<p>High-energy neutrinos are capable of carrying information over vast distances, and neutrino telescopes such as AMANDA-II provide the means to probe deep inside the violent and energetic interior of the universe. AMANDA-II is located in the glacial ice at South Pole in Antarctica and is optimised to detect Cherenkov emission from neutrino-induced muon tracks with energies above 100 GeV. </p><p>Data acquired in 2003 with the AMANDA-II detector were searched for a non-localised flux of neutrinos with energies in excess of 1 PeV. Because of the energy dependence of the neutrino mean free path, the Earth is essentially opaque to neutrinos above PeV energies. Combined with the limited overburden of the AMANDA-II detector (about 1.5 km), this means that a potential ultra-high energy neutrino signal will be concentrated at the horizon. The background for the analysis consists of large bundles of muons produced in atmospheric air showers. Owing to their energy losses, muons cannot penetrate the Earth, and the background will be downwards moving. </p><p>After applying different selection criteria, one event was observed in the final data sample, while 0.16±0.04 background events are expected. The corresponding 90% confidence level upper limit is 4.3. The expected number of neutrino signal events for a 10^-6 <i>E</i>^-2 GeV/(s sr cm² ) flux assuming a Φ(ν_e) : Φ(ν_μ) : Φ(ν_τ) = 1:1:1 flavour ratio is 4.1±0.2, yielding an upper limit on the all-flavour neutrino flux of <i>E</i>² Φ₉₀ ≤ 1.1∙10^-6 GeV/(s sr cm² ), including systematics and with the central 90% of the signal found in the energy range 480 TeV - 1.6 EeV. </p>

Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2008.

Galaxy clusters are the most massive bound systems known in the Universe and are believed to have formed through large scale structure formation. They host relativistic cosmic-ray (CR) populations and are gravitationally bound by large amounts of Dark Matter (DM), both providing conditions in which high-energy gamma rays may be produced either via CR interactions with the intracluster medium or through the annihilation or decay of DM particles. Prior to the launch of the Fermi satellite, predictions were optimistic that these sources would be established as γ-ray-bright objects by observations through its prime instrument, the Large Area Telescope (LAT). Yet, despite numerous efforts, even a single firm cluster detection is still pending. This thesis presents a number of studies based on data taken by the LAT over its now seven year mission aiming to discover these γ rays. Using a joint likelihood technique, we study the γ-ray spectra of a sample of nearby clusters searching for a CR-induced signal due to hadronic interactions in the intracluster medium. While we find excesses in some individual targets, we attribute none to the cluster. Hence, we constrain the maximum injection efficiency of hadrons being accelerated in structure formation shocks and the fraction of CR-to-thermal pressure. We also perform a refined search targeting the Coma cluster specifically due to its large variety of existing observations in other wavebands. In the latter case we find weak indications of an excess which however falls below the detection threshold. Because the cluster emission we consider is inherently extended, we need to take into account the imperfect modeling of the foreground emission, which may be particularly difficult such as is the case with the Virgo cluster. Here, we assess the systematics associated with the foreground uncertainties and derive limits based on an improved background model of the region. For the first time we derive limits on the γ-ray flux from CR and DM-interactions in which we take into account the dynamical state of the system. For DM we also include the contribution from substructure. The DM domain is further explored by searching for line-like features as they arise from the annihilation of DM into two photons in a large sample of clusters, including Virgo and Coma. Finding no evidence for γ-ray lines, we derive limits on the DM annihilation cross section that are roughly a factor 10 (100) above that derived from observations of the galactic center assuming an optimistic (conservative) scenario regarding the boost due to DM substructure. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Submitted. Paper 4: Submitted.</p>

