No description available.

The morphological analysis of galaxy clusters in X-rays allows a reliable determination of their dynamical state. Substructures on (sub-)Mpc scale influence the gravitational potential of a cluster and manifest themselves in the X-ray surface brightness distribution as secondary peaks or overall irregular shape. They lead to deviations from the hydrostatic equilibrium and spherical shape, two assumptions which are widely used in galaxy cluster studies to derive global astrophysical properties. Analyzing the X-ray morphology of clusters thus yields valuable information, provided that the employed substructure measures are well-tested and well-calibrated. In this work, the X-ray morphology of galaxy clusters is quantified using three common substructure parameters (power ratios, center shift and the asymmetry parameter), which are subsequently employed to study the disturbed cluster fraction as a function of redshift. To ensure a reliable application of these substructure parameters on a variety of X-ray images, a detailed parameter study is conducted. It focuses on the performance and reliability of the parameters for varying data quality using simulated and observed X-ray images. In particular, when applying them to X-ray images with low photon counts such as observations of distant clusters or survey data, it is important to know the characteristics of the parameters. Comparing the three substructure measures, the center shift parameter is most robust against Poisson noise and allows a reliable determination of the clusters' dynamical state even for low-count observations. Power ratios, especially the hexapole P3/P0, and the asymmetry parameter, on the other hand, are severely affected by noise, which results in spuriously high substructure signals. Furthermore, this work presents methods to minimize the noise bias. The results of the parameter study provide a step forward in the morphological analysis of high-redshift clusters and are employed in the framework of this thesis to quantify the evolution of the disturbed cluster fraction. The sample used for this analysis comprises 78 low-z (z < 0.3) and 51 high-z (0.3 < z < 1.08) galaxy clusters with varying photon statistics. The low-redshift objects were observed with the XMM-Newton observatory, contain a high number of photon counts and are part of several well-known and representative samples. For z > 0.3, the high-redshift subsets of the 400d2 and SPT survey catalog are used. These objects were mainly observed with the Chandra observatory and have low photon counts. To ensure a fair comparison, which is independent of the data quality, the photon statistics of the low- and high-redshift observations are aligned before performing the morphological analysis. In agreement with the hierarchical structure formation model, a mild positive evolution with redshift, i.e. a larger fraction of clusters with disturbed X-ray morphologies at higher redshift, is found. Owing to the low photon counts and small number of high-redshift observations, the statistical significance of this result is low. For two of the three substructure parameters (power ratios and center shift) the findings are also consistent within the significance limits with no evolution, but a negative evolution of the disturbed cluster fraction can be excluded for all parameters.

No description available.

The search for exo-planets is currently one of the most exiting and active topics in astronomy. Small and rocky planets are particularly the subject of intense research, since if they are suitably located from their host star, they may be warm and potentially habitable worlds. On the other hand, the discovery of giant planets in short-period orbits provides important constraints on models that describe planet formation and orbital migration theories. Several projects are dedicated to discover and characterize planets outside of our solar system. Among them, the Wide-Field Camera Transit Survey (WTS) is a pioneer program aimed to search for extra-solar planets, that stands out for its particular aims and methodology. The WTS has been in operation since August 2007 with observations from the United Kingdom Infrared Telescope, and represents the first survey that searches for transiting planets in the near-infrared wavelengths; hence the WTS is designed to discover planets around M-dwarfs. The survey was originally assigned about 200 nights, observing four fields that were selected seasonally (RA = 03, 07, 17 and 19h) during a year. The images from the survey are processed by a data reduction pipeline, which uses aperture photometry to construct the light curves. For