Im Rahmen eines selbst-konsistenten Outer-Gap-Modells der Pulsar-Magnetosphäre wurde die elektromagnetische sehr hochenergetische Strahlung des Crab-Pulsars simuliert. Dies wurde parallel anhand zweier verschiedener Fälle getan, die sich in den angenommenen Gleichungen für die elektrische Feldstärke und für den Krümmungsradius der magnetischen Feldlinien unterscheiden. Die Kinetik der geladenen Teilchen bei ihrer Propagation durch die Outer Gap wurde unter Einbeziehung von Krümmungsstrahlung, inverser Compton-Streuung und Triple Paarbildung betrachtet. Das theoretisch simulierte Spektrum wird mit von Fermi-LAT und von den MAGIC Teleskopen gemessenen Daten verglichen. / In a self-consistent model of outer vacuum gaps of pulsar magnetospheres, the gamma-ray output of the Crab pulsar is determined and compared to observational data by the Fermi Large Area Telescope and the Major Atmospheric Gamma-ray Imaging Cherenkov telescopes. Incorporating curvature radiation, inverse-Compton scattering and triplet pair production, the kinetics of the accelerated electrons are considered, energy losses are compared and the emerging spectra are computed. It is found that losses are dominated by curvature radiation emission. Triplet pair production may be relevant concerning particle multiplication and inverse-Compton up-scattering of the curvature photons is forming the spectral energy distribution at very high energies.

The Fermi Large Area Telescope has observed a gamma-ray excess toward the center of the Galaxy at ~ GeV energies. The spectrum and intensity of the excess are consistent with the annihilation of dark matter with a mass of ~100 GeV and a velocity-averaged cross section of ~ 1e-26 cubic centimeter per second. In the meantime, a population of unresolved millisecond pulsars (MSPs) in the Galactic center remains a possible source of the excess. Furthermore, recent analyses have shown that the excess prefers the spatial morphology of the stellar bulge distribution in the Galactic center, supporting a MSP origin. The new discovery makes it imperative to further study the signals from MSPs. This dissertation studies the gamma-ray emission from Galactic millisecond pulsars to provide new insights into the origin of the Galactic center excess. Using the GALPROP code, we simulate the propagation of e± injected by the putative MSPs in the Galactic bulge and calculate the inverse Compton (IC) emission caused by the e± losing energy in the interstellar radiation field. We find recognizable features in the spatial maps of the IC. Above TeV energies, the IC morphology tends to follow the distribution of the injected e±. Then, we study the Cherenkov Telescope Array (CTA) sensitivity to the IC signal from MSPs. We find that the CTA has the potential to robustly discover the IC signature when the MSP e± injection efficiencies are in the range ≈ 2.9-74.1%. The CTA can also discriminate between an MSP and a dark matter origin for the radiating e± based on their different spatial maps. Next, we analyze the Fermi data from directions of Galactic globular clusters. The globular clusters are thought to be shining in gamma rays because of the MSP population they host. By analyzing their gamma-ray spectra, we reveal evidence for an IC component in the high-energy tail of Fermi data. Based on the IC component in the globular cluster spectra, the e± injection efficiency of millisecond pulsars is estimated to be slightly smaller than 10%. Finally, we study the spatial morphology of the 511 keV signal toward the Galactic center using data from INTEGRAL/SPI. We confirm that the 511 keV signal also traces the old stellar population in the Galactic bulge, which is similar to the Fermi GeV excess. Using a 3D smoothing kernel, we find that the signal is smeared out over a characteristic length scale of 150 ± 50 pc. We show that positron propagation prior to annihilation can explain the overall phenomenology of the 511 keV signal. / Doctor of Philosophy / Dark matter means matter that does not interact with light; therefore, they are invisible to traditional observations. We know that dark matter exists based on plenty of gravitational evidence: the motions of stars in galaxies, the large-scale structure of the Universe, the temperature fluctuations in the cosmic microwave background. However, we still know very little about the particle nature of dark matter. Detecting dark matter is one of the most extensive missions of modern physics. In indirect detection, the dark matter particles are expected to annihilate or decay in the cosmos, producing messenger particles that include gamma rays, cosmic rays, and neutrinos. Astronomical observations could detect those signals and confirm the nature of dark matter. However, understanding the astrophysical sources is essential for indirect detection of dark matter as they may emit similar signals. For a recent example, the Fermi Large Area Telescope launched by NASA is the most sensitive gamma-ray telescope in the energy range of ~ 100 MeV to ~ 100 GeV. It has detected an excess of gamma-ray signals toward the Galactic center consistent with what we expect from dark matter annihilation. However, millisecond pulsars, a type of fast rotating neutron stars, may also generate similar gamma-ray signals. Therefore, the origin of the signal remains unsettled. In this dissertation, we study different prospective of the gamma-ray emission from the millisecond pulsars in the Milky Way. We first study the inverse Compton signal from the millisecond pulsars in the Galactic bulge, caused by the relativistic e± injected by the millisecond pulsars. We find that the signal traces the original distribution of the e± above TeV energies. Next generation ground-based gamma-ray observatories like the Cherenkov Telescope Array (CTA) could be used to detect the signal. We study the CTA sensitivity to such an inverse Compton signal. We find that CTA can detect the inverse Compton signal from millisecond pulsars and discriminate it from a dark matter signal. We also study the gamma-ray emission from globular clusters in the Milky Way. They are dense collections of old stars orbiting our Galaxy, and they are known for hosting many millisecond pulsars. We reveal evidence for inverse Compton emission from the gamma-ray data of globular clusters. Our discovery helps us better understand the high-energy property of millisecond pulsars. Last, we study the morphology of the Galactic 511 keV signal caused by positron annihilation. Compact objects including millisecond pulsars are potential sources of the positrons. We find that the old stellar distribution with a smearing scale of ~ 150 pc best describes the 511 keV signal. Positron propagation from their sources prior to annihilation could explain the measured smearing scale.