Time-dependent cosmic ray modulation in the heliosphere is studied by using a two-dimensional time dependent modulation model. To compute realistic cosmic ray modulation a compound approach is used, which combines the effect of the global changes in the heliospheric magnetic field magnitude and the current sheet tilt angle to establish realistic time dependent diffusion and drift coefficients. This approach is refined by scaling down drifts additionally (compared to diffusion) towards solar maximum. The amount of drifts needed in the model to realistically compute 2.5 GV proton and electron and 1.2GV electron and helium intensities, as measured by Ulysses from 1990 to 2004, is established. It is shown that the model produces the correct latitudinal gradients evident from the observations during both the Ulysses fast latitude scan periods. Also, much can be learned on the magnitude of perpendicular diffusion in the polar direction, K┴θ, especially for solar minimum conditions and for polarity cycles when particles drift in from the poles. For these periods K┴θ = 0.12K║ in the polar regions (with K║ the parallel diffusion coefficient)and K┴θ /K║ can vary between 0.01 to even 0.04 in the equatorial regions depending on the enhancement factor toward the poles. The model is also applied to compute radial gradients for 2.5 GV cosmic ray electrons and protons in the inner heliosphere. It is shown that, for solar minimum, and in the equatorial regions, the protons (electrons) have a radial gradient of 1.9 %/AU (2.9 %/AU), increasing for both species to a very fluctuating gradient varying between 3 to 4 %/AU at solar maximum. Furthermore, the model also computes realistic electron to proton and electron to helium ratios when compared to Ulysses observations, and charge-sign dependent modulation is predicted up to the next solar minimum expected in 2007. Lastly the model is also applied to model simultaneously galactic cosmic ray modulation at Earth and along the Voyager 1 trajectory, and results are compared with> 70 MeV count rates from Voyager 1 and IMP8. To produce realistic modulation, this model gives the magnitude of perpendicular diffusion in the radial direction as K┴r/K║= 0.035 and that the modulation boundary seemed to be situated between at 120 AU and 140 AU. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.

The propagation of high-energy cosmic rays is influenced by the time-varying heliospheric magnetic field embedded in the solar wind, and by the geomagnetic field. To penetrate through this geomagnetic field, they must have a rigidity that exceeds the geomagnetic cutoff rigidity for a given position on the earth. In the atmosphere, the primary cosmic rays interact with atmospheric nuclei, to form a cascade of secondary particles. Neutron monitors record these secondary cosmic rays, mainly the neutrons, with energies about a decade higher than detected by most spacecraft. Since neutron monitors are integral detectors, each with its own detection efficiency, energy spectra cannot readily be derived from their observations. One way to circumvent this is by conducting latitudinal surveys with mobile neutron monitors. Another way is to use the worldwide stationary neutron monitor network, but then the counting rates of these monitors must be normalised sufficiently accurate against one another. For this reason two portable calibration neutron monitors were built at the Potchefstroom campus of the North-West University and completed in 2002. To achieve sufficient calibration accuracy, several properties of the calibrator are investigated in this work. Effects such as atmospheric pressure variations, diurnal variations, short-term scintillations, and multiplicity, contribute to the fluctuations of the counting rate of a neutron monitor. Due to these effects, the coefficient of variation of the calibrator is determined to be -40% larger than the Poisson deviation. The energy response of the calibrator over the cutoff rigidity interval from the poles to the equator is investigated, with the result that it is almost 4% larger than that of a standard 3NM64 neutron monitor. It is also determined that not only the calibrator, but also the stationary NM64 and IGY neutron monitors, have fairly large instrumental temperature sensitivity, which must be accounted for in calibration procedures. Furthermore, the calibrator has a large sensitivity to the type of surface beneath it, influencing its counting rate by as much as 5%. This investigation is incomplete and requires further experimentation before the calibration of the stationary neutron monitors can start. When calibrations of a significant number of the worldwide neutron monitors are done, their intensity spectra as derived from differential response functions, will provide experimental data for modulation studies at rigidities above 1 GV. / Thesis (Ph.D. (Physics))--North-West University, Potchefstroom Campus, 2006.

