Dissertations / Theses on the topic 'Pulsars – Observations'

The study of pulsars in binary systems produces a wide variety of scientific results, including unique tests of general relativity and constraints on the equation of state of matter at extremely high densities. Through pulsar timing (which utilizes the fact that pulsars are precise clocks), it is possible to precisely measure the orbital parameters and masses of some binary pulsars, which can place constraints on their mass transfer histories. We present timing results for two binary pulsars. The intermediate-mass binary pulsar (IMBP) PSR J0621+1002 is a mildly recycled pulsar in an 8.3-day orbit around a massive white dwarf (WD) companion. It is one of only two known IMBPs with a precise mass measurement. We collected 9 days' worth of data using the 305-metre Arecibo telescope (allowing for full orbital coverage), which we used to improve constraints on the advance of periastron, and in turn, the pulsar and companion masses (to 1.53^{+0.10}_{-0.20}M☉ and 0.76^{+0.28}_{-0.07} solar masses, respectively) and inclination angle of the system. These results, combined with the relatively long orbital period, suggest a disk accretion evolutionary scenario involving short-lived periods of hyper-accretion, in which a moderate amount of mass has been transferred to the neutron star (NS). PSR J1906+0746 is a young pulsar in a compact ~4-hour orbit around a companion that was discovered in the early stages of the PALFA survey using the 305-metre Arecibo telescope. We present the timing results using data collected between 2005 to 2009 using the Green Bank, Arecibo, and Nançay telescopes. We have measured the advance of periastron, the time dilation and gravitational redshift parameter, and the orbital decay, and we find the pulsar and companion masses to be 1.323^{+0.011}_{-0.011}M☉ and 1.290^{+0.011}_{-0.011} solar masses, respectively. Although the companion may be a second NS, it is more likely to be a massive WD. The system's evolution probably involved a substantial transfer of mass from the WD progenitor onto the NS progenitor through Roche-lobe overflow accretion, followed by the formation of the WD, and a short common envelope phase, and finally the ejection of the envelope and the pulsar-forming supernova.

In this thesis, X-ray observations of four accretion powered pulsars are presented. Using RXTE observations of 4U 1907+09, we found three new pulse periods of the source. We found that the source spun-down almost at a constant rate of $dot nu$ = (-3.54 $pm 0.02) times 10^{-14}$ Hz s$^{-1}$ for more than 15 years. Using RXTE observations, X-ray flux related spectral and timing features in 2S 1417-62 were, in general, interpreted as a sign of a disc accretion with a similar geometry with a varying mass accretion rate, whereas spectral and timing features of the low X-ray flux regions were interpreted as a sign of possible temporary accretion geometry change prior to the next periastron. Using XMM-Newton and RXTE observations of SAX J2103.5+4545, we discovered quasi periodic oscillations around 0.044 Hz (22.7 sec) while the source was spinning-up with a rate of $(7.4pm0.9)times10^{-13}$Hz s$^{-1}$. In the X-ray spectrum, we also found a soft component consistent with a blackbody emission with ${rm{kT}}sim1.9$keV. Using RXTE observations, we also studied spectral evolution of Her X-1

The technique of pulsar timing reveals a wealth of new information when a precision of ∼1 μs or better is reached, but such precision is difficult to achieve. This thesis describes a series of very high precision timing observations that improve our knowledge of the targeted pulsar systems. We begin by describing a newly-developed baseband recording and coherent dedispersion system (CPSR2), along with a new object-oriented software development environment for pulsar data processing. Data obtained with this new instrument during a 3 year observing campaign at the Parkes 64m radio telescope are analysed in a number of novel ways. The mean profile of PSR J1022+1001 is shown to be stable on timescales of a few minutes, in contrast with previously published claims. We obtain a level of precision an order of magnitude better than any previous timing of this pulsar. In addition, we observe dramatic changes in the mean profile of the relativistic binary pulsar J1141?6545, which broadens by ∼50% over the time span of our observations. This is interpreted as evidence for secular evolution of the line of sight to the emission cone, caused by General relativistic geodetic precession which tilts the spin axis of the pulsar. High precision CPSR2 observations of the extraordinary double pulsar binary system are presented and we construct calibrated, mean polarimetric profiles for PSR J0737?3039A, in two frequency bands. These profiles provide a reference against which future profile evolution may be detected, given that we expect geodetic precession to alter the observed mean profile on an even shorter time scale than for PSR J1141?6545. The bulk of this thesis involves timing a selection of millisecond pulsars whose physical characteristics should allow the highest precision to be obtained. We measure several new proper motions and parallax distances. Shapiro delay is used to constrain the inclination angles and component masses of several of the binary systems in our source list. In addition, subtle periodic variations of the orbital parameters of two nearby binary millisecond pulsars are detected and attributed to annual orbital parallax, providing additional constraints on their three-dimensional orbital geometries. Future observations of these two sources may lead to more stringent tests of post-Keplerian gravitational theories. Finally, we use the timing residuals of one very stable source (PSR J1909?3744) as a reference against which we time PSR J1713+0747 with a root-mean-square precision of 133 ns, amongst the best timing residuals ever obtained. This result is an important step in the search for long-period gravitational waves using pulsar timing arrays.

Thesis (PhD) - Swinburne University of Technology, 2006.
A dissertation presented in fulfillment of the requirements for the degree of Doctor of Philosophy, Swinburne University of Technology - 2006. Typescript. Includes bibliographical references (p. 182-190).

Les données de haute précision de chronométrage des pulsars rapides vont contribuer significativement à l'astrophysique des étoiles à neutrons, à la dynamique du système solaire, à l'astrométrie, à la métrologie du temps et à la cosmologie. Deux années de données de chronométrage du pulsar rapide PSR1937+214 ont été analysées afin de déterminer d'éventuelles erreurs systématiques introduites par les modèles d'analyse. Un logiciel d'analyse a été développé à cette fin. Des erreurs systématiques bien supérieures aux incertitudes théoriques calculées ont ainsi été révélées. Les paramètres du pulsar déterminés par la procédure d'analyse, comme sa période et sa position, dépendent alors fortement des théories du mouvement de la Terre et des échelles de temps utilisées dans l'analyse. D’autre part, des observations de chronométrage de pulsars rapides ont débuté à l'Observatoire de Nançay. Celles-ci nécessitent un dispositif spécial afin de compenser l'effet de dispersion de la matière ionisée interstellaire. Le dispositif adopte et construit à Nançay à cet effet est décrit ainsi que les programmes permettant son fonctionnement. Les méthodes d'observation et les résultats préliminaires d'une campagne d'observation effectuée au cours de l'été 1988 sont donnés.

An eleven-year series of radio timing observations of 0833- 45 (Vela) and PSR 1641- 45 is presented. During this time, five large spin-ups ("glitches") were observed in 0833- 45 and one in 1641-45. The stellar response to these events is investigated, and the three relat ively long complete inter-glitch intervals in 0833-45 are modeled. The results are of relevance to studies of the interiors of neutron stars. The initial aim of the project - to obtain good observational coverage of large glitches in the Vela pulsar - was successfully achieved, and high quality observations of the periods between glitches were obtained as a by-product. The results of the analysis presented here provide support for the existence of both linear and non-linear coupling in the Vela pulsar, and put a limit on the former in PSR 1641- 45. The recently observed existence of a rapidly recovering component of part of a glitch in Vela was verified in the subsequent glitch, although there is now evidence to contradict the suggestion that this component involves a particular region of the star that is implicated in every glitch. Observations of a recent glitch in the same pulsar have resolved a small component of the spin-up; such a component has not been reported for any other large glitch.

Le Large Area Telescope à bord du satellite Fermi, lancé le 11 juin 2008, est un télescope spatial observant l'univers des hautes énergies. L'instrument couvre l'intervalle en énergie de 20MeV à 300 GeV avec une sensibilité nettement améliorée et la capacité de localiser des sources ponctuelles. Il détecte les photons gamma par leur conversion en paire électron-positron, et mesure leur direction et leur énergie grâce à un trajectographe et un calorimètre. Cette thèse présente les courbes de lumières et les mesures spectrales résolues en phase des pulsars radio et gamma détectés par le LAT. La mesure des paramètres spectraux (flux, indice spectral, et énergie de coupure) dépend des fonctions de réponse de l'instrument (IRFs). Une méthode développée pour la validation en orbite de la surface ecace est présentée en utilisant le pulsar de Vela. Les efficacités des coupures entre les données du LAT et les données simulées sont comparées à chaque niveau de la rejection du fond. Les résultats de cette analyse sont propagés vers les IRFs pour évaluer les systématiques des mesures spectrales. La dernière partie de cette thèse présente les découvertes de nouveaux pulsars individuels tels que PSR J0205+6449, J2229+6114, et J1048-5832 à partir des données du LAT et des éphémérides radio et X. Des analyses temporelles et spectrales sont investies dans le but de contraindre les modèles d'émission gamma. Finalement, nous discutons les propriétés d'une large population de pulsars gamma détectés par le LAT, incluant les pulsars normaux et les pulsars milliseconde.

In this work, we study theoretical and observational issues about pulsars (PSRs), pulsar wind nebulae (PWNe) and supernova remnants (SNRs). In particular, the spectral modeling of young PWNe and the X-ray analysis of SNRs with magnetars comparing their characteristics with those remnants surrounding canonical pulsars. The spectra of PWNe range from radio to γ-rays. They are the largest class of identified Galactic in γ-rays increasing the number from 1 to ∼30 during the last years. We have developed a detailed spectral code which reproduces the electromagnetic spectrum of PWNe in free expansion (tage .10 kyr). We shed light and try to understand issues on time evolution of the spectra, the synchrotron self-Compton dominance in the Crab Nebula, the particle dominance in PWNe detected at TeV energies and how physical parameters constrain the detectability of PWNe at TeV. We make a systematic study of all Galactic, TeV-detected, young PWNe which allows to find correlations and trends between parameters. We also discuss about the spectrum of those PWNe not detected at TeV and if models with low magnetized nebulae can explain the lack of detection or, on the contrary, high-magnetization models are more favorable. Regarding the X-ray analysis of SNRs, we use X-ray spectroscopy in SNRs with magnetars to discuss about the formation mechanism of such extremely magnetized PSRs. The alpha-dynamo mechanism proposed in the 1990’s produces an energy release that should have influence in the energy of the SN explosion. We extend the work done previously done by Vink & Kuiper (2006) about the energetics of the SN explosion looking for this energy release and we look for the element ionization and the X-ray luminosity and we compare our results with other SNRs with an associated central source.