We used the first station of the Long Wavelength Array (LWA) to observe giant pulses (GPs) from pulsars and search for other radio transients. Using the LWA with a bandwidth of 16 MHz at 39 MHz, we made a 24-hour observation of pulsar radio pulses from PSR B0950+08. The average pulse ux density and pulse width (dominated by "normal" pulses) are consistent with previous studies by others. Using techniques we developed for searching for radio transients, in this observation we detected 119 giant pulses (with signal-to-noise ratios 10 times larger than for the mean pulse). The giant pulses have a narrower temporal width (17.8 ms, on average) than the mean pulse (30.5 ms). Giant pulses occur at a rate of about 5.0 per hour, or 0.035% of the total number of pulse periods. The strength and rate of giant pulses is less than observed by others at ~100 MHz. The probability distribution of the cumulative pulse strength is a power law, but deviates from the Gaussian distribution of normal pulses. These results suggest PSR B0950+08 produces less frequent and weaker giant pulses at 39 MHz than at 100 MHz. We detected no other transients in this observation within a dispersion measure (DM) range from 1 to 90 pc cm³. Furthermore, we conducted observations of giant pulses from PSR B0950+08 in a separate set of observations of 12 hours made simultaneously at 42 and 74 MHz. In these observations we detected a total of 275 at 42 MHz and a total of 465 giant pulses at 74 MHz. Giant pulses with double-peak temporal structure have a shorter peak-to-peak separation compared to the average pulse. Once again, PSR B0950+08 appears to produce less frequent and weaker giant pulses than reported at 100 MHz. Giant pulses are identified with signal-to-noise ratios 10 times larger than for the mean pulse, and the probability distribution of the cumulative pulse strength is a power law, but deviate from the Gaussian distribution of normal pulses, for both frequencies. There were only 128 giant pulses detected simultaneously at 42 and 74 MHz, which implies that more than half of them are narrow-band radio pulses. Using these observations we analyzed the effect of scattering due to the interstellar medium on pulses with signal-to-noise ratio > 7 and the average pulse using a CLEAN-based algorithm, assuming a thin-screen scattering model. The scatter-broadening time constant τ ∝ ν^α, where ν is the observing frequency. The resulting α as calculated from pulses with signal-tonoise ratio > 7 and for the average pulses is found to be α = −1.45±0.14 and −0.14±0.03, respectively. These results indicate differences along the line of sight from a Kolmogorov spectrum for electron density uctuations. We calculated the altitude of the emission region for the pulsar using the dipolar magnetic field model. We found a similar magnitude for the emission region altitudes of normal and giant pulses. We detected no other transient pulses in a wide DM range from 1 to 4990 pc cm⁻³. We also conducted another a 12-hour observational study of PSR B0031−07 at 38 and 74 MHz, simultaneously. Giant pulses were identified with ux densities of a factor of ≥ 90 and ≥ 80 times that of an average pulse, at 38 and 74 MHz. The cumulative pulse strength distribution follows a power law, and has a much more gradual slope than a Gaussian distribution for the normal pulses. We found 158 of the observed pulses at 38 MHz qualified as giant pulses. At 74 MHz a total of 221 of the observed pulses were giant pulses. Only 12 giant pulses were detected within the same pulse period at both 38 and 74 MHz, meaning that the majority of them are narrow-band radio pulses. No other radio transients were detected within a DM range 1 to 4990 pc cm⁻³. We used the same data processing pipeline for observations of pulsar GPs to search within the pulsar observations for fast radio bursts (FRBs). We did not detect any nonpulsar signals with signal-to-noise ratio larger than 10. When the radio transient signals propagate through the interstellar medium, they are affected by propagation effects such as dispersion and scattering. Scattering may limit the detectability of radio transients. By examination of archived pulsar profiles, we investigated the impact of scattering on observed pulses. We utilized a CLEAN-based strategy to decide the scatter-broadening time, τ , under both the thin-screen and uniform-medium scattering models and to determine the scatter-broadening time frequency scaling index, α, where τ ∝ ν^α. In most cases the scattering tail was not large compared to the pulse width at half maximum. Still, we deconvolved 1342 pulse profiles from 347 pulsars assuming a Kolmogorov spectrum of the interstellar medium turbulence. For a subset of 21 pulsars the scattering-boarding tails were suficiently long to be estimated at the lowest frequencies. Since the scatter-broadening times were only determined distinctly for the subset of pulsars at the lowest observed frequency, we were restricted to utilizing upper limits on scatter-broadening times at higher frequencies for the assessment of the scatter-broadening-time frequency dependence. We include three new direct scatter-broadening time measurements at low frequencies and they are consistent with previous studies which were scaled from higher frequencies. Our findings are consistent with a relationship between the DM and scatter-broadening time which can range over more than two orders of magnitude in DM. One of the potential reasons that we did not detect FRBs is that transients may be highly scatter-broadened at low frequencies for high DM values. / Ph. D.