Well motivated theoretical models predict the annihilation of dark matter (DM) into standard model particles, a phenomenon which could be a significant source of photons in the gamma-ray sky. With its unprecedented sensitivity and its broad energy range (20 MeV to more than 300 GeV) the main instrument on board the Fermi satellite, the Large Area Telescope (LAT), might be able to detect an indirect signature of DM annihilations. In this work we revisit several interesting claims of extended dark matter emission made from analyses of Fermi-LAT data: First, based on three years of Fermi Large Area Telescope (LAT) gamma-ray data of the Virgo cluster, evidence for an extended emission associated with dark matter pair annihilation in the bb̄ channel has been reported by Han et al. (arxiv:1201.1003). After an in depth spatial and temporal analysis, we argue that the tentative evidence for a gamma-ray excess from the Virgo cluster is mainly due to the appearance of a population of previously unresolved gamma-ray point sources in the region of interest. These point sources are not part of the LAT second source catalogue (2FGL), but are found to be above the standard detection significance threshold when three or more years of LAT data is included. Second, we confirm the detection of a spatially extended excess of 2-5 GeV gamma rays from the Galactic Center (GC), consistent with the emission expected from annihilating dark matter or an unresolved population of about 10³ milisecond pulsars. However, there are significant uncertainties in the diffuse galactic background at the GC. We have performed a revaluation of these two models for the extended gamma ray source at the GC by accounting for the systematic uncertainties of the Galactic diffuse emission model. We also marginalize over point source and diffuse background parameters in the region of interest. We show that the excess emission is significantly more extended than a point source. We find that the DM (or pulsars population) signal is larger than the systematic errors and therefore proceed to determine the sectors of parameter space that provide an acceptable fit to the data. We found that a population of order a 10³ MSPs with parameters consistent with the average spectral shape of Fermi-LAT measured MSPs was able to fit the GC excess emission. For DM, we found that a pure τ⁺τ⁻ annihilation channel is not a good fit to the data. But a mixture of τ⁻τ⁻ and bb̄ with a (σν) of order the thermal relic value and a DM mass of around 20 to 60 GeV provides an adequate fit. We also consider the possibility that the GeV excess is due to nonthermal bremsstrahlung produced by a population of electrons interacting with neutral gas in molecular clouds. The millisecond pulsars and dark matter alternatives have spatial templates well fitted by the square of a generalized Navarro-Frenk-White (NFW) profile with inner slope γ = 1.2. We model the third option with a 20-cm continuum emission Galactic Ridge template. A template based on the HESS residuals is shown to give similar results. The gamma-ray excess is found to be best fit by a combination of the generalized NFW squared template and a Galactic Ridge template. We also find the spectra of each template is not significantly affected in the combined fit and is consistent with previous single template fits. That is, the generalized NFW squared spectrum can be fit by either of order 10³ unresolved MSPs or DM with mass around 30 GeV, a thermal cross section, and mainly annihilating to bb̄ quarks. While the Galactic Ridge continues to have a spectrum consistent with a population of nonthermal electrons whose spectrum also provides a good fit to synchrotron emission measurements. We also show that the current DM fit may be hard to test, even with 10 years of Fermi-LAT data, especially if there is a mixture of DM and MSPs contributing to the signal, in which case the implied DM cross section will be suppressed.

