This MSc report discusses the attributes of W Ursae Majoris (W UMa) stars and an investigation into the Minimum Entropy (ME) method, a digital technique applied to the determination of their periods of variability. A Python code programme was written to apply the ME method to photometric data collected on W UMa stars by the All Sky Automated Survey (ASAS). Starting with the orbital period of the binaries estimated by ASAS, this programme systematically searches around this period for the period which corresponds to the lowest value of entropy. Low entropy here means low scatter (or spread) of data across the phase-magnitude plane. The ME method divides the light curve plot area into a number of elements of the investigators choosing. When a particular orbital period is applied to this photometric data, the resulting distribution of this data in the light curve plane corresponds to a speci c number of data points in each element into which this plane has been divided. This data spread is measured and calculated in terms of entropy and the lowest value of entropy corresponds to the lowest spread of data across the light curve plane. This should correspond to the best light curve shape available from the data and therefore the most accurate orbital period available. Subsequent to the testing of this Python code on perfect sine waves, it was applied, and its results compared, to the 62 ASAS eclipsing binary stars which were investigated by Deb and Singh (2011). The method was then applied to selected stars from the ASAS data base. / Environmental Sciences / M. Sc. (Astronomy)

Gamma-ray burst light curves are notoriously irregular, yet a significant number consists of a single fast-rising, exponentially decaying pulse. These are called single-peaked light curves. The goal of this thesis is to analyse a sample of 2710 GRBs collected by the Fermi space telescope by identifying single-peaked bursts and comparing their properties to those of the multi-peaked bursts. Furthermore, the validity of the relativistic shock breakout theory as an explanation for single-peaked, low-luminosity GRBs is investigated using a closure relation. For this investigation, the Fermi sample wascomplemented by low-luminosity GRBs observed by other instruments. A criterion for selecting single-peaked bursts was successfully developed, yielding 48% long and 79% short, single-peaked GRBs. Significant differences between the populations were found in multiple properties. In general, single-peaked GRBs appear to be weaker and more slowly varying than multi-peaked ones; however, a larger sample of GRBs with redshift measurements is needed to draw conclusions about possible intrinsic differences in energy connected to the progenitor systems. The investigation of low-luminosity GRBs’ compliance with the shock breakout closure relation showed that 64% of the low-luminosity GRBs were within a factor 5 of fulfilling the relation as opposed to only 24% of high-luminosity GRBs. It was further shown that only a small number (< 5%) of Fermi GRBs without redshift measurements could be low-luminosity shock breakout GRBs according to this theory. In conclusion, while the shock breakout closure relation does hold for a greater proportion of low-luminosity GRBs than high-luminosity GRBs, there is still a large number of low-luminosity GRBs left unexplained by this theory. / Gammablixtljuskurvor är ökänt oregelbundna men en betydande andel består av en enda snabbt stigande och exponentiellt avtagande puls. Dessa kallas singelpeakade ljuskurvor. Målet med detta examensarbete är att analysera de 2710 gammablixtar som Fermirymdteleskopet har observerat genom att identifiera singelpeakade blixtar och jämföra deras egenskaper med multipeakade blixtars. Dessutom undersöks den relativistiska shockbreakoutteorin som förklaringsmodell för singelpeakade lågluminositetsgammablixtar. I denna undersökning kompletterades fermiblixtarna med lågluminositetsblixtar från andra instrument. Ett kriterium för identifikation av singelpeakade gammablixtar utvecklades och detta resulterade i 48% långa och 70% korta, singelpeakade gammablixtar. Flertalet egenskaper uppvisade signifikanta skillnader mellan populationerna. I allmänhet verkar singelpeakade gammablixtar vara svagare och variera långsammare än multipeakade. Dock behövs en större population av gammablixtar med uppmätta rödskift för att med säkerhet kunna avgöra om singelpeakade blixtar verkligen släpper ut mindre energi. Undersökningen av huruvida lågluminositetsgammablixtar kan förklaras med shockbreakoutteorin visade att 64% av lågluminositetsblixtarna uppfyllde kravet upp till en faktor fem medan bara 24% av högluminositetsblixtarna gjorde det. Vidare visades att endast ett litet antal (<5%) av fermiblixtarna utan uppmätta rödskift skulle kunna vara lågluminositetsshockbreakoutblixtar enligt denna teori. Även om shockbreakoutteorin kan förklara en större andel av lågluminositetsblixtarna än högluminositetsblixtarna återstår ett stort antal oförklarade lågluminositetsblixtar.