Les pulsars sont des étoiles à neutron hautement magnétisées en rotation rapide produisant un rayonnement pulsé. Cette thèse est dédiée à leur magnétosphère, c'est à dire la zone proche de l'étoile à neutron, remplie d'un plasma entraîné par la rotation rapide de celle-ci. Il a été montré dès 1969 que la magnétosphère doit avoir des zones très peu denses arborant des champs électriques intenses capables d'accélérer le plasma raréfié de ces régions à des énergies très élevée le long du champ magnétique. La courbure des lignes de champ, couplé avec la rotation d'une particule autour du champ, cause un rayonnement dit de « synchro-courbure ». L'énergie est rayonnée essentiellement en photons gamma (g). Ces photons peuvent ensuite être convertis par interaction quantique photon γ-champ magnétique ou γ-γ en une paire électron-positron e+e- dont chaque composante rayonne à son tour, résultant en une cascade qui alimente la magnétosphère en plasma. Cette thèse traite particulièrement de deux phénomènes clefs de ces cascades : le rayonnement de synchro-courbure et la création de paires par interaction γγ.La théorie quantique du rayonnement de synchro-courbure est développée pour la première fois à partir des principes de base de l'électrodynamique quantique. Les paramètres compatibles avec les approximations du calcul correspondent à une large gamme de conditions physiques typiques des magnétosphères de pulsars. Les transitions quantiques sont considérées dans l'approximation continue lorsqu'elles impliquent un saut de l'impulsion de la particule dans la direction parallèle au champ, et discrète dans la direction perpendiculaire. Il en résulte un spectre tendant asymptotiquement vers les descriptions classiques des rayonnement de courbure et de synchro-courbure mais présentant des déviations très importantes lorsque les transitions discrètes dominent le rayonnement.L’interaction γγ→e+e- a été étudiée dans le cas où un gamma réagit sur un fond de photons de basse énergie. Ce mécanisme est considéré comme potentiellement important lorsque le champ magnétique n'est pas assez fort pour produire des paires par le mécanisme γ-champ magnétique. Tout indique que le fond est anisotrope, c'est pourquoi nous avons développé un formalisme permettant de prendre en compte arbitrairement les anisotropies et de produire les spectres des particules produites. Appliqué à un modèle simple d'étoile rayonnant thermiquement en X, il en résulte une dépendance forte du taux de réaction sur la direction du photon gamma.Cette thèse comprend également un modèle de chronométrage du pulsar milliseconde dans un système triple J0337+1715. Ce pulsar orbite avec deux étoiles naines blanches dont les interactions mutuelles ne sont pas négligeables. Une intégration numérique, à l'ordre newtonien et post-newtonien, a été développée pour déterminer les orbites. Un modèle complet incluant le calcul des retards du système du pulsars au télescope a été réalisé. Le modèle s'ajuste aux données de chronométrage provenant du radiotélescope de Nançay avec des résidus d'écart-type inférieur à 2 µs. Un tel système permet en principe le test du principe d'équivalence fort gravitationnel par une technique similaire à celle employée lors des expériences de laser-lune, mais avec une précision sans précédent en régime de champ fort. Ce test requiert une évaluation rigoureuse des incertitudes sur chaque paramètre, échantillonnées grâce à un code MCMC. La validation du code et l'évaluation des incertitudes sont en cours. / Pulsars are highly magnetized fast rotating neutron stars producing a pulsed radiation. This thesis is dedicated to their magnetosphere, namely the zone surrounding the star and filled with a plasma dragged by the rotation of the star. It was shown as soon as 1969 that the magnetosphere must have vacuum gaps, where intense electric fields develop that are capable of accelerating the rarefied plasma to very high energies along the magnetic field. The curvature of the field lines, together with the rotation around the magnetic field, results in the so-called «  synchrocurvature » radiation. The energy is mostly radiated in gamma photons (γ). These photons may then be converted by the quantum processes γ photon-magnetic field or γ-γ in an electron-positron pair e+e-, each component of which then radiates at its turn which results in a cascade that provides plasma to the magnetosphere. This thesis particularly deals with two key phenomena of these cascades : synchrocurvature radiation and γγ pairs.The quantum theory of synchrocurvature radiation is developed for the first time from the first principles of quantum electrodynamics. The range of parameters compatible with the approximations of the derivation covers a wide range of physical conditions typical of pulsar magnetospheres. Quantum transitions are considered in the continuous limit when they imply a jump of the particle impulsion parallel to the magnetic field, and discrete when the jump is in the perpendicular direction. It results in a spectrum that asymptotically tends to the classical descriptions of curvature and synchrocurvature radiations but that presents very important deviations when the discrete transitions dominate the radiation.The γγ→e+e- process was studied in the case of the reaction of a gamma photon on a soft photon background. This mechanism is considered as potentially important when the magnetic field is nopt strong enough for the γ-magnetic field process to efficiently produce pairs. The soft background is most likely anisotropic, and that is why we developed a formalism allowing to arbitrarily take into account anisotropies, as well as produce the spectra of the outgoing particles so as to be able to feed the subsequent cascade consistently. Applied to a simple model of a star radiating thermal X rays, it results in a strong dependence of the reaction rate on the direction of the gamma photon.This thesis also includes a timing model of the millisecond pulsar in a triple system J0337+1715. This pulsar orbits with two white-dwarf stars, and their mutual interactions are not negligible. It follows that a numerical integration of the orbits was developed at Newtonian and first post-Newtonian orders. A complete model including the computation of delays from the star to the telescope was realized. This model is able to fit the timing data from the Nançay (France) radiotelecope with a standard deviation of less than 2µs. In principle, such a system allows to test the strong equivalence principle by a technique similar to that employed in Lunar-laser-ranging experiments, but with an unprecedented accuracy in the strong-field regime. This test demands a careful estimate of the uncertainties on each parameter, which we sample using a MCMC code. The validation of the code and the evaluation of the uncertainties are ongoing.