One of the most significant current discussions in Astrophysics relates to the origin of high-energy cosmic rays. According to our current knowledge, the abundance distribution of the elements in cosmic rays at their point of origin indicates, within plausible error limits, that they were initially formed by nuclear processes in the interiors of stars. It is also believed that their energy distribution up to 1018 eV has Galactic origins. But even though the knowledge about potential sources of cosmic rays is quite poor above „ 1015 eV, that is the “knee” of the cosmic-ray spectrum, up to the knee there seems to be a wide consensus that supernova remnants are the most likely candidates. Evidence of this comes from observations of non-thermal X-ray radiation, requiring synchrotron electrons with energies up to 1014 eV, exactly in the remnant of supernovae. To date, however, there is not conclusive evidence that they produce nuclei, the dominant component of cosmic rays, in addition to electrons. In light of this dearth of evidence, γ-ray observations from supernova remnants can offer the most promising direct way to confirm whether or not these astrophysical objects are indeed the main source of cosmic-ray nuclei below the knee. Recent observations with space- and ground-based observatories have established shell-type supernova remnants as GeV-to- TeV γ-ray sources. The interpretation of these observations is however complicated by the different radiation processes, leptonic and hadronic, that can produce similar fluxes in this energy band rendering ambiguous the nature of the emission itself. The aim of this work is to develop a deeper understanding of these radiation processes from a particular shell-type supernova remnant, namely RX J1713.7–3946, using observations of the LAT instrument onboard the Fermi Gamma-Ray Space Telescope. Furthermore, to obtain accurate spectra and morphology maps of the emission associated with this supernova remnant, an improved model of the diffuse Galactic γ-ray emission background is developed. The analyses of RX J1713.7–3946 carried out with this improved background show that the hard Fermi-LAT spectrum cannot be ascribed to the hadronic emission, leading thus to the conclusion that the leptonic scenario is instead the most natural picture for the high-energy γ-ray emission of RX J1713.7–3946. The leptonic scenario however does not rule out the possibility that cosmic-ray nuclei are accelerated in this supernova remnant, but it suggests that the ambient density may not be high enough to produce a significant hadronic γ-ray emission. Further investigations involving other supernova remnants using the improved back- ground developed in this work could allow compelling population studies, and hence prove or disprove the origin of Galactic cosmic-ray nuclei in these astrophysical objects. A break- through regarding the identification of the radiation mechanisms could be lastly achieved with a new generation of instruments such as CTA. / Eine der gegenwärtigen bedeutendsten geführten Diskussionen in der Astrophysik bezieht sich auf den Ursprung der hochenergetischen Kosmischen Strahlung. Nach unserem heutigen Verständnis weist die am Ort des Ursprungs elementare Zusam- mensetzung der Kosmischen Strahlung darauf hin, dass diese zu Beginn mittels nuklearer Prozesse im Inneren von Sternen gebildet wurde. Weiterhin wird ange- nommen, dass die Kosmische Strahlung bis 1018 eV galaktischen Ursprungs ist. Auch wenn das Verständnis über die potentiellen Quellen der Kosmischen Strahlung ober- halb von 1015 eV, dem sogenannten „Knie“ des Spektrums der Kosmischen Strah- lung, lückenhaft ist, so liegt doch der Konsens vor, dass Supernovaüberreste (SNR) die wahrscheinlichsten Quellen für Energien bis 1015 eV sind. Unterstützt wird die- ser Sachverhalt durch Beobachtungen von nichtthermischer Röntgenstrahlung von SNR, deren Emission Elektronen mit Energien bis zu 1014 eV erfordern. Jedoch gibt es bis heute keinen überzeugenden Beweis, dass SNR zusätzlich zu den Elektronen auch Atomkerne, die den dominierenden Anteil in der Kosmischen Strahlung bilden, beschleunigen. Trotz fehlender überzeugender Beweise ermöglichen nun Beobachtungen von SNR im γ-Strahlungsbereich einen vielversprechenden Weg zur Aufklärung der Fra- ge, ob diese astrophysikalischen Objekte in der Tat die Hauptquelle der Kosmischen Strahlung unterhalb des Knies sind. Kürzlich durchgeführte Beobachtungen im Welt- raum und auf der Erdoberfläche haben zu der Erkenntnis geführt, dass schalenartige SNR γ-Strahlung im GeV- und TeV-Bereich emittieren. Die Interpretation dieser Beobachtungen ist jedoch schwierig, da sowohl Atomkerne als auch Elektronen im betrachteten Energiebereich zu ähnlichen γ-Emissionen führen. Dadurch wird die eindeutige Identifizierung der Emission als das Resultat hadronischer oder leptoni- scher Emissionsprozesse erschwert. Das Ziel dieser Arbeit ist es, am Beispiel des schalenartigen SNR RX J1713.7- 3946 ein tieferes Verständnis über die Strahlungsprozesse zu erhalten, indem vom γ-Weltraumteleskop Fermi durchgeführte Beobachtungen analysiert werden. Um ge- naue Spektren und die Ausdehnung der Region der Emission zu erhalten, wird ein verbessertes Modell für die diffuse galaktische γ-Hintergrundstrahlung entwickelt. Die mit diesem verbesserten Hintergrund durchgeführte Analyse von RX J1713.7- 3946 zeigt, dass das vom Fermi-Satelliten beobachtete Spektrum nicht dem hadro- nischen Szenario zugeschrieben werden kann, sodass das leptonische Szenario für die γ-Emissionen von diesem SNR verantwortlich ist. Das leptonische Szenario schließt jedoch nicht die Möglichkeit aus, dass auch Atomkerne in diesem SNR beschleu- nigt werden. Aber es deutet darauf hin, dass die umgebende Teilchendichte nicht ausreichend hoch genug ist, um zu einer signifikanten hadronischen γ-Emission zu führen. Weitere Untersuchungen, die andere SNR in Kombination mit dem hier ent- wickelten verbesserten Modell der Hintergrundstrahlung beinhalten, können Popu- lationsstudien erlauben. Dies könnte klären, ob die SNR tatsächlich die Quellen der galaktischen Kosmischen Strahlung sind. Ein Durchbruch bezüglich der Identi- fikation des Strahlungsmechanismus könnte auch durch eine neue Generation von Beobachtungsinstrumenten, wie das Cherenkov Telescope Array, erreicht werden.

A proper understanding of the effects of turbulence on the diffusion and drift of cosmic-rays in the heliosphere is imperative for a better understanding of cosmic-ray modulation. This study presents an ab initio model for cosmic-ray modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. The latter model is solved for solar minimum heliospheric conditions, utilizing boundary values chosen in such a way that the results of this model are in fair to good agreement with spacecraft observations of turbulence quantities, not only in the ecliptic plane, but also along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence energy spectra, which in turn are used as inputs for parallel mean free paths based on those derived from quasi-linear theory, and perpendicularmean free paths from extended nonlinear guiding center theory. The modelled 2D spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently exist no models or observations for this quantity, and it is the only free parameter in this study. The use of such a spectrum yields a non-divergent 2D ultrascale, which is used as an input for the reduction terms proposed to model the effects of turbulence on cosmic-ray drifts. The resulting diffusion and drift coefficients are applied to the study of galactic cosmic-ray protons, electrons, antiprotons, and positrons using a three-dimensional, steady-state numerical cosmic-ray modulation code. The magnitude and spatial dependence of the 2D outerscale is demonstrated to have a significant effect on computed cosmic-ray intensities. A form for the 2D outerscale was found that resulted in computed cosmic-ray intensities, for all species considered, in reasonable agreement with multiple spacecraft observations. Computed galactic electron intensities are shown to be particularly sensitive to choices of parameters pertaining to the dissipation range of the slab turbulence spectrum, and certain models for the onset wavenumber of the dissipation range could be eliminated in this study. / Thesis (PhD (Physics))--North-West University, Potchefstroom Campus, 2013