Aquesta tesi està centrada en l’estudi de les nebuloses produïdes pel vent dels pulsars (acrònim anglès, PWNe), que es formen com a resultat de que la major part de l’energia rotacional del púlsar es perd per l’emissió d’un vent relativista de partícules. Els vents, supersònics respecte al medi interestel·lar, produeixen un xoc terminal on les partícules s’acceleren. Atès que la nebulosa conté fotons i camps magnètics, les partícules poden emetre a totes les freqüències, des de ràdio fins a energies TeV, mitjançant processos no tèrmics com el sincrotró i el Compton invers. Aquesta tesi estudia l’evolució d’aquesta radiació no tèrmica al llarg de la vida del púlsar, analitzant els canvis produïts a la distribució d’energia espectral com a resultat de l’expansió i la contracció del PWN a causa dels equilibris de pressió i la interacció amb l’entorn. La tesi considera en primer lloc el cas del complex format pel púlsar/PWN, PSR J0205 + 6449/3C 58, que és especial per la seva curta edat, la seva significativa potència i la seva similitud amb la nebulosa del Cranc (la millor estudiada). Es presenten els resultats de l’anàlisi de 8 anys de dades de Fermi-LAT. Utilitzant una efemèride contemporània per a la pulsació, s’ha pogut detectar significativament 3C 58 durant l’interval de fase fora del pic de PSR J0205 + 6449. He analitzat les dades amb un model de PWNe en funció del temps basat en el codi TIDE, desenvolupat pel grup en el que he treballat durant els darrers 8 anys. El meu model proporciona un ajust raonable a les dades; un en el que el PWN 3C 58 encara no reverbera. La reverberació és el període de l’evolució de les nebuloses quan el xoc invers creat per l’explosió de la supernova viatja cap al púlsar, comprimint-les. És un període relativament curt, però significatiu i que no ha estat gaire estudiat. La resta de la tesi estudia PWNe més antigues, o més joves com 3C 58, però evolucionant-les cap al futur per a analitzar el seu comportament durant la reverberació. Aquest estudi ha conduït al descobriment i caracterització de la supereficiència. La supereficiència succeeix quan, a causa de la seva compressió deguda al xoc invers que torna de l’explosió de la supernova, la nebulosa està sotmesa a un escalfament adiabàtic significatiu. Degut a la compressió, el camp magnètic de la PWN també augmenta, així com hi ha més partícules a energies més altes que les que hi havia abans. Vaig trobar que aquest procés pot produir PWNe que durant poc temps emeten més energia en raigs X i en altres freqüències que el que tenen com a energia de rotació en aquell moment. Aquesta aparent contradicció és conseqüència del fet que potència rotacional del púlsar ja no és el dipòsit d’energia del sistema. Aquest període finalitza quan la pressió del camp magnètic, que augmenta significativament, és capaç d’equilibrar la pressió cinemàtica proporcionada pel xoc invers. He estudiat tant PWNe ben caracteritzades com una àmplia gamma de models de PWN representatius dels púlsars observats per estudiar les seves propietats de reverberació i supereficiència. Finalment, he estimat, mitjançant simulacions de Monte Carlo, quantes PWNe galàctics s’espera que reverberin o estiguin en una etapa de supereficiència en un moment donat i he realitzat prediccions per a possibles deteccions futures amb la propera generació d’instruments. Aquesta tesi es presenta com un compendi de resultats publicats. Tres articles publicats a The Astrophysical Journal, The Astrophysical Journal Letters i Monthly Notices of the Royal Astronomical Society conformen els capítols 2, 3 i 4 de la tesi.
Esta tesis se centra en el estudio de las nebulosas producidas por los vientos de púlsares (de su acrónimo en inglés, PWNe), que se forman como resultado de que la mayor parte de la energía de rotación del púlsar se pierde a través de la emisión de un viento relativista de partículas. Los vientos, al ser supersónicos con respecto al medio interestelar, producen un choque terminal donde las partículas se aceleran. Dado que la nebulosa contiene campos magnéticos y de fotones, las partículas pueden emitir en todas las frecuencias, desde radio hasta las energías TeV, a través de procesos no térmicos como sincrotrón y Compton inverso. Esta tesis estudia la evolución de esta radiación no térmica a lo largo de la vida del púlsar, analizando los cambios producidos en la distribución espectral de energía como resultado de la expansión y contracción del PWN debido a los equilibrios de presión y la interacción con el entorno. La tesis primero considera el caso del complejo formado por el púlsar/PWN, PSR J0205 + 6449/3C 58, que es especial por su corta edad, significativa potencia y parecido con la Nebulosa del Cangrejo (la mejor estudiada). Se presentan los resultados del análisis de 8 años de datos de Fermi-LAT. Utilizando una efeméride contemporánea para la pulsación, se ha podido detectar significativamente 3C 58 durante el intervalo de fase fuera del pico de PSR J0205 + 6449. He analizado estos datos con un modelo de PWNe dependiente del tiempo basado en el código TIDE, desarrollado por el grupo en el que he trabajado durante los últimos 8 años. Mi modelo proporciona un ajuste razonable a los datos; uno en el que el PWN 3C 58 aún no está en reverberación. La reverberación es el período de evolución de las nebulosas que ocurre cuando el choque inverso creado por la explosión de la supernova viaja hacia el púlsar, comprimiéndolas. Es un período relativamente corto pero significativo y apenas estudiado. El resto de tesis estudia PWNe más viejas, o más jóvenes como 3C 58 pero evolucionándolas hacia el futuro para analizar el comportamiento que muestran durante reverberación. Este estudio ha llevado al descubrimiento y caracterización de la supereficiencia. La supereficiencia ocurre cuando, debido a su compresión debido al impacto del choque inverso de la explosión de la supernova, la nebulosa está sujeta a un calentamiento adiabático significativo. Debido a la compresión, el campo magnético del PWN también aumenta, así como también hay más partículas a energías más altas que antes. Descubrí que tal proceso puede producir PWNe que durante un corto tiempo emiten más rayos X y fotones a otras frecuencias de lo que existe como energía de rotación en ese momento. Esto es consecuencia de que la potencia rotacional del púlsar ya no es la reserva de energía del sistema. Este período de reverberación termina cuando la presión del campo magnético, aumentada debido al incremento del propio campo, es capaz de equilibrar la presión cinemática proporcionada por el choque inverso. He estudiado tanto varias PWNe bien caracterizadas como una amplia gama de modelos PWN representativos de los púlsares observados para estudiar sus propiedades de reverberación y supereficiencia. Finalmente, estimé a través de simulaciones de Monte Carlo cuántas PWNe galácticas se espera que estén reverberando o en una etapa de supereficiencia en un momento dado y realicé predicciones para posibles detecciones futuras con la próxima generación de instrumentos. Esta tesis se presenta como un compendio de resultados publicados. Tres artículos publicados en The Astrophysical Journal, The Astrophysical Journal Letters y Monthly Notices of the Royal Astronomical Society conforman correspondientemente los Capítulos 2, 3 y 4 de la tesis.
This thesis focuses on the study of Pulsar wind Nebulae (PWNe), which form as a result of the bulk of the pulsar rotational energy begin lost via the emission of a relativistic wind of particles. The winds, being supersonic with respect to the interstellar medium, produce a termination shock where particles are accelerated. Since the nebula is threaded with photon and magnetic fields, particles are able to emit at all frequencies, from radio to TeV energies, via non-thermal processes such as synchrotron and inverse Compton. This thesis zooms into studying the evolution of this non-thermal radiation along the pulsar lifetime, analyzing the changes produced to the spectral energy distribution as a result of the expansion and contraction of the PWN due to pressure balances and interaction with the environment. The thesis first considers the case of the complex formed by the pulsar/PWN, PSR J0205+6449/3C 58, which is especial due to its young age, significant power, and similarity to the Crab Nebula (the best studied PWN). The thesis presents the results of the analysis of 8 years of Fermi-LAT data. The main aspect is that using a contemporaneous ephemeris for the pulsation, we could significantly detect 3C 58 during the off-peak phase interval of PSR J0205+6449. I analyzed the observed data with a time-dependent model of PWNe based on the code TIDE, developed by the group in which I worked over the last 8 years. My model provides a reasonable fit to data; one in which the PWN 3C 58 is not yet reverberating. Reverberation is the period of PWN evolution when the reverse shock created by the supernova explosion travels back towards the pulsar, compressing the wind bubble. It is a relatively short but significant period, barely studied. The rest of the thesis studies older PWNe, or younger ones like 3C 58 but evolved into the future so as to grasp the behavior of reverberation when they pass through it. This study has led to the discovery and characterization of superefficiency. Superefficiency happens when, due to its compression because of the returning reverse shock of the supernova explosion, the nebula is subject to significant adiabatic heating. To what extent this heating affected the luminosities of the PWN at different energy ranges was not clear. The thesis describes in detail how due to the compression, the magnetic field of the PWN also increases, as well as there are more particles at higher energies than there were previously. I found that such process can produce PWNe that for a short time emit more in X-rays and other frequencies than what they have as rotational energy at the time. The former is not a paradox, but the consequence of the fact that the rotational spinning down of the pulsar is no longer the energy reservoir of the system. This period ends when the magnetic field pressure, increased because of the magnetic field, significantly risen up, is able to detain the kinematic pressure provided by the reverse shock. I took on both, several well-characterized PWNe and a broad range of PWN models representative of the observed pulsars to study their reverberation and superefficiency properties. Having attained such modelling, I estimated via Monte Carlo simulations how many Galactic PWNe are expected to be reverberating or in a superefficiency stage at any given time and realized predictions for possible future detections with the next generation of instruments. This thesis is presented as a compendium of published results. Three papers published in The Astrophysical Journal, The Astrophysical Journal Letters, and Monthly Notices of the Royal Astronomical Society correspondingly conform Chapters 2, 3 and 4 of the thesis.

In this thesis, RXTE observations of 4U 1907+09 are presented. Timing analysis of these data sets have yielded quasi periodic oscillations (QPOs) at orbital phases corresponding to the two flares in every orbital period. Known continuous spin down trend and QPO behaviour at the flares strongly suggest that a transient accretion disk occurs at the flares. Our findings strongly suggested that neutron star passes through the equatorial wind of Be companion star. During these passages a transient disk forms around Be neutron star.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2000.
Includes bibliographical references.
This dissertation concerns the use of charge coupled devices (CCDs) for X-ray astronomy. Part I of this thesis focuses on the calibration of the Advanced CCD Imaging Spectrometer (ACIS), one of the two primary focal plane instruments on the Chandra X-ray Observatory. I begin by presenting a series of measurements made using undispersed synchrotron radiation. I analyzed the results of these experiments to extract parameters necessary to model the absolute detection efficiency of CCDs used as reference standards for ACIS characterization. I also discuss a novel "mesh" technique I pioneered to non-destructively measure the sub-pixel structure of CCDs, including the channel stops and gates. This work also provided a valuable tool for understanding the way charge is collected inside certain regions of the ACIS detectors. Guided by the results of the mesh experiments, I performed additional measurements to constrain recombination effects in the channel stops. Part II of this thesis concerns the X-ray properties of rotation-powered (radio) pulsars. High energy observations serve as a powerful diagnostic of the energetics and emission mechanisms of neutron stars. Associations between pulsars and the remnants of their supernovae provide independent measures of the distance and age of neutron stars, as well as constraints on their initial velocities, magnetic fields, and spin periods. I analyze ASCA and ROSAT observations of three young pulsars which are particularly well-suited for addressing these issues. I also present observations of the rotation-powered pulsar with the highest known magnetic field, and describe how the results relate to anomalous X-ray pulsars, another type of neutron star. Finally, I analyze Chandra observations of the supernova remnant Cas A. These data fully demonstrate the capabilities of ACIS and reveal a previously unknown point source at the center of the remnant.
by Michael James Pivovaroff.
Ph.D.