the most complete field (19h-1145 epochs) in the survey, we produce an alternative set of light curves by using the method of difference imaging, which is a photometric technique that has shown important advantages when used in crowded fields. A quantitative comparison between the photometric precision achieved with both methods is carried out in this work. We remove systematic effects using the sysrem algorithm, scale the error bars on the light curves, and perform a comparison of the corrected light curves. The results show that the aperture photometry light curves provide slightly better precision for objects with J < 16. However, difference photometry light curves present a significant improvement for fainter stars. In order to detect transits in the WTS light curves, we use a modified version of the box-fitting algorithm. The implementation on the detection algorithm performs a trapezoid-fit to the folded light curve. We show that the new fit is able to produce more accurate results than the box-fit model. We describe a set of selection criteria to search for transit candidates that include a parameter calculated by our detection algorithm: the V-shape parameter, which has proven to be useful to automatically identify and remove eclipsing binaries from the survey. The criteria are optimized using Monte-Carlo simulations of artificial transit signals that are injected into the real WTS light curves and subsequently analyzed by our detection algorithm. We separately optimize the selection criteria for two different sets of light curves, one for F-G-K stars, and another for M-dwarfs. In order to search for transiting planet candidates, the optimized selection criteria are applied to the aperture photometry and difference imaging light curves. In this way, the best 200 transit candidates from a sample of ~ 475 000 sources are automatically selected. A visual inspection of the folded light curves of these detections is carried out to eliminate clear false-positives or false-detections. Subsequently, several analysis steps are performed on the 18 best detections, which allow us to classify these objects as transiting planet and eclipsing binary candidates. We report one planet candidate orbiting a late G-type star, which is proposed for photometric follow-up. The independent analysis on the M-dwarf sample provides no planet candidates around these stars. Therefore, the null detection hypothesis and upper limits on the occurrence rate of giant planets around M-dwarfs with J < 17 mag presented in a prior study are confirmed. In this work, we extended the search for transiting planets to stars with J < 18 mag, which enables us to impose a more strict upper limit of 1.1 % on the occurrence rate of short-period giant planets around M-dwarfs, which is significantly lower than other limit published so far. The lack of Hot Jupiters around M-dwarfs play an important role in the existing theories of planet formation and orbital migration of exo-planets around low-mass stars. The dearth of gas-giant planets in short-period orbit detections around M stars indicates that it is not necessary to invoke the disk instability formation mechanism, coupled with an orbital migration process to explain the presence of such planets around low-mass stars. The much reduced efficiency of the core-accretion model to form Jupiters around cool stars seems to be in agreement with the current null result. However, our upper limit value, the lowest reported sofar, is still higher than the detection rates of short-period gas-giant planets around hotter stars. Therefore, we cannot yet reach any firm conclusion about Jovian planet formation models around low-mass and cool main-sequence stars, since there are currently not sufficient observational evidences to support the argument that Hot Jupiters are less common around M-dwarfs than around Sun-like stars. The way to improve this situation is to monitor larger samples of M-stars. For example, an extended analysis of the remaining three WTS fields and currently running M-dwarf transit surveys (like Pan-Planets and PTF/M-dwarfs projects, which are monitoring up to 100 000 objects) may reduce this upper limit. Current and future space missions like Kepler and GAIA could also help to either set stricter upper limits or finally detect Hot Jupiters around low-mass stars. In the last part of this thesis, we present other applications of the difference imaging light curves. We report the detection of five faint extremely-short-period eclipsing binary systems with periods shorter than 0.23 d, as well as two candidates and one confirmed M-dwarf/M-dwarf eclipsing binaries. The etections and results