on the size of the emission region and on the Doppler factor of the relativistic jet. Additionally, the long-term variability was studied using nine years of Fermi-LAT data. This brought out new flux enhancements, which characterize the long-term lightcurve from 100 MeV up to 500 GeV. Other striking characteristics are a steady linear increase of the yearly average flux, together with a hardening of the spectral index. The investigation of the light curve indicates a hint of quasi-periodic behavior with a period of around 1083±32 days.This work includes spectrum and flux variability studies for the well-studied but ever-surprising Crab Nebula at TeV energies with more than a decade of H.E.S.S. observations. The spectrum measured in this work goes from 280 GeV to 62 TeV, making this the first measurement tha textends to such very-high-energies. Considered as a standard candle for ground-based gamma-ray astronomy, the Crab Nebula is also used for calibration and instrument studies. The detection of GeV flares by the Fermi-LAT were unexpected and motivated the search of flux variations at TeVenergies with the H.E.S.S. experiment. The position of the Crab Nebula in the northern hemisphere makes this investigation challenging due to the large systematic uncertainties introduced by the non-optimal observation conditions. This work showed that the systematic uncertainties can be reduced by taking into account the atmospheric transparency. No flux variations were found at energies above 1 TeV from the H.E.S.S. I data. A flare reported by the Fermi-LAT in October 2016 was also investigated. This analysis showed the GeV flare lasting for one month, while the flux withH.E.S.S. II had an excess variance of 15 %. This should be compared to the commonly quoted 20% systematic uncertainty by H.E.S.S. experiment.