Les pulsars millisecondes permettent de relier le système de référence céleste extragalactique au système dynamique en comparant leurs positions VLBI à celles issues du chronométrage. La position du pulsar milliseconde PSR 1937+21 a été déterminée avec une précision de 0,004 seconde de degré par une expérience VLBI. Le corrélateur spécialise qui a été développe pour traiter ce type de données est décrit, ainsi que la technique de cartographie à référence de phase utilisée pour obtenir la position du pulsar par rapport à un quasar angulairement proche. Une technique de chronométrage des pulsars millisecondes utilisant les données VLBI a été développée. Les résultats présentés montrent que la précision du chronométrage est largement inférieure à une microseconde. En chronométrant régulièrement plusieurs pulsars, on peut obtenir une information sur la stabilité à long terme de l'échelle de temps atomique utilisée comme référence. On montre également comment cette technique peut être utilisée pour rechercher de nouveaux pulsars et pour étudier la scintillation dans le milieu interstellaire.

This Thesis presents the prospects for investigating gamma-ray pulsars with future Cherenkov facilities, the Cherenkov Telescope Array (CTA) and ASTRI mini-array (possible precursor for CTA). Gamma-ray pulsars are compact astrophysical objects which emit photons with energies up to ~100 GeV. The nature of the gamma-ray emission from these sources is not fully understood. In addition, the recent detection of the Crab pulsar with Cherenkov telescopes such as MAGIC and VERITAS at very high energies (VHE, >100 GeV) challenged current theoretical models. CTA will be a next-generation ground-based VHE gamma-ray instrument, designed to achieve a sensitivity of the order of one magnitude better than that of currently operating Cherenkov installations. It will consist of two arrays, one in northern and one in southern hemisphere, each with a large number of different-sized telescopes. Early observations can be carried out with CTA precursors, such as the ASTRI mini-array. I simulated the VHE emission from the 12 most energetic Fermi pulsars. I analyzed the Fermi-LAT data of these pulsars above 10 GeV and extrapolated their gamma-ray spectra up to ~160 TeV, in order to estimate how many of them will be detectable with CTA. In addition, I performed a detailed investigation of the pulsed VHE emission from the Crab pulsar, simulating the light curve detectable with CTA. I calculated with which accuracy it will be possible to study the timing properties of this pulsar with CTA and the ASTRI mini-array. Finally, I investigated VHE gamma rays from the Vela X region. Assuming different spatial distributions for the emission from the Vela pulsar wind nebula, I calculated more realistic estimates of the significance of the Vela pulsar detection with CTA. Using different software packages (ctools and Asrtisim), I also studied the extended Vela X emission and tested the resolving capabilities of CTA and the ASTRI mini-array.
Questa Tesi contiene i risultati di uno studio dell'emissione delle pulsar a raggi gamma osservate con futuri telescopi Cherenkov, il Cherenkov Telescope Array (CTA) ed il mini-array ASTRI (uno dei possibili precursori per CTA). Le pulsar a raggi gamma sono oggetti astrofisici compatti che emettono fotoni con energie fino a ~100 GeV. La natura dell'emissione di raggi gamma da queste sorgenti non è chiara. Inoltre, la recente rivelazione di emissione di altissima energia (VHE, >100 GeV) da parte della Crab pulsar con i telescopi Cherenkov MAGIC e VERITAS reppresenta una sfida per attuali modelli teorici. CTA sarà uno strumento nuova generazione, progettato per raggiungere una sensibilità un'ordine di grandezza migliore di quella dei telescopi Cherenkov attualmente in funzione. Esso comprenderà un array in ciascun emisfero con un gran numero di telescopi di dimensioni diverse. Le prime osservazioni verranno eseguite con precursori di CTA, come il mini-array ASTRI. Ho simulato l'emissione VHE dalle 12 Fermi pulsar più energiche. Ho analizzato i dati Fermi-LAT di queste pulsar ad energie superiori a 10 GeV ed estrapolato i loro spettri gamma fino a ~160 TeV, per stimare quante di loro saranno rivelabili con CTA. Inoltre, ho eseguito un esame più dettagliato dell'emissione VHE pulsata dalla Crab pulsar, simulando la curva di luce osservabile con CTA. Ho calcolato con quali accuratezza sarà possibile studiare le proprietà del timing di questa pulsar con CTA ed il mini-array ASTRI. Infine, ho studiato l'emissione di altissima energia (VHE) dalla sorgente Vela X. Assumendo diverse distribuzioni spaziali per l'emissione della wind nebula della Vela pulsar, ho calcolato stime più realistiche delle significatività della Vela pulsar con CTA. Utilizzando diversi pacchetti software (ctools e Asrtisim), ho anche studiato l'emissione estesa della Vela X e verificato la risoluzione angolare ottenibile con CTA ed il mini-array ASTRI.

In this thesis, X-ray and optical observations of accretion powered pulsars are presented. By using archival RXTE observations we work on the X-ray spectral and pulse timing analysis of 4U 1538-52, 4U 1907+09, SMC X-1 to have more detailed information about their orbital and spin parameters. For 4U 1538-52 and SMC X-1, we determined new orbital epochs. By using long term pulse history of 4U 1907+09, we were able to work spin-down trend of the system and also calculate the change in the spin-down rate. Using Fermi/GBM observations we can monitor bright accreting pulsar systems. We are producing long term histories of pulse frequency and flux of 20 continuosly monitoring systems. Adding Swift/BAT observations to GBM observations, for 4U 1626-67, we did reveal the characteristics belong to spin-down trend before and spin-up behaviour after torque reversal seen in 2008 February. Two newly discovered IGRJ06074+2205 and IGRJ01583+6713 sources are identified as X-ray binary systems and we found parameters of them like distance, magnitudes, by using both optical photometric and spectroscopic observations.

This thesis presents the results of a comprehensive multi-wavelength study of the high mass X-ray binary 2S 0114+650. This enigmatic source has previously been proposed to be the first in a new class of super-slow X-ray pulsars, containing a neutron star revolving once every 2.7 h. The 11.6 d orbital period of this system has been well established in both X-ray and optical wavelengths. During the initial stages of the research presented in this thesis we discovered an additional 30.7 d ???super-orbital??? modulation in the X-ray emission from this source. While similar periodicities seen in other X-ray binaries are commonly attributed to the precession of a warped accretion disc, the observational evidence suggests the absence of such a disc in 2S 0114+650. The purpose of this project is thus to determine the nature of the super-orbital modulation and to better constrain the astrophysical parameters of this system. To investigate the long-term variability we analysed ~8.5 yr of archived data from the Rossi X-ray Timing Explorer space telescope. The problem of the spurious ~24 h periods in this data was solved as a by-product of these studies. Follow-up pointed observations were obtained with this satellite in order to examine the spectral and temporal behaviour over the spin, orbital and super-orbital timescales. Independent confirmation of the super-orbital modulation was performed using ~2 yr of data from the INTEGRAL satellite obtained during a long-term monitoring campaign of the Cassiopeia region. The evolution of the spin, orbital and super-orbital periods over ~10 yr was examined using archived data from the Rossi X-ray Timing Explorer satellite. Radio observations were performed with the Giant Meterwave Radio Telescope to search for any radio emission associated with this source and to determine whether it is variable over the known periodicities. Near infrared observations were performed with the Mt Abu telescope to determine whether a Be star nature can be ruled out for the optical component. Finally, a statistical analysis of the properties of the confirmed super-orbital X-ray binaries was performed in order to search for commonalities between these systems.

L'origine de la variabilité observée dans l´émission X et Gammas des binaires X reste encore à ce jour un problème largement débattu en astrophysique des hautes énergies. Ces objets présentent notamment des fortes variations apériodiques et quasi-périodiques de leur luminosité sur d´aussi petites échelles de temps que la milliseconde et ce comportement erratique doit permettre de contraindre les modèles d´émission des zones les plus rapprochées des étoiles à neutrons ou des trous noirs de masse stellaire les constituant. Nous nous proposons dans ce manuscrit d´étudier le comportement de ces objets sous 3 angles différents : tout d´abord nous suivons l´évolution d´une source découverte par le satellite INTEGRAL, IGR J19140+0951. Par une étude spectro-temporelle et multi-instruments nous montrons que celle-ci doit vraisemblablement appartenir à la classe des binaires X de grande masse et abriter une étoile à neutron. Ensuite, nous proposons une nouvelle technique adaptée à l´étude temporelle des données provenant d´instruments à masques codés. Nous mettons en évidence par ce biais la présence de comportement périodiques et quasi-périodiques dans les données INTEGRAL/ISGRI de certains pulsars ou microquasars, jusqu'à des énergie proches de la centaine de keV. Enfin, nous proposons un modèle pour décrire la variabilité à basse fréquence des états les plus durs des binaires-X. Celui-ci est basé sur la comptonisation thermique de photons mous par une couronne chaude soumise à une onde de pression en géométrie cylindrique. Nous montrons alors par des solutions analytiques et une résolution numérique du problème qu´un tel modèle pourrait permettre de décrire certaines composantes observées dans les spectres de puissance des états durs ainsi que leur évolution tels que le bruit apériodique et certaines oscillations périodiques de basses fréquences
The exact origin of the observed X and Gamma ray variability in X-ray binaries is still an open debate in high energy astrophysics. Among others, these objects are showing aperiodic and quasiperiodic luminosity variations on timescales as small as the millisecond. This erratic behavior must put constraints on the proposed emission processes occuring in the vicinity of the neutron star or the stellar mass black-hole held by these objects. We propose here to study their behavior following 3 different ways: first we examine the evolution of a particular X-ray source discovered by INTEGRAL, IGR J19140+0951. Using timing and spectral data given by different instruments, we show that the source type is plausibly consistent with a High Mass X-ray Binary hosting a neutron star. Subsequently, we propose a new method dedicated to the study of timing data coming from coded mask aperture instruments. Using it on INTEGRAL/ISGRI real data, we detect the presence of periodic and quasi-periodic features in some pulsars and microquasars at energies as high as a hundred keV. Finally, we suggest a model designed to describe the low frequency variability of X-ray binaries in their hardest state. This model is based on thermal comptonization of soft photons by a warm corona in which a pressure wave is propagating in cylindrical geometry. By computing both numerical simulations and analytical solution, we show that this model should be suitable to describe some of the typical features observed in X-ray binaries power spectra in their hard state and their evolution such as aperiodic noise and low frequency quasi-periodic oscillations

L'origine de la variabilité observée dans l´émission X et Gammas des binaires X reste encore à ce jour un problème largement débattu en astrophysique des hautes énergies. Ces objets présentent notamment des fortes variations apériodiques et quasi-périodiques de leur luminosité sur d´aussi petites échelles de temps que la milliseconde et ce comportement erratique doit permettre de contraindre les modèles d´émission des zones les plus rapprochées des étoiles à neutrons ou des trous noirs de masse stellaire les constituant.