The detailed nature of radio emission processes of pulsars and the exact location and distribution of the pulse emitting regions are still shrouded with mystery. Pulsars with drifting subpulses are considered as an important key for unlocking the mystery of how radio pulsars work. The phenomenon of subpulse drifting (first reported by Drake & Craft (1968)) is manifested as an organized subpulse behavior - subpulses appear at progressively changing longitude in the pulse window following some particular path. The path followed by the subpulses is specific to the individual pulsar concerned and is known as drift band. Drifting is generally characterized by Pm2(horizontal separation between the drift bands, i.e. in pulse longitude) and Pm3(vertical separation between the drift bands, i.e. in pulse numbers). The subpulse drifting phenomenon finds a natural explanation in the model of Ruderman & Sutherland (1975). According to this model, the subpulse drifting is produced from a system of sub-beams (subpulse associated plasma columns). Sparks (sparking discharges within the vacuum gap) rotating around the magnetic axis under the action of an E x B drift, give rise to a circulating pattern of sub-beams, and the time for one full circulation is referred to as the carousel rotation period, which we designate as P4. As pulsar radiation beams are widely believed to be arranged in concentric cones, it is natural to expect the circulating sparks to be distributed in annular rings on the polar cap. Each of these rings gives rise to one cone in the nested cones of emission. It has been recently shown that, subpulse drift may be fairly common among pulsars (Weltevrede et al. (2006) and (2007)). Hence, the pulsar radio emission mechanism is most likely closely connected with mechanism for drifting. In spite of significant progress both in high quality observations of drifting (e.g. Weltevrede et al. (2006) and (2007)) and attempts for confronting the results from the observations with existing models (e.g. Deshpande & Rankin (1999) and Gupta et al. (2004) etc), the pulsar emission mechanism is still an unsolved puzzle. Backer (1970b) first reported that emission from certain pulsars abruptly switches off for several periods, and suddenly comes back. This phenomenon is known as nulling. Nulling appears to be random, broadband and intrinsic to the concerned pulsar. Nulling is quite common in pulsars (Biggs, 1992). Although different aspects of the phenomenon of nulling are investigated in detail for many pulsars by several authors using high sensitivity observations, nulling is not yet explained by the existing theoretical models for pulsar radio radiation. In this thesis, I have mainly studied phenomenon of subpulse drifting and nulling, with the aim to probe the radio emission processes of pulsars. Most of the pulsars have a narrow duty cycle of emission (5-10 % of pulsar period). This is generally consistent with the expectations of the angular width of the polar cap, for typical viewing geometries. However, there are small but significant number of pulsars with unusually wide profiles where the emission is seen for a wide range of longitude (≥ 90 degrees). These are expected to be pulsars which are highly aligned, i.e. the magnetic dipole axis is almost parallel to the spin axis. In such a case, the line of sight (LOS) is very close to both the rotation and the magnetic axes, and consequently, we sample a large region of the polar cap. This has the exciting potential to allow a detailed study of the distribution and behavior of emission regions located in an annular ring around the magnetic axis. The study of pulsars showing systematic subpulse drift patterns provides important clues for the understanding of the unsolved problems of pulsar emission mechanism. Constraints provided by such observations can have far reaching implications for the theoretical models, as exemplified by some of the recent results in this area (e.g. Deshpande & Rankin (1999) and Gupta et al. (2004)). In this context, wide profile drifting pulsars can provide extra insights because of the presence of simultaneous multiple drift bands. During the thesis period, I have mainly concentrated on the study of single pulse properties of two wide profile drifting pulsars, PSR B0818-41 and PSR B0826-34. In depth study of PSR B0818-41 We have studied single pulse properties of a relatively less studied wide profile pulsar, B0818-41 using highly sensitive multi-frequency observations with the GMRT in full polar mode. Detailed investigation of PSR B0818-41 are reported in Chapters 2, 3 and 4 of this thesis. New results from our study are described in the following. We estimate the mean flux of PSR B0818-41 at 5 different frequencies and show that the spectrum flattens at frequencies lower than 325 MHz (at 244 or 157 MHz), providing indication of a low frequency turn-over. Significant linear polarization is observed at 325, 610 and 1060 MHz. Average linear polarization falls off much faster than the total intensity and decreases to zero near the outer edge of the profile. This can be explained by the orthogonal polarization mode jump at the edges of the profile observed at 325 MHz. Polarization angle sweep across the pulse profile evolves remarkably with frequency (between 325, 610 and 1060 MHz), which is not generally observed in other pulsars. Very less circular polarization without any signature of changing handedness is observed at 325 and 610 MHz. But circular polarization changes sign at the middle of the pulse profile at 1060 MHz. We report the discovery of a remarkable subpulse drift pattern in PSR B0818-41, using the high sensitivity GMRT observations. We find simultaneous occurrence of three drift regions with two different drift rates: an inner region with steeper apparent drift rate flanked on each side by a region of slower apparent drift rate. The closely spaced drift bands always maintain a constant phase relationship: the subpulse emission from the inner drift region is in phase with that from the outer drift region on the right hand side, and at the same time the emission in the inner drift region is out of phase with the outer drift region situated on the left hand side. This phase locked relationship (hereafter PLR) is maintained for the entire stretch of the data (for all the epochs of observations at 325 and 610 MHz) and does not appear to get perturbed after intermittent nulling or during changes in the drift rate. We observe frequent changes of drift rates. We see extreme examples of changing drift rates such as transitions from negative to stationary or stationary to negative drift