Cherenkov-Teleskope sind in der Lage, das schwache Cherenkovlicht aus Teilchenschauern zu detektieren, die von kosmischen Teilchen mit Energien von ca. 100 GeV bis 100 TeV in der Erdatmosphäre initiiert werden. Das Ziel ist die Detektion von Cherenkovlicht aus Schauern, die von Gammastrahlen erzeugt wurden, der größte Teil der Schauer stammt jedoch von geladenen Teilchen. Im Jahr 2012 wurde das H.E.S.S.-Observatorium in Namibia, bis dahin bestehend aus vier Teleskopen mit 100 m²-Spiegeln, um ein fünftes Teleskop mit einer Spiegelfläche von ca. 600 m² ergänzt. Aufgrund der großen Spiegelfläche besitzt dieses Teleskop die niedrigste Energieschwelle aller Teleskope dieser Art. In dieser Dissertation wird ein schneller Algorithmus namens MonoReco präsentiert, der grundlegende Eigenschaften der Gammastrahlen wie ihre Energien und Richtungen rekonstruieren kann. Dieser Algorithmus kann weiterhin unterscheiden, ob Schauer von Gammastrahlen oder von geladenen Teilchen der kosmischen Strahlung initiiert wurden. Diese Aufgaben werden mit mithilfe von künstlichen neuronalen Netzwerken erfüllt, welche ausschließlich die Momente der Intensitätsverteilungen in der Kamera des neuen Teleskops analysieren. Eine Energieschwelle von 59 GeV und Richtungsauflösungen von 0.1°-0.3° werden erreicht. Das Energiebias liegt bei wenigen Prozent, die Energieauflösung bei 20-30%. Unter anderem mit dem MonoReco-Algorithmus wurden Daten, die in der Zeit um das Periastron des Binärsystems PSR B1259-63/LS 2883 im Jahre 2014 genommen wurden, analysiert. Es handelt sich hierbei um einen Neutronenstern, der sich in einem 3,4-Jahres-Orbit um einen massereichen Stern mit einer den Stern umgebenden Scheibe aus Gas und Plasmen befindet. Zum ersten Mal konnte H.E.S.S. das Gammastrahlenspektrum dieses Systems bei Energien unterhalb von 200 GeV messen. Weiterhin wurde bei erstmaligen Beobachtungen zur Zeit des Periastrons ein lokales Flussminimum gemessen. Sowohl vor dem ersten als auch nach dem zweiten Transit des Neutronensterns durch die Scheibe wurden hohe Flüsse gemessen. Im zweiten Fall wurden Beobachtungen erstmals zeitgleich mit dem Fermi-LAT-Experiment durchgeführt, das wiederholt sehr hohe Flüsse in diesem Teil des Orbits messen konnte. Ein Vergleich der gemessenen Flüsse mit Vorhersagen eines leptonischen Modells zeigt gute Übereinstimmungen. / Cherenkov telescopes can detect the faint Cherenkov light emitted by air showers that were initiated by cosmic particles with energies between approximately 100 GeV and 100 TeV in the Earth's atmosphere. Aiming for the detection of Cherenkov light emitted by gamma ray-initiated air showers, the vast majority of all detected showers are initiated by charged cosmic rays. In 2012 the H.E.S.S. observatory, until then comprising four telescopes with 100 m² mirrors each, was extended by adding a much larger fifth telescope with a very large mirror area of 600 m². Due to the large mirror area, this telescope has the lowest energy threshold of all telescopes of this kind. In this dissertation, a fast algorithm called MonoReco is presented that can reconstruct fundamental properties of the primary gamma rays like their direction or their energy. Furthermore, this algorithm can distinguish between air showers initiated either by gamma rays or by charged cosmic rays. Those tasks are accomplished with the help of artificial neural networks, which analyse moments of the intensity distributions in the camera of the new telescope exclusively. The energy threshold is 59 GeV and angular resolutions of 0.1°-0.3° are achieved. The energy reconstruction bias is at the level of a few percent, the energy resolution is at the level of 20-30%. Data taken around the 2014 periastron passage of the gamma-ray binary PSR B1259-63/LS 2883 were analysed with, among others, the MonoReco algorithm. This binary system comprises a neutron star in a 3.4 year orbit around a massive star with a circumstellar disk consisting of gas and plasma. For the first time the gamma-ray spectrum of this system could be measured by H.E.S.S. down to below 200 GeV. Furthermore, a local flux minimum could be measured during unprecedented measurements at the time of periastron. High fluxes were measured both before the first and after the second transit of the neutron star through the disk. In the second case measurements could be performed for the first time contemporaneously with the Fermi-LAT experiment, which has repeatedly detected very high fluxes at this part of the orbit. A good agreement between measured fluxes and predictions of a leptonic model is found.

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