Nous nous proposons dans ce manuscrit d´étudier le comportement de ces objets sous 3 angles différents : tout d´abord nous suivons l´évolution d´une source découverte par le satellite INTEGRAL, IGR J19140+0951. Par une étude spectro-temporelle et multi-instruments nous montrons que celle-ci doit vraisemblablement appartenir à la classe des binaires X de grande masse et abriter une étoile à neutron. Ensuite, nous proposons une nouvelle technique adaptée à l´étude temporelle des données provenant d´instruments à masques codés. Nous mettons en évidence par ce biais la présence de comportement périodiques et quasi-périodiques dans les données INTEGRAL/ISGRI de certains pulsars ou microquasars, jusqu'à des énergie proches de la centaine de keV.

Enfin, nous proposons un modèle pour décrire la variabilité à basse fréquence des états les plus durs des binaires-X. Celui-ci est basé sur la comptonisation thermique de photons mous par une couronne chaude soumise à une onde de pression en géométrie cylindrique. Nous montrons alors par des solutions analytiques et une résolution numérique du problème qu´un tel modèle pourrait permettre de décrire certaines composantes observées dans les spectres de puissance des états durs ainsi que leur évolution tels que le bruit apériodique et certaines oscillations périodiques de basses fréquences.

Les pulsars sont des étoiles à neutrons qui tournent rapidement, émettent des ondes électromagnétiques et dont nous recevons des impulsions périodiques, agissant ainsi comme des phares cosmiques. L'extrême stabilité de ces signaux périodiques en font des outils inestimables, par exemple pour tester la théorie de la relativité générale ou détecter les ondes gravitationnelles basse fréquence. Cependant, leur observation en radio peut être rendue difficile à cause d'interférence radioélectrique (RFI) d'origine diverse. Ces interférences peuvent masquer les signaux des pulsars, ce qui complique significativement la détection et l'extraction précise de leur signature. Cette thèse vise à proposer des techniques d'apprentissage profond pour atténuer l'impact des RFI dans les observations de pulsars. En particulier, cette thèse illustre l'intérêt de ces méthodes dans le cas d'observations menées par NenuFAR, un précurseur pour le segment basse fréquence du futur Square Kilometre Array (SKA).La première partie de cette thèse commence par présenter le contexte astrophysique des pulsars, insistant sur leur importance et leur rôle dans l'astronomie contemporaine. Elle présente ensuite le radiotélescope NenuFAR, en détaillant ses spécifications techniques, ses capacités d'observation et ses avantages uniques dans les observations des pulsars à basse fréquence. Enfin, elle passe en revue et résume les méthodes d'atténuation des RFI en radioastronomie, en analysant les limitations des technologies existantes et en identifiant les principaux problèmes auxquels cette thèse vise à répondre. En particulier, les méthodes actuelles de traitement des RFI conduisent souvent à la suppression des données identifiées comme affectées par des RFI, ce qui entraîne inévitablement une perte d'informations précieuses.Dans la deuxième partie de cette thèse, nous présentons d'abord un cadre de simulation conçu pour générer des données d'observation de pulsars corrompues par des RFI. L'objectif principal de ce cadre est de créer un ensemble de données qui permettant l'entraînement des modèles d'apprentissage profond, contournant alors le manque récurrent de données réelles correctement étiquetées.Après une revue approfondie de l'apprentissage profond et des réseaux de neurones convolutifs, nous soulignons les insuffisances des méthodes de traitement des RFI existantes, qui impliquent généralement l'identification et la suppression des données suspectées d'être perturbées par les RFI. Pour éviter cette perte d'information relever ce défi, nous proposons une nouvelle approche qui consiste à formuler le problème d'atténation des RFI comme une tâche de restauration d'image, visant à reconstruire les signaux altérés par les RFI. En capitalisant sur des avancées récentes en apprentissage profond, nous montrons que cette tâche peut être réalisée efficacement grâce à un réseau, baptisé RFI-DRUnet, dont l'architecture est inspirée d'un réseau débruiteur performant.Dans les chapitres suivants, nous illustrons l'applicabilité et l'efficacité de l'approche proposée sur des données d'observation de pulsars simulées et réelles, la comparant aux approches actuelles basées sur les réseaux neuronaux convolutifs profonds qui détectent les RFI. Les résultats montrent que la méthode proposée peut restaurer efficacement les signaux d'observation altérés par les RFI sous diverses conditions. En outre, nous discutons l'impact potentiel de cette méthode pour améliorer la chronométrie des pulsars
Pulsars are rapidly rotating neutron stars that emit electromagnetic radiation signals received as periodic pulses, thereby acting as cosmic lighthouses. The excellent stability of these periodic signals makes them invaluable tools for assessing the theory of general relativity and for detecting low frequency gravitational waves. However, their observation by radio telescopes can be made difficult by radio frequency interference (RFI) of various origins. These interferences can alter the pulsar signals, which may significantly complicate the detection and precise extraction of their signatures. This thesis aims to propose deep learning techniques to mitigate the impact of RFI in pulsar observations. In particular, this thesis illustrates the interest of these methods in the case of observations carried out by NenuFAR, a pathfinder for the low-frequency segment of the upcoming Square Kilometre Array (SKA).The first part of this thesis begins presenting the astrophysical background of pulsars, emphasizing their importance and role in contemporary astronomy. It then introduces the NenuFAR radio telescope, detailing its technical specifications, observational capabilities, and unique advantages in low-frequency pulsar observations. Lastly, it reviews and summarizes RFI mitigation methods in radio astronomy, analyzing the limitations of existing technologies and identifying the key issues this thesis aims to address. In particular, current RFI mitigation methods often lead to the deletion of data identified as affected by RFI, which inevitably results in a loss of valuable information.In the second part of this thesis, we first present a simulation framework designed to generate RFI-corrupted pulsar observation data. The main goal of this framework is to build a dataset that allows the training of deep learning models, thus circumventing the recurring lack of properly labeled real data.After a thorough review of deep learning and convolutional neural networks, we highlight the shortcomings of existing RFI processing methods, which typically involve identifying and removing data suspected of being corrupted by RFI. To avoid this information loss and address this challenge, we propose a novel approach that consists in formulating the RFI attenuation problem as an image restoration task, aiming at reconstructing RFI-altered signals. Capitalizing on recent advances in deep learning, we show that this task can be efficiently achieved using a network, named RFI-DRUnet, whose architecture is inspired by a high-performance denoising network.In the following chapters, we illustrate the applicability and effectiveness of the proposed approach on simulated and real pulsar observation data, comparing it to current approaches based on deep convolutional neural networks that detect RFI. The results show that the proposed method can effectively restore RFI-corrupted observational signals under various conditions. Furthermore, we discuss the potential impact of this method for improving pulsar chronometry

La thèse porte sur la recherche avec le réseau de télescopes H.E.S.S. de pulsations en rayons gamma en provenance de pulsars.Une première partie est dédiée à l’étude de tests statistiques de périodicité à base de simulations Monte Carlo, notamment dans un régime de basse statistique et/ou de faible rapport signal sur bruit. Dans une deuxième partie, ces tests sont utilisés dans l’analyse des données de H.E.S.S. du pulsar de Vela et de deux autres pulsars, PSR B1706-44 et PSR J1509-5850. Les pulsations de Vela ont un spectre très dur et sont observées aux énergies allant de 3 TeV jusqu’au-delà de 20 TeV. Il s’agit des rayons gamma les plus énergétiques jamais détectés en provenance d’un pulsar. C’est également la première découverte d’une composante spectrale pulsée en discontinuité avec celle déjà connue des pulsars à plus basse énergie, au GeV. Le pulsar B1706-44 est détecté dans l’intervalle 10-80 GeV depuis le sol, avec un indice et un flux compatibles avec ceux de l’instrument LAT embarqué à bord du satellite Fermi. Un autre pulsar, J1509-5850, présente une indication pour une émission au TeV mais celle-ci reste à confirmer avec plus de données.Dans une troisième partie, la phénoménologie de la composante spectrale 3-20 TeV de Vela est étudiée en détail. Une revue critique de deux modèles récents ayant proposé une interprétation possible est suivie par un travail de modélisation sur la base d’une autre catégorie de modèles plus récente. Celle-ci repose sur l’accélération par reconnexion magnétique dans la feuille de courant et sur un boost de Lorentz, a contrario des modèles précédents. Nous démontrons qu’il est possible de reproduire nos données dans ce cadre où l’émission est produite par une diffusion Compton inverse sur les photons thermiques provenant de la surface de l’étoile. Cette découverte ouvrant de nouveaux horizons, nous terminons avec l’étude des perspectives du domaine avec le futur observatoire gamma CTA
The thesis focuses on the search of gamma rays pulsations coming from pulsars with the H.E.S.S. telescopes.The first section is dedicated to the study of statistical tests of periodicity based on Monte Carlo simulations, in particular with scarce data and/or a small signal to noise ratio.In the second section, these tests are used in the analysis of H.E.S.S. observations of Vela and two other pulsars, PSR B1706-44 and PSR J1509-5850. The pulsations of Vela exhibit a very hard spectrum and are observed at energies ranging from 3 TeV to beyond 20 TeV. These are the most energetic gamma rays ever detected from a pulsar. It is also the first discovery of a pulsed spectral component in discontinuity with that already known at lower energies (GeV) from pulsars. The pulsar B1706-44 is detected from the ground in the 10-80 GeV range, with an index and a flux compatible with those of the LAT instrument onboard the Fermi satellite. Another pulsar, J1509-5850, shows a hint for a TeV emission but it still needs to be confirmed with further data.In the third part, the phenomenology of the spectral component between 3 and 20 TeV of Vela is studied in detail. After a critical review of two recent models suggesting possible interpretations, we explore another and more recent category of models. These rely on magnetic reconnection in the current sheet for the acceleration and on a Lorentz boost, as opposed to previous models. We demonstrate that it is possible to reproduce our data in this framework where the emission is due to an inverse Compton scattering of thermal photons from the neutron star surface.With this discovery opening new horizons, we conclude with the study of the perspectives of the field with the future gamma observatory CTA