rates, many of which appear to have some connection with nulls. We investigate changes in drift rates for about 10,000 pulses from two different epochs of observations at 325 MHz and observe frequent occurrences of small changes in the drift rate, seven transitions from negative to stationary drift rates, five transitions from stationary to negative drift rates, and two possible signatures of curved drift bands. In addition to the remarkable subpulse drift observed at 325 MHz, we report subpulse drifting at 244 and 610 MHz. At 244 MHz subpulse drifting is observed only in the leading and trailing outer regions, but not in the inner region. Though the drift bands are weaker, subpulse drifting is observed in both inner and outer region at 610 MHz. Pm2, Pm3 and ΔΦs(subpulse width) are determined for the inner and the outer drift regions for different frequencies. Though Pm3 is observed to be the same for the inner and outer drift regions, pm2 and ΔΦsare different for different drift regions. The unique drift pattern of this pulsar can be naturally explained as being created by the intersection of our LOS with two conal rings on the polar cap of a fairly aligned rotator. Based on the frequency evolution of the average profile, observed polarization angle (PA) swing and results from subpulse drifting, we converged on two possible choices of emission geometry: G-1 (inclination angle α= 11 deg and impact angle β= -5.4 deg; which incidentally reproduces the middle part of the PA sweep at 610 MHz) and G-2 (α=175.4 deg and β=- 6.9 deg; geometry derived from RVM fit to 325 MHz PA sweep). Pulsar radiation pattern simulated with both the geometries reproduces the average profile as well as the observed features in the drift pattern quite well. However, G-2 fits the PA sweep much better. We report that the peaks of the emission from the trailing and leading outer regions, as a function of the pulse number, are offset by a constant interval, P5~9P1. We also report a phase locked relation (PLR) between the inner and outer drift regions for PSR B0818-41. A new technique is introduced by us for resolving aliasing, using this constant offset (P5 ~ 9P1) between the peak emission from the leading and trailing outer regions. From the result of this technique, we propose that the subpulse drifting for PSR B0818 -41 is most likely first order aliased, and the corresponding carousel rotation period 4 =10 s. This implies that PSR B0818 -41 has the fastest known carousel. The drift pattern in the inner and outer rings are always phase locked for PSR B0818 -41. This could be a significant constraint for the theoretical models of pulsar radio emission, and favors a pan magnetospheric emission mechanism. We observe frequent nulling for PSR B0818-41. We calculate a nulling fraction ~30% at 325 MHz for this pulsar. Lengths of neighboring nulls and bursts are found to be independent. For the inner drift region, our investigations bring out the fact that the nature of the transitions from burst to null are different from the transitions from null to burst. Switching off of pulsar radiation during nulling for PSR B0818 -41 is not abrupt, but is gradual, whereas the transitions from null to burst are found to be rather abrupt for the inner drift region. This effect is not prominent in the outer drift regions. Although, the inner region of the last active pulses before nulls are dimmer, the first active pulses after nulls outshines the normal ones. The intensity of the inner region is maximum for the average profile from the first active pulse immediately after the nulls and then gradually goes down. This is consistent with the behavior of the individual nulls described above. However, this is not the case for the leading and trailing outer regions. The average profiles from the first active pulse immediately after the nulls follows similar shape as the normal profile but shows an increased intensity (in the form of a bump) in the inner region which is not present in the normal average profile. In addition, the leading and the trailing peaks appear to be of similar intensity, while trailing peak is significantly more intense for the normal profile. The average profiles from the pulses immediately after the nulls are wider than the normal profile. The average profiles of the first active pulses after the nulls are drastically similar between two epochs of observations. This is a very unique result which is not reported for any other pulsar so far and may imply that the phenomenon of nulling is associated with some systematic energy re-distribution in the pulsar magnetosphere. In depth study of PSR B0826-34 PSR B0826-34 is a pulsar with one of the widest known profile. The earlier studies of this pulsar (Durdin et al. (1979), Biggs et al. (1985) and Gupta et al. (2004)) have brought out some unique properties : strong evolution of the average profile with frequency, apparent nulling for 70% of time and a remarkable subpulse drift property- multiple curved drift bands with frequent changes and sign reversals of drift rate. We studied PSR B0826 -34 using the GMRT, simultaneously at 303 and 610 MHz, and individually at 157, 325, 610 and 1060 MHz. Detailed investigation of PSR B0826- 34 are reported in Chapter 5 of this thesis. Some of the interesting new results from our work are, As a natural out-come of the simultaneous dual frequency observations, we obtain an accurate DM value, equal to 52.2(6) pc/cm3, for this pulsar. Unlike most normal pulsars the DM determination for this pulsar is a difficult and trick exercise, mainly because the profile is quite complex, very wide and strongly evolving with frequency. The advantage of our method of DM determination is that the observations at a single epoch are self sufficient for obtaining the DM value at that epoch. Contrary to the earlier study by Esamdin et al. (2005), we find no evidence of weak emission during the typical long null states of this pulsar, simultaneously at 303 and 610 MHz, as well as from non simultaneous observations at 157, 325, 610 and 1060 MHz at separate epochs. We have also obtained absolute flux limits for the non-detection at various frequencies, which should be a useful comparison standard for any more sensitive studies in the future. We present the average profiles at five different frequencies. Main pulse (MP) and inter pulse (IP) emission observed for this pulsar span over wide pulse longitude. There is a remarkable frequency-evolution of pulse profile: IP becomes stronger with increasing frequency. We estimated the mean flux of the MP, IP and the full pulse region of PSR B0826- 34 at different frequencies of