A proper understanding of the effects of turbulence on the diffusion and drift of cosmic-rays in the heliosphere is imperative for a better understanding of cosmic-ray modulation. This study presents an ab initio model for cosmic-ray modulation, incorporating for the first time the results yielded by a two-component turbulence transport model. The latter model is solved for solar minimum heliospheric conditions, utilizing boundary values chosen in such a way that the results of this model are in fair to good agreement with spacecraft observations of turbulence quantities, not only in the ecliptic plane, but also along the out-of-ecliptic trajectory of the Ulysses spacecraft. These results are employed as inputs for modelled slab and 2D turbulence energy spectra, which in turn are used as inputs for parallel mean free paths based on those derived from quasi-linear theory, and perpendicularmean free paths from extended nonlinear guiding center theory. The modelled 2D spectrum is chosen based on physical considerations, with a drop-off at the very lowest wavenumbers commencing at the 2D outerscale. There currently exist no models or observations for this quantity, and it is the only free parameter in this study. The use of such a spectrum yields a non-divergent 2D ultrascale, which is used as an input for the reduction terms proposed to model the effects of turbulence on cosmic-ray drifts. The resulting diffusion and drift coefficients are applied to the study of galactic cosmic-ray protons, electrons, antiprotons, and positrons using a three-dimensional, steady-state numerical cosmic-ray modulation code. The magnitude and spatial dependence of the 2D outerscale is demonstrated to have a significant effect on computed cosmic-ray intensities. A form for the 2D outerscale was found that resulted in computed cosmic-ray intensities, for all species considered, in reasonable agreement with multiple spacecraft observations. Computed galactic electron intensities are shown to be particularly sensitive to choices of parameters pertaining to the dissipation range of the slab turbulence spectrum, and certain models for the onset wavenumber of the dissipation range could be eliminated in this study. / Thesis (PhD (Physics))--North-West University, Potchefstroom Campus, 2013

Time-dependent cosmic ray modulation in the heliosphere is studied by using a two-dimensional time dependent modulation model. To compute realistic cosmic ray modulation a compound approach is used, which combines the effect of the global changes in the heliospheric magnetic field magnitude and the current sheet tilt angle to establish realistic time dependent diffusion and drift coefficients. This approach is refined by scaling down drifts additionally (compared to diffusion) towards solar maximum. The amount of drifts needed in the model to realistically compute 2.5 GV proton and electron and 1.2GV electron and helium intensities, as measured by Ulysses from 1990 to 2004, is established. It is shown that the model produces the correct latitudinal gradients evident from the observations during both the Ulysses fast latitude scan periods. Also, much can be learned on the magnitude of perpendicular diffusion in the polar direction, K┴θ, especially for solar minimum conditions and for polarity cycles when particles drift in from the poles. For these periods K┴θ = 0.12K║ in the polar regions (with K║ the parallel diffusion coefficient)and K┴θ /K║ can vary between 0.01 to even 0.04 in the equatorial regions depending on the enhancement factor toward the poles. The model is also applied to compute radial gradients for 2.5 GV cosmic ray electrons and protons in the inner heliosphere. It is shown that, for solar minimum, and in the equatorial regions, the protons (electrons) have a radial gradient of 1.9 %/AU (2.9 %/AU), increasing for both species to a very fluctuating gradient varying between 3 to 4 %/AU at solar maximum. Furthermore, the model also computes realistic electron to proton and electron to helium ratios when compared to Ulysses observations, and charge-sign dependent modulation is predicted up to the next solar minimum expected in 2007. Lastly the model is also applied to model simultaneously galactic cosmic ray modulation at Earth and along the Voyager 1 trajectory, and results are compared with> 70 MeV count rates from Voyager 1 and IMP8. To produce realistic modulation, this model gives the magnitude of perpendicular diffusion in the radial direction as K┴r/K║= 0.035 and that the modulation boundary seemed to be situated between at 120 AU and 140 AU. / Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2005.