Diese Arbeit berichtet über die Suche nach gepulster und ungepulster hochenergetischer (VHE) Gammastrahlung mit dem High Energy Stereoscopic System (H.E.S.S.) im Energiebereich von 100 GeV bis 100 TeV von drei Pulsaren. Gepulste VHE Gammastrahlung wurde bisher nur fuer den jungen Krebspulsar gefunden. Eine besondere Gruppe von Pulsarwindnebeln (PWN) sind die zusammengesetzten Supernovaüberreste (SNR), bei denen sich ein PWN im Zentrum einer expandierenden SNR Schale befindet. Die Resultate der Suche nach gepulster und ungepulster VHE Gammastrahlung von zwei Millisekundenpulsaren, PSR J0437-4715 und PSR J1824-2452, werden im ersten Teil dieser Arbeit vorgestellt. Teile der Beobachtungen wurden in einer speziellen Triggerkonfiguration (dem Topologischen Trigger mit konvergenter Ausrichtung) durchgeführt, um die Energieschwelle des Instruments zu senken. Kein Hinweis auf gepulste oder ungepulste Emission wurde gefunden und obere Grenzen auf den gepulsten und ungepulsten Fluss wurden bestimmt. Die oberen Grenzen auf den gepulsten Fluss werden mit bestehenden Modellvorhersagen verglichen und erlauben für PSR J1824-2452 den Bereich möglicher Geometrien in einigen Modellen einzuschränken. Die Resultate der Suche nach gepulster und ungepulster VHE Gammastrahlung aus der Richtung des zusammengesetzten SNR Kes 75 werden im zweiten Teil dieser Arbeit präsentiert. Der PWN im Zentrum von Kes 75 wird von einem sehr jungen und energiereichen Pulsar, PSR J1846-0258, angetrieben, der ein aussergewöhnlich starkes Magnetfeld besitzt. Während kein Hinweis auf gepulste Strahlung gefunden wurde, konnte ungepulste Emission von VHE Gammastrahlung von einer Punktquelle mit einer statistischen Signifikanz von 10 sigma nachgewiesen werden. Die VHE Gammastrahlung ist räumlich koinzident mit dem PWN und mit der SNR Schale. Beide werden als mögliche Quelle für die beobachtete Emission diskutiert. Der Pulsar von Kes 75 wäre der jüngste bisher bekannte Pulsar, der einen Pulsarwindnebel antreibt.
This work reports on the search for pulsed and steady very-high energy (VHE) gamma-ray emission in the energy range extending from 100 GeV up to 100 TeV from the direction of three pulsars with the High Energy Stereoscopic System (H.E.S.S.). Pulsed gamma-ray radiation from pulsars with energies beyond 100 GeV was found thus far only for the young and energetic Crab pulsar. A special class of pulsar wind nebulae (PWNe) is associated with composite supernova remnants (SNRs) where the PWN is centered in an expanding SNR shell. In the first part of this thesis, the results on the search for pulsed and steady VHE gamma-ray emission from the two millisecond pulsars, PSR J0437-4715 and PSR J1824-2452, are presented. Parts of the observations were conducted in a special trigger setup (the topological trigger with convergent pointing) to reduce the energy threshold of the instrument. No signal of pulsed or steady emission is found and upper limits on the pulsed and steady gamma-ray flux are derived. The upper limits on the pulsed gamma-ray flux are compared to existing model predictions and, in the case of PSR J1824-2452, allow the range of possible viewing geometries in some models to be constrained. In the second part of this work, results on the search for pulsed and steady VHE gamma-ray emission from the direction of the composite SNR Kes 75 are presented. The PWN in the center of Kes 75 is powered by a very young and powerful pulsar, PSR J1846-0258, that has an exceptionally high magnetic field. While no hint for pulsed emission is found, steady VHE gamma-ray emission is detected with a statistical significance of 10 sigma from a point-like source. The VHE gamma-ray emission is spatially coincident with the PWN and the SNR shell. Both are discussed as a possible origin for the observed emission. The pulsar of Kes 75 would be the youngest pulsar known to date to power a VHE PWN.

This thesis is concerned with the detection of very high energy 7-rays from isolated pulsars and objects that are not known to emit pulsed radiation, using the atmospheric Cerenkov technique. The first chapter summarises the processes by which celestial 7-rays can be generated and absorbed. A discussion of the criteria for selecting objects for study is given. The second chapter describes the atmospheric Cerenkov technique and gives a brief outline of its historical development. Details of the design and operation of the current Durham university V.H.E. 7- ray telescopes are given. The third chapter describes the standard analysis techniques used to search for pulsed and unpulsed emission. The fourth chapter presents results of new observations of four isolated pulsars. Upper limits to the V.H.E. 7-ray flux are derived for each pulsar. A summary of the results is given and conclusions drawn regarding the consequences of the observations for the most popular pulsar model. The fifth chapter deals with new analysis techniques that have been developed to test data for the presence of a signal which is not necessarily periodic. A number of different approaches is described. The results of these techniques when applied to simulated data are presented and conclusions are drawn regarding the relative effectiveness of the tests. The sixth chapter applies the most successful test to data from three objects. Upper limits to the flux from Centaurus A and SN1987aare derived from our observations. Evidence for V.H.E. 7-ray emission from Scorpius X-1 is presented, together with evidence that this emission is modulated with the period of the binary orbit of this system. The final chapter summarises the preceding results and conclusions and indicates areas where current research may lead to substantial improvements in telescope design and analysis techniques.

I present an analysis of the dispersion measure (DM) variations for 68 pulsars. The observations were taken using six International LOFAR Stations in Europe over the period of 3.5 years (between June 2014 and November 2017) at the centre frequency of 150 MHz with 80 MHz of bandwidth. During this time each pulsar was observed on weekly basis resulting in an average of 160 observations per source. I show that, the variations of the DM measurements show various trends along the span of the observation: increasing or decreasing, and in some cases more changes from one trend to another. I perform the structure function analysis for each of observed pulsar included in the study, in order to check if the DM variations follow the Kolmogorov power spectrum which describes the turbulence structure of the interstellar medium (ISM). I find that for a number of pulsars results show consistency with the Kolmogorov distribution (e.g. PSRs J1913−0440 and J2157+4017) while other sources show significant difference (e.g. PSRs J0108+6608 and J0614+2229). I also obtain the DM derivatives (i.e. dDM/dt) for each pulsar, in order to examine the correlation between the DM and its derivative. The result of this correlation shows a best-fit with a square-root dependence of 0.6±0.2, which is comparable with the result that was previously obtained by Hobbs et al. (2004), who shows a dependence of square-root between the DM and its derivative; with a gradient of 0.57±0.09. Also, one of the major results of this study that, thanks to the timing analysis, allowed me to produce a new timing solution for three pulsars: PSRs J0613+3731, J0815+4611 and J1740+27. This study concludes in that: i) the DM variations can be used to understand the general properties of the ISM ii) the low-frequency observations can enable us to study the dispersion effect on pulsar signals, which can be very useful for the effort of the pulsar timing array (PTA) project iii) IISM studies using pulsar timing is a powerful technique requiring careful approach to data reduction and analysis due to characteristic of the pulsars.

This thesis reports on the recent results of a continuing, high-precision pulsar timing project, currently focused on the nearby, binary millisecond pulsar, PSR J0437_4715. Pulse arrival time analysis has yielded a remarkable series of constraints on the physical parameters of this system and evidence for the distortion of space-time as predicted by the General Theory of Relativity. Owing to the proximity of the PSR J0437_4715 system, relative changes in the positions of the Earth and pulsar result in both annual and secular evolution of the line of sight to the pulsar. Although the changes are miniscule, the effects on the projected orbital parameters are detectable in our data at a high level of significance, necessitating the implementation of an improved timing model. In addition to producing estimates of astrometric parameters with unparalleled precision, the study has also yielded the first three-dimensional orbital geometry of a binary pulsar. This achievement includes the first classical determination of the orbital inclination, thereby providing the unique opportunity to verify the shape of the Shapiro delay and independently confirm a general relativistic prediction. With a current post-fit arrival time residual RMS of 130 ns over four years, the unrivaled quality of the timing data presented herein may eventually contribute to the most stringent limit on the energy density of the proposed stochastic gravitational wave background. Continuing the quest for even greater timing precision, a detailed study of the polarimetry of PSR J0437_4715 was undertaken. This effort culminated in the development of a new, phase-coherent technique for calibrating the instrumental response of the observing system. Observations were conducted at the Parkes 64-m radio telescope in New South Wales, Australia, using baseband recorder technologies developed at York University, Toronto, and at the California Institute of Technology. Data were processed off-line at Swinburne University using a beowulf-style cluster of high-performance workstations and custom software developed by the candidate as part of this thesis.

Le 11 juillet 1990, durant un vol stratospherique, l'experience figaro (0,1-4 mev) a observe le pulsar du crabe et le pulsar binaire a0535+26. L'analyse du spectre du pulsar du crabe a montre une structure fine, en emission a 0,44 mev qui a ete interpretee comme une raie d'annihilation electronique, decalee gravitationnellement. Sa distribution en phase est hautement structuree, avec trois positions aux phases absolues 0,1, 0,3 et 0,45. Interprete comme une raie d'emission etroite, le flux emis correspond a 3110##4 photons cm##2 sec##1. Ce flux et la distribution en phase permettent de tester et de quantifier les modeles de production de positrons dans les sites actifs autour du pulsar. Lors du meme vol, le pulsar binaire/be a0535+26 a ete detecte pour la premiere fois dans la gamme d'energie 150 kev-300 kev, avec une periode de 103,200,02 sec. Malgre la similarite des courbes de lumiere, le spectre pulse est significativement au-dessus des extrapolations, montrant soit que les mecanismes d'emission ont un spectre plus dur que les modeles thermiques couramment employes, soit que differentes regions actives co-existent dans la magnetosphere. D'autre part, la possibilite d'une raie d'absorption cyclotron est discutee. A bord de la plate-forme orbitale gamma, l'experience gamma-1, une chambre a etincelles operant de 50 a 5000 mev, a observe le pulsar de vela dans un etat bas caracterise par une intensite faible et un spectre dur. Cette observation permet de confirmer la stabilite de la partie haute energie (300-5000 mev) du spectre du pulsar, tandis qu'a plus basse energie (50 a 300 mev) le pulsar de vela apparait variable sur des echelles de temps de plusieurs mois

The main topic of this thesis is analysis of an unidentified Galactic TeV gamma-ray source, MGRO J1908+06, discovered by Milagro instrument in 2007. We analyzed 54 hours of observational data from the Very Energetic Radiation Imaging Telescope Array System (VERITAS), a ground-based gamma-ray observatory in southern Arizona comprised of an array of four Cherenkov Telescopes that reconstructs the energy and direction of astrophysical gamma-rays by imaging Cherenkov light emitted by energetic particles in air showers produced by the primary gamma-rays. MGRO J1908+06 is located between a supernova remnant SNR G40.5-0.5 and a young, energetic pulsar PSR J1907+0602. We studied the energy dependent morphology of the TeV emission from the source and measured the source extent and spectrum. The source extends well past the boundary of the SNR and is not correlated with strong radio continuum or molecular line emission which likely excludes an origin for the emission as solely due to the SNR. While emission in the 0.5-1.25 TeV band was centered around the pulsar, higher energy emission was observed near the supernova remnant. This morphology is opposite that observed in other pulsar wind nebulae. We proposed two models for the high energy emission located well away from the pulsar but close to the SNR: (1) shock acceleration at the shock front created by an interaction between the pulsar wind and the dense gas at the edge of the SNR or (2) molecular clouds around the SNR provides seed photons with energies higher than those from Cosmic Microwave Backgrounds for inverse Compton scattering. The former model can be tested by looking for molecular emission lines that trace shocks and by measuring the pulsar velocity. In addition, we investigated the gamma-ray emission from the nova explosion of V407 Cygni that occurred in March 2010. The Fermi-LAT observed this event in the energy range of E >100 MeV. The origins of the gamma-ray emission that the Fermi-LAT team proposed are either protons (hadronic model) or electrons (leptonic model), both of which were accelerated at the nova shock via the Fermi acceleration mechanism. We did not consider their leptonic model because no TeV gamma-ray emission is predicted. Their hadronic model can generate TeV gamma-rays with the modeled parameters. We found no evidence for TeV emission. We showed that with the flux upper limit calculated using the VERITAS data imposes constraints on the extension of the proton spectrum at high energies.