observation. Significant correlation in the total intensity of the individual pulses between 303 and 610 MHz is reported from the simultaneous dual frequency observations, which is indicative of the broad-band nature of the emission. The intensity correlations are positive for large lags, indicating that there is some kind of memory in the underlying structure. This memory is the longest for PSR B0826- 34, amongst all known cases. Our study of this pulsar brings out insight into simultaneous behavior of the single pulses from PSR B0826- 34 at 303 and 610 MHz, which has not been examined so far. We see about 6 -7 drift bands in the MP region at 303 and at 610 MHz. At 610 MHz we see about 2 -3 drift bands in the IP region. We observe wide variations in the drift rates, including positive and negative drift rates and curved drift bands, which are simultaneous for both frequencies. We have noticed coherence between simultaneous multiple drift bands - at some given instant of time all the drift bands (6 -7 drift bands) under the MP window show similar kind of drift. Though we find the drift pattern to be very similar in the simultaneous 303 and 610 MHz data, we observe that the drift band separation (Pm2) evolves significantly between these two frequencies, and in a manner opposite to the average profile evolution. In addition, we confirm the dependence of Pm2 on pulse longitude at 303 MHz and find indications for the same at 610 MHz. Significant linear polarization is observed in the MP region which drops abruptly at the edges of the pulse profile. Two orthogonal mode jumps are seen at the edges of the MP for both 325 and 610 MHz. We observe somewhat non orthogonal mode jump at the edges of IP for 610 MHz. Significant circular polarization in the MP along with the sense reversal near the center is observed for both the frequencies. The PA curve shows typical ”S” shaped swing (though there is some hint of a kink in the central part of the PA curve). RVM fit (Radhakrishnan & Cooke, 1969) to the PA curve is obtained with α~ 9.8 deg, β~3.2 deg, at both 325 and 610 MHz. The detailed study of two unique wide profile pulsars, PSR B0818-41 and PSR B0826 -34, was very rewarding and provided fair amount of insight towards the emission properties of pulsars. We broadly conclude that the emission from simultaneous multiple drift bands are coherent. In other words the emission mechanism responsible for generation of the drift bands is heavily correlated in the whole on pulse window. Also the equispaced sparks argues for a more isotropic arrangement of sparks which is favored by the conal model (Rankin, 1983). Drifting from more than one rings are observed only for two pulsars, PSR B0818-41 and PSR B0826-34. For PSR B081841 we observe that the emission from different rings are always locked in phase. This constant phase relation is maintained even during sequences of irregular drifting as well as after nulling. PSR B0826 -34 is another wide profile pulsar for which presence of simultaneous multiple drift regions are observed. For this pulsar the MP and the IP emission are interpreted to be coming from two concentric rings of emission. The drift bands in these regions are locked in phase implying that the emission from the inner and the outer rings are in phase. For PSR B0826- 34 we observe frequent nulling and changes of drift rates which are simultaneous for both the inner and outer rings. Hence for all pulsars for which we know drifting from more than one ring, the drift pattern in the inner and outer rings are always phase locked. No counter example is observed. This requires common drift rate in the inner and outer rings, implying that emission in the two rings are not independent, and the conditions responsible for drifting are similar in both rings. Our finding of PLR between the emission from the inner and the outer rings puts constraints on the theoretical models of pulsar emission mechanism and favors a pan magnetospeheric radiation mechanism. Preliminary study of single pulse properties of six other pulsars Inspired by the success of our study of PSR B0818- 41 and PSR B0826-34 we carried out single pulse study of few other pulsars with diverse profile. Preliminary results from this study are presented in Chapter 6. However, some of the new results form this work are highlighted in the following. We report occasional nulling for PSR B0540+23 which is important in the sense that nulls are not commonly seen in the core components. We observe simultaneous two drift bands for B1819- 22 at 325 and 610 MHz. We observe some kind of mode changing between stronger and weaker modes with changes of drift rates, which are probably associated with occasional nulling observed in this pulsar. For PSR B1839 -04 subpulse drifting is observed under the two peaks of the profile. The emission under the leading and trailing peaks appear to be in phase. Determination of the orbital parameters of binary pulsars Apart from the above work, I got interested in determination of the orbital parameters of the binary pulsars. This work was triggered by the discovery of a binary pulsar PSR J0514- 4002 (the first known pulsar in the globular cluster NGC 1851) at the GMRT in 2004 (Freire et al., 2004). We present a novel method for determination of the orbital parameters of binary pulsars, using data on the pulsar period at multiple observing epochs in contrast to the method described by Freire, Kramer & Lyne (2001) which requires both pulsar period and period derivatives at particular observing epochs. This method uses the circular nature of the velocity space orbit of Keplerian motion and produces preliminary values based on two one dimensional searches. Preliminary orbital parameter values are then refined using a computationally efficient linear least square fit. This method works for random and sparse sampling of the binary orbit. Unlike the method used by Freire, Kramer & Lyne (2001), which works for nearly circular binary orbits, this method works for binary orbit with any eccentricity. We demonstrate the technique on (a) the highly eccentric binary pulsar PSR J0514- 4002 (the first known pulsar in the globular cluster NGC 1851) and (b) 47 Tuc T, a binary pulsar with a nearly circular orbit. Our result agrees with the earlier determination of the orbital parameters of the binary pulsars done with coherent multi-epoch timing (Freire, Kramer & Lyne (2001) and Freire et al. (2007)). In our method the computation involves only one dimensional searches and linear least square fits. This study is reported in Chapter 7. The main conclusions and the possible future works are presented in Chapter 8.