La historia de la astronomía de los rayos gammas en los últimos veinte años está marcada por el éxito que los telescopios Cherenkov han tenido en la exploración de la banda de muy altas energías. La ultima generación de telescopios Cherenkov, que incluye los telescopios HESS, VERITAS y MAGIC, ha logrado, en sólo siete años, aumentar el número total de fuentes conocidas de muy altas energías, desde unas pocas hasta más de un centenar. Esta populación de fuentes incluye objetos galácticos y extragalácticos. Los telescopios Cherenkov se han demostrado muy eficientes en el descubrimiento de nuevas emisoras de rayos gammas y también en el estudio detallado de la propiedades físicas de las fuentes bien conocidas, como por ejemplo, la Nebulosa del Cangrejo, el prototipo de los pleriones o Nebulosas de Viento de Púlsar ("pulsar wind nebulae"). Gracias a su brillo a todas las longitudes de onda, el Cangrejo es una fuente astrofísica de referencia. Su espectro ha sido medido en veinte ordenes de magnitud en energía, desde las frecuencias radio hasta las muy altas energías, pero nuevas medidas resultan aún esenciales para entender las contradicciones que surgen cuando se combinan los resultados de diferentes longitudes de onda. Finalizado del commissioning del segundo telescopio en el Otoño del 2009, y tras el comienzo de la toma de datos en modalidad estereoscopica, MAGIC ha mejorado significantivamente su rendimiento y alcanza el umbral en energía más bajo de todos los telescopios Cherenkov. Por lo tanto MAGIC es un instrumento ideal para medir el espectro de la Nebulosa del Cangrejo con una precisión sin precedentes a partir de 50 GeV. Este logro tiene una importancia extraordinaria para el astrofísica de muy altas energías y tiene el potencial de solucionar unos de los misterios aun no resueltos de esta fuente. Este es uno de los asuntos centrales de este trabajo de tesis. Por otro lado, MAGIC ha tenido también un impacto fundamental en el descubrimiento de nuevos objetos astrofísicos, y, con la mejora en sensibilidad del nuevo sistema de dos telescopios , este impacto será aún mas significativo. Entre las fuentes galacticas, los microcuásares constituyen unos de los mejores candidatos a emisores de rayos gammas. Su caracterización a muy alta energía aportaría información muy útil para entender la formación de chorros de partículas en objetos astronomicos. Aunque muchos modelos teóricos predicen esta emisión, aún no ha sido detectada. De hecho hay evidencias de que los tres sistemas binarios que se han detectado hasta la fecha por encima de 300 GeV son binarias de púlsares más que microcuásares. Sin embargo, el hecho que el microcuásar Cygnus X-3 haya sido detectado por encima de 100 MeV por los satélites AGILE y Fermi/LAT confirma que esta clase de objetos sigue siendo un objetivo interesente por la astrofísica de muy alta energía. En los últimos años, MAGIC ha dedicado un esfuerzo muy importante búsqueda de señales desde los microcuásares. Esta tesis presenta los resultados de las observaciones de dos de ellos: Cygnus X-3 y Scorpius X-1. Se han logrado extraer los limites superiores al flujo integral de estas dos fuentes más restrictivos hasta la fecha por encima de unos centenares de GeV. Para Cygnus X-3 en particular, se presentan además las primeras observaciones a estas energías totalmente simultáneas con la emisión detectada por los satélites.
of the Imaging Atmospheric Cherenkov Telescopes (IACTs) in the exploration of the Very- High-Energy (VHE) band. The last generation of IACTs have been capable to increase the total number of known VHE emitting sources from a few to almost one hundred in just seven years of operation. This population comprises galactic and extragalactic objects. IACTs have proved to be very effective in both the discovery of new emitters, as well as in the fine analysis of the physics properties of well established sources. Among them, the Crab Pulsar Wind Nebula is probably the best studied astrophysical object and the archetypal PWN. Due to its brightness at almost all wavelenghts, it is considered as an astrophysical candle. Despite the Crab Nebula broad-band spectrum has been thoroughly studied across twenty orders of magnitudes, from radio frequencies to VHE, further effort is needed to resolve the contradictions in the combination of all the multiwavelenght results. With the commissioning of the second MAGIC telescope in 2009 and the beginning of the operations in stereoscopic mode, the performance of the instrument improved dramatically, allowing MAGIC to reach the lowest ever energy threshold among all the existing IACTs, and describe the Crab Nebula spectrum with unprecedented precision down to 50 GeV. This achievement is of crucial importance for the VHE !-ray astrophysics in the pre-CTA era, since it can cast new light on some of the unsolved mysteries of one of its most established sources. On the other hand, MAGIC made a strong impact in the discovery of new VHE sources and, with the improved sensitivity of the stereoscopic mode, this will be even more so in the future. Among the galactic objects, Microquasars (MQs) constitute some of the best candidates for VHE emission, but despite several well accepted models predict such signal, it has not been detected. There are, in fact, evidences that the three binary systems which have been unambiguously detected at energies above few hundreds of GeV are binary pulsars rather than accreting microquasars. Nevertheless, the recent detection of the microquasar Cygnus X-3 above 100 MeV by both Agile and Fermi satellites, and the claim of short oneday flares from Cygnus X-1 reported by Agile confirmed that microquasars remain interesting targets for VHE telescopes. MAGIC made a strong effort in searching for VHE signals from microquasars, but found only a non-significant evidence of signal from Cygnus X-1 in 80 minutes of observation on September 24, 2006. MAGIC tried to detect similar flares in the following four years but the subsequent hundred more hours of observations were unsuccessful. Besides Cygnus X-1, MAGIC pointed at two other microquasar candidates, whose results are presented in this thesis: Cygnus X-3 and Scorpius X-1. The most constraining UL to the integral flux of these sources at the energy above few hundred GeV are provided. Further investigations are being planned to discover these sources at VHE in the next years.

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.

Gamma-ray binaries are systems that comprise a compact object orbiting a companion star and display the maximum of the non-thermal Spectral Energy Distribution (SED) in gamma rays. Currently we know five gamma-ray binaries. All of them host an early type star and a compact object of unknown nature, either a black hole or a neutron star, except for one of them where radio pulsations have been detected. The optical emission received from gamma-ray binaries is produced by the optical star and its environment. If the optical star is a Be star, then it presents a circumstellar decretion disk, being its size traced by the Equivalent Width of the Halpha line (EW). Numerical simulations of gamma-ray binaries hosting a Be star suggest that the circumstellar decretion disk is perturbed/disrupted during the periastron passage of the compact object by the tidal forces and/or the putative pulsar wind. The circumstellar disk contribution to the optical photometry is a significant fraction of the total optical emission. The observed optical photometric flux from it will be proportional to the projected area of the optically emitting disk. Therefore, any orbital variability in the optical light curves can be associated to changes in the circumstellar disk. We conducted long-term optical photometric and EW observations of the gamma-ray binary LS I +61 303, aimed to unveil the optical superorbital variability seen at other wavelengths. We obtained the following results from the observations: 1) The optical photometry and EW present a superorbital variability of the orbital phase of the maximum, as seen in radio and X-rays, providing an evidence of the coupling between the thermal and non-thermal emission processes in LS I +61 303. 2) The optical observations present a superorbital variability of the flux compatible with the 1667 d superorbital period. 3) This superorbital variability is attenuated or missing in some orbital phases. 4) Orbital variability in a multi-wavelength context presents lags that can be naturally interpreted considering different emitting regions. 5) The observations seem to be only compatible with an extended and (quasi)-coplanar circumstellar disk. 6) The observations are compatible with a density wave scenario, and with a very restrictive precessing-disk model. Gamma-ray binaries hosting a massive star and a young non-accreting pulsar present strong interaction between the relativistic pulsar wind, and the wind of the stellar companion, resulting in efficient particle acceleration and in the production of non-thermal radiation, from radio to gamma rays. The study of the dynamical interaction between the winds can be conducted through numerical simulations, allowing a qualitative analysis of the radiative output of the system. Furthermore, the two-wind interaction region might suffer the impact of an inhomogeneous stellar wind (hereafter clumps), making its dynamics and hence its radiative output more complex. We conducted RHD simulations of the interaction of relativistic winds from young pulsars with inhomogeneous stellar winds aimed to provide a plausible framework for the high-energy variability observed in gamma-ray binaries. We obtained the following results from the numerical simulations: 1) The two-wind interaction structure is very unstable and sensitive to the tiniest perturbations, which lead to quick Rayleigh-Taylor (RT) and in particular Kelvin-Helmholtz (KH) instability growth. 2) The arrival of clumps can have a very strong impact on the whole interaction structure, which is expected to strongly affect the non-thermal radiation as well. 3) The clumps trigger violent RT/KH instabilities leading to quick changes of the shocked pulsar-wind region. 4) Clumps generate quick and global variations in the shocked pulsar wind. This can lead to strong short time-scale flux variability in the non-thermal radiation of gamma-ray binaries.

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is an atmospheric Cherenkov telescope that detects cosmic gamma-rays using the wavefront-sampling technique. STACEE uses the large mirror area of the National Solar Thermal Test Facility (NSTTF) to achieve an energy threshold below 200 GeV. This telescope was used to search for high-energy gamma-ray emission from the Crab Nebula and pulsar. A statistical excess of 4.07sigma in the number of on-source events compared to off-source events was detected in 15 hours of on-source observing time, corresponding to an integral flux above the energy threshold (Ethr = 185 +/- 35 GeV) of I(E > Ethr) = (2.5 +/- 0.6) x 10-10 photons cm -2 s-1. The observed flux is in agreement with the previous result obtained by STACEE-32 and consistent with the power law spectrum seen at higher energies. A special instrument was developed to make simultaneous observations of the Crab in the optical and gamma-ray bands. Pulsed emission was detected in the optical band, demonstrating the accuracy of the barycentering and epoch folding analysis tools. After barycentering the arrival times and calculating the rotational phases of gamma-ray events, no evidence for pulsed emission from the Crab pulsar was found. The upper limit on the pulsed fraction of the signal was 16.4% at the 99.9% confidence level. Unfortunately, neither the polar cap model, nor the outer gap model is excluded by this new upper limit.

Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2011.
ENGLISH ABSTRACT: The Square Kilometre Array (SKA) is an international project to build the world’s largest and most sensitive radio telescope interferometer. It will consist of thousands of antennas distributed over many kilometers, with the hosting country being either South Africa or Australia. Various pathfinders have been initiated to demonstrate the technologies needed to implement the SKA. The astronomy community has defined five Key Science Projects (KSP), which are the drivers for the SKA. The science goals determine the technical requirements needed to design SKA systems. Many years of planning and development are still needed in order to meet to the requirements of such a large and complex project. This thesis aims to present the various SKA pathfinder systems in terms of their application to the SKA key science projects, with main focus that of pulsar observations. In order to achieve this, a thorough overview of basic radio astronomy principles and techniques is presented, followed by a detailed look at the various pathfinders. Throughout the work, aspects of pulsar observations are given particular attention with the goal of designing optimal systems for the SKA.
AFRIKAANSE OPSOMMING: Die Square Kilometre Array (SKA) is ’n internasionale projek om die wêreld se grootste en mees sensitiewe radio teleskoop interferometer te bou. Dit sal bestaan uit duisende antennas versprei oor etlike kilometers, met die gasheer land óf Suid-Afrika óf Australië. Verskeie koersbepalers is geïnisieer om die tegnologie te demonstreer wat benodig is om die SKA te implementeer. Die astronomie gemeenskap het vyf sleutel wetenskap projekte (KSP) gedefinieër wat dien as die drywers vir die SKA. Die wetenskap doelwitte bepaal die tegniese vereistes wat nodig is om SKA stelsels to ontwerp. Talle jare van beplanning en ontwikkeling word steeds benodig om te voldoen aan die vereistes van so ’n groot en komplekse projek. Hierdie tesis het ten doel die aanbieding van verskeie SKA koersbepaler stelsels in terme van hulle aanwending tot die SKA sleutel wetenskap projekte, met hoof fokus op die van pulsar waarnemings. Om dit te behaal, word ’n deeglike oorsig van basiese radio astronomie beginsels en tegnieke aangebied, gevolg deur ’n gedetailleerde kyk na die verskeie koersbepalers. Regdeur die werk word spesifieke aandag gegee aan aspekte van pulsar waarnemings met die doel om optimale stelsels vir die SKA te ontwerp.

La thèse présentée dans ce manuscrit est consacrée à l'étude des pulsars brillants en rayons 'y, aux énergies allant de 100 MeV à quelques TeV. Elle repose sur les données de l'instrument LAT à bord du stallite Fermi, et sur celles obtenues avec le système de télescopes Tchernekov au sol H. E. S. S. Installé en Namibie. La mesure spectrale résolue en phase des pulsars avec l'instrument Fermi-LAT, et en particulier celui de Vela (PSR B0835-45) a constitué la première étape de ce travail. La mise au point d'une méthode d'analyse dédiée au cinquième et le plus grand télescope de H. E. S. S. A permis ensuite de détecter l'émission pulséé de PSR B0835-45 depuis le sol, dès 10 GeV, puis d'en mesurer le spectre. Le très bon accord avec celui obtenu avec le LAT a démontré la validité du modèle numérique du télescope et de la méthodologie. Il s'agit du premier pulsar détecté par H. E. S. S. , et du seuil en énergie le plus bas atteint en astronomie Tcherenkov à ce jour. Dans une troisième étape, l'analyse des données de Vela obtenues avec l'ensemble des télescopes du réseau H. E. S. S. A permis de mettre en évidence, avec un bon niveau de confiance (4. 5 écarts standard), l'indication d'un signal pulsé au TeV en provenance de PSR B0835-44. L'évaluation spectrale montre que ce signal, si il est confirmé, représente une seconde composante. Une étude rapide et simple montre que cette composante pourrait être le résultat de la diffusion Compton-inverse des leptons du vent du pulsar, accélérés aux énergies ultra-relativistes au-delà du cylindre de lumière, sur le champ des photons X thermiques de l'étoile à neutrons. Ces résultats ouvrent les perspectives de la physique des pulsars au-delà de la dizaine de GeV et surtout au TeV avec le future observatoire CTA
The thesis presented in this manuscript is devoted to the study of bright pulsars shilling in -y-rays, at energies ranging from 100 MeV to several TeV. It is based on data from the LAT instrument aboard the Fermi satellite, as well as those obtained with the ground-based system of Cherenkov telescopes, H. E. S. S. , * installed in Namibia. The phased-resolved spectral measurements of pulsars with the Fermi-LAT, and in particular that of Vela (PSR B0835-45) constitutes the first step of this work. The development of an analysis method dedicated to the fifth and the largest H. E. S. S. Telescope has enabled us to detect the pulsed emission of PSR B0835-44 from ground, starting as low as 10 GeV on, and then to measure its spectrum. The very good agreement with the results obtained from the LAT data demonstrates the validity of the numerical model of the telescope and of the methodology. This is the first pulsar detected by H. E. S. S. , and the lowest energy threshold reached in Cherenkov astronomy as of today. In a third step, the analysis of data obtained on Vela with all H. E. S. S. Telescopes has resulted in an indication of a pulsed signal in the TeV range from PSR B0835-45, at a good level of confidence (4. 5 standard deviations). The spectral evaluation shows that this signal, if confirmed, represents a second component. A quick and simple study shows that this component could result from inverse Compton scattering of leptons in the pulsar wind, accelerated to ultra-relativistic energies beyond the light cylinder, on neutron star's thermal X-ray photons. These results open the perspectives for pulsar physics at tens of GeV and especially above TeV energies with the future CTA observatory

It has been more than 60 years since astronomers turned their attention towards the 7֊ray window (> 100 TeV). Nowadays, 7֊ray astronomy has won its place as a separate branch of astronomy in its own right. The present thesis introduces the reader to 7-ray observations in the 〜 100 GeV-100 TeV energy window, but focuses, in particular, on the efforts to describe and detect the pulsed, Very High-Energy (VHE) 7-ray emission from pulsars. Pulsars are highly magnetised {B 〜 101շ G) ， rapidly rotating (P ~ 10—2 s) neutron stars. Periodic radio emission from pulsars has been detected in more than 1,500 cases, in contrast to their 7-ray signature which has been confirmed for only six of them and only up to a few GeV. There are many models in existence which attempt to reproduce the observed pulsed profiles and energy spectra in high energies (optical, X and ๆ rays). Nevertheless, two classes of models are the most popular: the Polar Cap and the Outer Gap models. Both predict spectral cut-offs at tens of GeV, which are consistent with previous upper limits in the VHE range. The six most energetic pulsars have been detected with the EGRET (Energetic Gamma Ray Experiment Telescope) instrument on-board the с GRO (Compton Gamma Ray Observatory) satellite. Probing the universe at higher energies requires a different detection technique. The Imaging Atmospheric Technique (lACT) exploits the Earth's atmosphere with the use of large, ground-based reflectors that are very sensitive to Cherenkov light (300-600 nm). The latter is produced during electromagnetic particle cascades, triggered by the interaction of VHE 7 rays with the top atmospheric layers. So far there has not been a confirmed pulsar detection using Cherenkov astronomy. The High Energy Stereoscopic System (H.E.S.S.) in Namibia is an array of four telescopes, which is sensitive above 100 GeV. H.E.S.S. uses the lACT to reject the lO3 times more abundant cosmic-ray events that suppress the 7-ray signal. The system is capable of stereoscopic observations of the same source with all four telescopes, which further eliminates background events. Despite the fact that imaging with H.E.S.S. is not effective below 100 GeV, lower energy events can still be recorded, along with a large portion of the background. The present thesis deals with the least energetic events (< 100 ĢeV) detectable with H.E.ร.ร., where pulsar 7-ray emission is likely to be present. A very sensitive temporal analysis has been performed in order to identify the potentially periodic events in the large background. The necessary procedures and parameters of the analysis are described in detail, prior to the results. The author has analysed data from two 7-ray pulsars, the Crab and PSR B1706-44, which were seen with EGRET up to ~ 20 GeV, as well as the binary radio pulsar PSR B1259-63, which has not been detected at high energies (> 1 eV). The data were optimised for the lowest energies, and the lowest energy threshold achieved was 75 GeV (in the case of PSR B1706-44). In all cases studied, the author coded and applied a number of periodicity tests that check for significant deviations from random noise. The resulting probabilities were not significantly low to support signal presence. Based on the background levels in the data sets, the author derived upper limits on the integral and differential flux. These upper limits were consistent with the Polar Cap and Outer Gap scenarios, within statistical errors, but constrain the alternative model of a spectrum with a simple exponential cut-off in the case of PSR B1706—44. Despite the lack of detection, these results represent the lowest energies explored with H.E.S.S.， yet.

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.

The research presented in this thesis utilises available near and far-field sea- level records to provide constraints on the major rapid sea-level rise events that occurred during the most recent period of deglaciation (Termination 1).The far-field modelling results show that previous, large discrepancies between predictions and observations of sea-level at Barbados, Huon, Tahiti and Sunda Shelf can be resolved by utilising a model of glacial isostatic adjustment characterised by a high viscosity lower mantle (4 X lO22 Pa s) and a major Antarctic contribution (-15 m) to meltwater pulse lA (-14.5-13.5 cal. kyr BP). The latter constraint is contrary to previous suggestions that this event was sourced from northern hemisphere ice sheets, and adds further to the hypothesis that an Antarctic source for mwp-IA is a possible mechanism to explain the progression of millennial scale climatic events that occurred during Termination 1. Furthermore, the far-field sea-level records preclude the existence of the smaller meltwater pulse IB event (-11 cal. kyr BP) and can not conclusively rule out a meltwater pulse event at 19 cal. kyr BP. Modelling of all available near-field sea-level data from the coast of Antarctica supports the far-field results in that the data do not preclude a dominant Antarctic source to mwp-IA and indicate that this event may have been caused by rapid melting of the Weddell and Ross Sea regions.