Dans le domaine de l’astrophysique des hautes énergies, de nombreuses questions restent à ce jour sans réponse et, parmi elles se trouve l’origine des rayons cosmiques Galactiques. La première preuve observationnelle de ces particules accélérées a été apportée au sein d’un vestige de supernova il y a seulement vingt ans. Depuis, nous savons que les vestiges de supernova, les pulsars et leurs nébuleuses accélèrent efficacement des particules mais de nombreuses interrogations subsistent encore. Les preuves directes concernant l’accélération de protons (constituant 90% du rayonnement cosmique) sont rares et de nombreuses sources nouvellement détectées en gamma sont de nature inconnue. Les rayonnements produits au sein des accélérateurs Galactiques fournissent d’importants éléments de réponse quant à la nature des particules accélérées. En particulier, alors que les domaines de la radio et des rayons X ne tracent que les électrons accélérés, les rayons gamma peuvent inférer la présence d’électrons et également de protons (et noyaux en général) mais l’émission s’avère le plus souvent difficile à interpréter.Les mesures des rayons gamma de très hautes énergies dépendent de notre connaissance de l’atmosphère terrestre, dans lequel ils se propagent avant d’être détectés par les télescopes Tcherenkov au sol tels que le réseau H.E.S.S. La partie technique de cette thèse concerne l’étude de l’impact des profils d’atmosphère sur les données H.E.S.S. Grâce à des simulations et des analyses prenant en compte les caractéristiques propres à chaque prise de données, nous étudions l’impact des profils d’atmosphère mesurés sur les fonctions de réponse de l’instrument et sur la reconstruction spectrale.Le premier objectif scientifique de cette thèse est de comprendre la nature de l’émission gamma au sein de deux vestiges de supernova (G326.3-1.8 et RX J1713.7-3946) par le biais d’analyses spectro-morphologiques détaillées. L’analyse de G326.3-1.8, avec les données du Fermi-LAT, a mené à deux résultats importants: une nouvelle preuve d’accélération de protons et la première séparation morphologique et spectrale de deux composantes imbriquées en gamma. L’analyse de RX J1713.7-3946, avec les données H.E.S.S. et les outils d'analyse Ctools, a confirmé une extension plus importante en gamma qu’en rayons X mais dont l’origine reste encore incertaine.La deuxième partie de cette thèse entreprend de discuter la nature des sources Galactiques non-associées au TeV. Pour ce faire, nous présentons un code générique visant à rechercher des contreparties multi-longueurs d’onde sur ces sources au TeV, et permettant de poser des contraintes sur des paramètres physiques tels que le champ magnétique moyen et l’indice spectral en radio. En appliquant ce code sur cinq sources non-identifiées du relevé du plan Galactique de H.E.S.S., nous apportons des arguments quant à leur origine. En particulier, nous étudions deux sources, dont l’émission au TeV provient probablement de multiples contributions, soulevant ainsi l’importance des données multi-longueurs d’onde pour comprendre la nature de l’émission en gamma. / In high energy astrophysics, several questions are still open and amongst them is the origin of Galactic cosmic rays. The first observational evidence of accelerated particles has only been revealed twenty years ago. Since then, supernova remnants, pulsars and their nebulae are known to efficiently accelerate particles but several questions still hold. In particular, evidence of accelerated protons (which consist on 90% of the cosmic-ray spectrum) is still elusive and several gamma-ray sources have unknown origin. Non-thermal emissions produced in Galactic accelerators provide insights about the nature of the accelerated particles. In particular, while radio and X-ray observations indicate the presence of accelerated electrons, gamma rays can be produced by both electrons and protons (or nuclei in general) but it may be difficult to assess the origin of the emission.The technical part of this thesis concerns the study of the impact on the reconstructed H.E.S.S. data when using atmospheric profiles measured with a lidar instead of a standard atmospheric model currently used. Very high energy gamma rays propagate into the atmosphere before reaching Cherenkov Telescopes and thus, the accuracy of our measurements depends on our understanding of the atmospheric composition. Using run-wise simulations based on lidar data, we study the impact on the instrument response functions and we analyse data to quantify the effect on the spectral reconstruction.The first scientific goal of this thesis is to understand the gamma-ray emission from two supernova remnants (G326.3-1.8 et RX J1713-3946) through detailed spectro-morphological analyses. The analysis of the composite supernova remnant G326.3-1.8, with Fermi-LAT data, has led to two major results: a new evidence of accelerated protons and the first morphological and spectral separation in gamma rays of two nested components. The study of RX J1713-3946, with H.E.S.S. data and using the Ctools package, confirms a significant gamma-ray extension beyond the X-ray emitting shell but its origin remains unclear.The second part of this thesis aims to constrain the nature of the unidentified TeV sources revealed in the H.E.S.S. Galactic Plane Survey. We thus present a generic code, based on a multi-wavelength approach, to find counterparts and estimate physical parameters like the radio spectral index and the mean magnetic field. We apply this code on five unidentified TeV sources and we put constraints on their nature. In particular, we present two high-confusion cases, for which the TeV emission is probably due to the contribution from different components, emphasizing the importance of multi-wavelength data to understand the origin of the gamma-ray emission.

AMS-02 est un spectromètre magnétique conçu pour la détection du rayonnement cosmique chargé et des photons gamma. La mesure en 2013 de la fraction de positons par AMS-02 a confirmé la présence d'une composante de positons primaires, nécessitant l'existence d'une source de positons dans notre Galaxie. Cette thèse s'est exclusivement consacrée à l'étude de ce phénomène, et à la recherche de cette source de positons. La fraction de positons, le flux de positons et le flux d'électrons sont mesurés à l'aide d'une méthode de fit multidimensionnel mis au point lors de cette thèse. Une étude particulière est réservé au phénomène de confusion de charge, important pour la mesure du signe de la charge à haute énergie. La fonction de réponse instrumentale du détecteur est évaluée à l'aide de simulations Monte Carlo et est corrigée par les données réelles du détecteur. Un travail phénoménologique est effectué afin d'interpréter les résultats mesurés. L'interprétation de ces résultats en termes de composante primaire issues d'un pulsar local et du halo galactique de matière noire est effectué. Les implications et l'espace des paramètres autorisé pour chacun des modèles est explicité. Une ré-évaluation des secondaires à l'aide des nouvelles données d'AMS-02, ainsi que l'étude de l'impact de l'erreur expérimentale d'AMS-02 sur ces résultats, est exécuté. / AMS-02 is a magnetic spectrometer design for the cosmic ray and gamma ray detection. The measurement in 2013 of the positron fraction by AMS-02 confirmed the presence of a primary positron component, which means the existence of a galactic primary source of positrons. This thesis is devoted exclusively to the study of this phenomenon. The positron fraction, the positron flux, and the electron flux are measured using a method of multidimensional fit developed during this thesis. The charge confusion phenomenon, which is important at the highest energies, is investigated. The instrumental response function of the detector is evaluated using Monte Carlo simulations and is corrected using the ISS data. A phenomenological work is done to interpret the measured results. The interpretation of these results in terms of primary component from a local pulsar and dark matter halo is done. The implications and the parameter space allowed for each model is detailed. A reevaluation of secondaries with the new AMS-02 data, and the study of the impact of the experimental error of AMS-02 on these results is performed.

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Die Erforschung von Gravitationswellen hat unsere Sicht auf das Universum revolutioniert. Mit dem bevorstehenden Start von LISA, einem Weltraum-Gravitationswellendetektor, und neuen Berichten über Hinweise auf einen Gravitationswellenhintergrund im Nanohertz-Bereich aus Pulsar Timing Array (PTA)-Experimenten, eröffnen sich neue Möglichkeiten und Herausforderungen. Diese Dissertation entwickelt innovative Datenanalysetechniken und Wellenformmodelle, um Erkenntnisse aus diesen Beobachtungen zu gewinnen. Ein Schwerpunkt liegt auf der Untersuchung von Extreme Mass Ratio Inspirals (EMRIs) durch LISA. Diese Quellen bestehen aus kleinen, kompakten Objekten, die sich um ein zentrales Schwarzes Loch bewegen. Die Wellenformen von EMRIs bieten die Möglichkeit präziser Parametermessungen, sind jedoch aufgrund ihrer langen Signaldauer und harmonischen Komplexität schwer zu berechnen. Wir präsentieren die Implementierung einsatzbereiter EMRI-Wellenformen im Frequenzbereich für Grafikprozessoren (GPUs) und zentrale Recheneinheiten (CPUs). Zudem untersuchen wir das wissenschaftliche Potenzial von EMRIs innerhalb von Akkretionsscheiben, erforschen den Einfluss von Umwelteffekten mittels bayesianischer Methoden und bewerten die Multimessenger-Aussichten dieser Systeme. Im PTA-Bereich entwickeln wir Methoden zur Optimierung der Datenkombinationen für PTA-Analysen und tragen zum European Pulsar Timing Array bei, indem wir alternative Sampling-Pipelines für die Analyse von Gravitationswellenhintergründen und individuellen Quellen implementieren. Mit transdimensionalen Sampling-Methoden suchen wir nach einzelnen supermassiven Schwarzen Löchern und bewerten deren Signifikanz. Diese Dissertation trägt zur Weiterentwicklung der Gravitationswellenastronomie bei, indem sie neue Methoden und Modelle entwickelt, die tiefere Einblicke in die kosmischen Phänomene ermöglichen, die von LISA- und PTA-Beobachtungen erfasst werden. / Gravitational wave astronomy has reshaped our understanding of the cosmos. As we look towards the future launch of LISA, a space-based gravitational wave detector, and analyze recent evidence of a nanohertz gravitational wave background from Pulsar Timing Array (PTA) experiments, new opportunities and challenges emerge. This thesis delves into developing novel data analysis techniques and waveform models to extract information from these observations. Focusing on LISA, we delve into Extreme Mass Ratio Inspirals (EMRIs). These sources consist of small compact objects spiralling into massive black holes at the centres of galaxies. Their observations are expected to provide precise parameter measurements for these systems. However, EMRI waveform generation poses challenges due to the long signal duration and large harmonic content. For the first time, we provide a fast implementation of EMRI waveforms in the frequency domain, suitable for both graphics processing units (GPUs) and central processing units (CPUs). In addition, we explore the scientific potential of EMRIs embedded in accretion disks. Employing Bayesian inference, we investigate the measurability of environmental effects and explore these systems' multimessenger prospects. Transitioning to PTA, we develop methods to optimize data combinations for PTA analyses. We present our contributions to the second data release of the European Pulsar Timing Array collaboration, which consists of implementing alternative sampling pipelines for gravitational wave background and individual source analyses. Using trans-dimensional sampling methods, we search for individual supermassive black hole binaries and assess their significance. The burgeoning field of gravitational wave astronomy has the potential to transform our understanding of the Universe. The work in this thesis develops new approaches that will facilitate the delivery of the best possible scientific results from current and future gravitational wave observations.

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