La historia de la astronomía es tan antigua como alcanzan nuestros registros. Todas las civilizaciones han estado interesadas en el estudio del cielo nocturno, sus objetos y fenómenos. Estas observaciones se realizaron a simple vista hasta el comienzo del siglo XVII, cuando Galileo Galilei empezó a usar un instrumento desarrollado recientemente llamado telescopio. Desde entonces, el rango de longitudes de onda accesible ha ido creciendo, con una explosión en el siglo XX gracias al desarrollo de instrumentos para observar los diferentes rangos: antenas (radio y submilimétrico), telescopios (óptico e infrarrojo) y satélites (ultravioleta, rayos X y rayos gamma de baja energía). Las últimas longitudes de onda del espectro electromagntico a las que se han accedido han sido los rayos gamma de muy alta energía. En este rango, los flujos son tan bajos que no pueden ser observados por instrumentos espaciales, cuyas áreas de colcción típicas son del orden de O(1) m2. Para detectar esta radiación, usamos la conocida como “imaging atmospheric Cherenkov technique”, basada en la detección de los flashes de luz Cherenkov que los rayos gamma de muy alta energía producen cuando interaccionan con la atmósfera. Este campo es muy joven, con la primera fuente descubierta en 1989 por el pionero telescopio Whipple. Es muy dinámico, con más de 150 fuentes detectadas hasta la fecha, la mayoría de ellas por MAGIC, HESS y VERITAS, que forman la generación actual de estos instrumentos. Finalmente, el campo es también muy prometedor, con la preparación de la siguiente generación de telescopios de este tipo: CTA, una matriz de telescopios que se espera completar en 2020. El trabajo presentado en esta tesis comprende todos los esfuerzos que he realizado para hacer que la astronomía de rayos gammas usando telescopios Cherenkov avance. La Parte I de la tesis es una introducción al universo no térmico, la técnica de “imaging atmospheric Cherenkov” y los IACTs MAGIC and CTA. La Parte II comprende todo el trabajo técnico realizado para conseguir bajar el umbral de energía de los IACTs. Esta parte incluye la simulación, caracterización y test del sistema de trigger analógico dise˜nado para alcanzar el umbral de energía más bajo posible con los LSTs de CTA. Junto a este trabajo, se ha buscado mejorar el sistema de trigger del telescopio MAGIC simulando, testando y poniendo en marcha un nuevo concepto de sistema de trigger estéreo para MAGIC . Este nuevo sistema, que usa la información de la posición de las cascadas en cada una de las cámaras de MAGIC para eliminar ruido, se denomina “Topo-trigger”. La parte científica de la tesis trata de fuentes galácticas observadas con los telescopiosMAGIC En la Parte III hablo del análisis de la emisión de rayos gamma de muy alta energía de pleriones: El descubrimiento de emisión de rayos gamma de muy alta energía del desconcertante plerión 3C 58, probable remanente de la SN 1181 AD y el plerión más débil detectado a estas energías hasta la fecha; la caracterización de la cola de emisión a las más altas energías detectadas desde la nebulosa del Cangrejo observándola a los más altos ángulos zenitales posibles; y el estudio de una componente adicional debido al efecto Compton inverso en los estallidos de rayos gamma reportados por el satélite Fermi-LAT en el régimen de sincrotrón. La Parte IV incluye la búsquedas de rayos gamma de muy alta energía de estrellas variables cataclísmicas. En un contexto multibanda, estudiamos la naturaleza de la emisión de rayos gamma de alta energía previamente declarada de la variable cataclísmica AE Aqr. También realizamos observaciones de novas y una nova enana para desvelar los mecanismos de aceleración que tienen lugar en este tipo de objetos y descubrir una componente hadrónica putativa de la emisión de rayos gamma de baja energía. Un capítulo de conclusiones resume todo el trabajo realizado y los prospectos relacionados con los temas tratados en esta tesis.
The history of astronomy is as ancient as the reach of our written records. All the human civilizations have been interested in the study and interpretation of the night sky and its objects and phenomena. These observations were performed with the naked eye until the beginning of the 17th century, when Galileo Galilei started to use an instrument recently developed called telescope. Since then, the range of accessible wavelengths has been increasing, with a burst in the 20th century with the developing of instruments to observe them: antennas (radio and submillimeter), telescopes (optical, IR) and satellites (UV, X-rays and soft gamma rays). The last wavelength range accessed was the Very-High-Energy (VHE) gamma rays. At this range fluxes are so low that it is not possible to use space-based instruments with typical collection areas of O(1) m2. We must resort to the imaging atmospheric Cherenkov technique, which is based on the detection of the flashes of Cherenkov light that VHE gamma rays produce when they interact with the Earth’s atmosphere. The field is very young, with the first source discovered in 1989 by the pioneering Whipple telescope. It is very dynamic with more than 150 sources detected to date, most of them by MAGIC, HESS and VERITAS, that make up the current generation of instruments. Finally, the field is also very promising, with the preparation of a next generation of imaging atmospheric Cherenkov telescopes: CTA, that is expected to start full operation in 2020. The work presented in this thesis comprises my e orts to take the ground-based -ray astronomy one step forward. Part I of the thesis is an introduction to the nonthermal universe, the imaging atmospheric Cherenkov technique and the Imaging Atmospheric Cherenkov Telescopes (IACTs) MAGIC and CTA. Part II deals with several ways to reduce the trigger threshold of IACTs. This includes the simulation, characterization and test of an analog trigger especially designed to achieve the lowest possible energy threshold with the LSTs of CTA. Together with this work, the trigger of the MAGIC telescopes was improved. We have simulated, tested and commissioned a new concept of stereoscopic trigger. This new system, that uses the information of the position of the showers on each of the MAGIC cameras, is dubbed “Topo-trigger”. The scientific fraction of the thesis deals with galactic sources observed with the MAGIC telescopes. In Part III, I talk about the analysis of the VHE -ray emission of Pulsar Wind Nebulae (PWNe): the discovery of VHE -ray emission from the puzzling PWN 3C 58, the likely remnant of the SN 1181 AD and the weakest PWN detected at VHE to date; the characterization of the VHE tail of the Crab nebula by observing it at the highest zenith angles; and the search for an additional inverse Compton component during the Crab nebula flares reported by Fermi-LAT in the synchrotron regime. Part IV is concerned with searches for VHE -ray emission of cataclysmic variable stars. I studied, on a multiwavelength context, the VHE -ray nature of the previously claimed pulsed -ray emission of the cataclysmic variable AE Aqr. I also performed observations of novae and a dwarf nova to pinpoint the acceleration mechanisms taking place in this kind of objects and to discover a putative hadronic component of the soft -ray emission. A conclusion chapter summarizes all the work performed and lists prospects related with the topics treated in this thesis.

L'expérience H. E. S. S. (High Energy Stereoscopic System) a pour objectif l'observation du ciel austral en rayons γ de très haute énergie. L'observatoire, constitué de quatre télescopes fonctionnant en stéréoscopie et situé en Namibie, est un des plus performants au monde. Enregistrant des données depuis 2003, c'est l'instrument idéal pour l'observation du plan galactique et la détection des nombreuses sources qui s'y trouvent. Trois méthodes d'analyse ont été mises en place, s'appuyant sur différentes propriétés des gerbes électromagnétiques issues de l'interaction des γ avec l'atmosphère. L'objectif premier de cette thèse était de combiner les informations issues de ces trois méthodes pour la sélection des événements et la reconstruction de la direction et de l'énergie des gamma. La nouvelle analyse, appelée Xeff, apporte un gain significatif sur la qualité de la sélection et sur la précision de reconstruction de l'énergie et de la direction. L'analyse Xeff a été utilisée pour l'étude de nébuleuses de pulsar telles que Vela X, GO. 9+0. 1 et MSH 15-52. De nouveaux résultats ont été mis en évidence concernant l'extension de la source (Vela X) ou la forme spectrale (GO. 9+0. 1 et MSH 15-52) grâce à de nouvelles observations et à l'efficacité de l'analyse mise en place. Dès 2010, l'expérience rentrera dans une nouvelle phase avec l'achèvement d'un cinquième télescope, dédié à l'observation des rayons gamma à partir de quelques dizaines de GeV. Les tests mis en place pour l'étalonnage des photomultiplicateurs et les résultats obtenus sont décrits dans le manuscrit
H. E. S. S. (High Energy Stereoscopic System) is one of the leading systems of four Imaging Atmospheric Cherenkov Telescopes that investigates very high energy (VHE) cosmic γ-rays in the 100 GeV to 100 TeV energy range. H. E. S. S. Is located in Namibia, near the Gamsberg mountain and operational since December 2003. The H. E. S. S. Experiment is mainly aimed to the observation of the southern sky including the galactic plan and the numerous astrophysics sources therein. Three analysis methods have been developed using various properties of the electromagnetic showers generated by the interaction of primary cosmic γ-rays within the Earth atmosphere. The first goal of this thesis was to combine the information from these methods for the selection and the energy and direction reconstruction of γ-ray events. The new analysis called Xeff improves significantly the quality of the selection and the precision of the reconstruction. This analysis has been afterwards applied to the study of pulsar wind nebulae like Vela X, GO. 9+0. 1 and MSH 15-52. New results were found concerning the source extension (Vela X) or spectral analysis (GO. 9+0. 1 and MSH 15-52) at TeV energies, thanks to additional data and to the improved efficiency of the new method. In 2010, a new phase will begin with the achievement of a fifth telescope dedicated to γ-ray observation from tens GeV. The calibration processes of the photomultipliers equipping the camera of the new telescope, as well as the results of the tests, are also described in this thesis

Die vorliegende Arbeit beschäftigt sich mit zwei Aspekten der Gammastrahlungsastronomie. Einerseits studiert sie die Anforderungen an das zukünftige CTA-Observatorium für Gammastrahlung und präsentiert insbesondere Abschätzungen der Datenmengen, die während des Betriebs des Observatoriums anfallen werden. Für das größere CTA-Teleskopfeld auf der Südhalbkugel werden demnach eine Triggerate von 13 kHz und Datenraten von bis zu 2500 MB/s erwartet. Unter der Annahme, dass 15% der Zeit für Beobachtungen genutzt werden können, ergibt sich in 15 Jahren ein Datenvolumen von bis zu 165 PB. Die Implementation eines entsprechenden Systems zur Datenerfassung und -speicherung stellt eine Herausforderung dar, die jedoch mit existierenden Technik bewältigt werden kann. Andererseits befasst sie sich mit dem Supernovaüberrest RX J0852.0-4622 (auch bekannt als Vela Junior), präsentiert die Ergebnisse einer Analyse von Daten, die mit dem H.E.S.S.-Experiment genommen wurden, und geht der Frage nach, ob RX J0852.0-4622 ein kosmischer Teilchenbeschleuniger ist. Dabei erlauben die präzisen Messungen eine im Vergleich zu früheren Veröffentlichungen verbesserte Bestimmung der Eigenschaften der emittierenden Teilchenpopulation. Es ergibt sich, dass das Energiespektrum von RX J0852.0-4622 ein Potenzgesetz ist, das zu hohen Energien hin mit einer Abschneideenergie von 7.2 TeV exponentiell unterdrückt wird. Abschließend wird anhand von Simulation gezeigt, dass CTA die Abschneideenergie von RX J0852.0-4622 signifikant besser bestimmen können wird. Diese genauere Vermessung des Energiespektrums sollte dazu beitragen, den hadronischen oder leptonischen Charakter der Emission aufzuklären.
This work focuses on two different aspects of gamma-ray astronomy. On the one hand, it studies the instrumental challenge posed by the future CTA Observatory by estimating the amount of data to be collected. Based on an analysis of simulated data, the more demanding southern array is expected to have an array trigger rate of 13 kHz, a data rate of up to 2500 MB/s and a data volume after 15 yr of operation and assuming a duty cycle of 15% of up to 165 PB. The design of the data acquisition and storage systems will be a challenge but should be manageable with existing technologies. On the other hand, it studies supernova remnants, by presenting analysis results of the gamma-ray data of the RX J0852.0-4622 supernova remnant (commonly known as Vela Junior) measured with the operating H.E.S.S. experiment and interpreting them in order to check the plausibility of RX J0852.0-4622 being a cosmic ray accelerator. The more precise measurements permit a better determination of the parent particle population properties with respect to previous publications. More precisely, a clear curvature of the spectrum of RX J0852.0-4622 is measured with an exponential energy cut-off at 7.2 TeV. Finally, the analysis of simulated data shows that CTA should be able to significantly improve the determination of the spectral energy cut-off of RX J0852.0-4622, which should help in identifying the nature of the gamma-ray emission.