presented in this work demonstrate the benefits of using the difference imaging light curves, especially when going to fainter magnitudes. / Die Suche nach Exoplaneten ist heute eine der interessantesten und aktivsten Forschungsgebiete in der Astronomie. Vor allem erdähnliche Planeten sind das Ziel diverser Forschungsprojekte, da diese, sofern sie in einem bestimmten Abstand um ihrem Mutter-Stern kreisen, eine Oberflächentemperatur aufweisen, die ein Vorkommen von Wasser in flüssiger Form ermöglicht und somit ``habitabel'' sind. Außerdem sorgt die Entdeckung von Gasriesen in kurzperiodischen Umlaufbahnen für wichtige Erkenntnisse zu Modellen, die die Planetenbildung und orbitale Migration beschreiben. Viele Projekte haben es sich zum Ziel gesetzt, Planeten außerhalb unseres Sonnensystems zu finden und zu charakterisieren. Eines dieser Projekte is der WFCam Transit Survey (WTS), ein Pionierprogramm, das sich durch eine besondere Zielsetzung und Methodik auszeichnet. Die Beobachtungen für WTS haben im August 2007 am United Kingdom Infrared Telescope in Hawaii begonnen. Der Survey unternimmt die erste Suche nach Exoplaneten im nah-infraroten Wellenlängenbereich, welcher für die Suche nach Planeten um M-Zwerge optimal ist. Ursprünglich waren für das Projekt etwa 200 Nächte geplant - verteilt auf insgesamt vier Felder, welche über das Jahr verteilt beobachtet wurden. Die gewonnen Daten werden in einer automatischen Pipeline prozessiert, um Lichtkurven mit Aperturphotometrie zu erstellen. Für das Feld mit den meisten Beobachtungen (``19h-Feld'' mit 1145 Belichtungen)erzeugen wir ein alternatives Set an Lichtkurven mit ``difference imaging'', einer photometrische Methode, die sich in der Vergangenheit für Felder mit hohen Sterndichten als überlegen herausgestellt hat. Ein quantitativer Vergleich zwischen der photometrischen Genauigkeit der beiden Methoden wurde in dieser Arbeit durchgeführt. Wir korrigieren ausserdem systematische Effekte unter Verwendung des ``sysrem'' Algorithmus, skalieren die unrealistischen Fehlerbalken in den Lichtkurven und vergleichen diese mit den ursprünglichen. Die Ergebnisse zeigen, dass die Lichtkurven der Aperturphotometrie geringfügig höhere Genauigkeit für Objekte mit J < 16 mag aufweisen. Difference imaging Lichtkurven zeigen dagegen eine deutliche Verbesserung für schwächere Sterne. Um Transits von Planeten zu detektieren, verwenden wir eine modifizierte Version des ``box-fitting'' Algorithmus. Unsere Implementierung erweitert den Algorithmus um einem Trapezoid-Fit des Transitbereichs in der gefalteten Lichtkurve. Wie wir zeigen, liefert dieser Fit bessere Resultate als der reine Box-Fit. Wir beschreiben ein Set von Selektionskriterien, mit welchem wir nach Transit-Kandidaten in den Lichtkurven suchen. Diese Kriterien beinhalten einen Parameter, der von unserem Detektionsalgorithmus berechnet wird: der V -Form Parameter. Dieser hat sich als sehr nützlich herausgestellt, um automatisch bedeckungsveränderliche Doppelsterne zu identifizieren und diese von der Suche nach Planeten auszuschliessen. Wir optimieren die Kriterien über Monte-Carlo Simulationen von künstlichen Transitsignalen, welche in die realen WTS Lichtkurven eingespeist und durch unseren Detektionsalgorithmus analysiert werden. Wir führen die Optimierung der Selektionskriterien separat für zwei getrennte Sets von Lichtkurven durch, jeweils eines für F-G-K Sterne und für M-Sterne. Um nach Transits von Planeten zu suchen, werden die optimierten Kriterien auf die Lichtkurven von Aperturphotometrie und difference imaging Photometrie angewendet. Auf diese Weise werden die 200 besten Transitkandidaten aus ~475 000 Quellen automatisch selektiert. Danach wird eine visuelle Untersuchung der gefalteten Lichtkurven vorgenommen, um klare Fehldetektionen zu entfernen. Anschließend werden weitere Analyseschritte für die vielversprechendsten 18 Kandidaten durchgeführt, was es uns ermöglicht, diese Objekte als Planeten-Kandidaten oder bedeckungsveränderliche Doppelsterne zu klassifizieren. Wir haben einen vielversprechenden Planetenkandidaten gefunden, der um einen späten G-Stern kreist, für welchen wir eine photometrische Nachbeobachtung vorschlagen. Die davon unabhängige Analyse der M-Stern Lichtkurven führte zu keiner Detektion. Damit können wir die Nullhypothese und die oberen Limits für die Häufigkeit von Gasriesen um M-Zwerge mit J < 17 mag aus einer früheren Studie bestätigen. In dieser Arbeit erweitern wir die Suche nach Planetentransits auf Sterne mit J < 18 mag, was es uns ermöglicht, ein Limit von 1.1 % für die Häufigkeit von kurzperiodischen Gasreisen um M-Zwerge zu setzen, was deutlich niedriger ist als die Limits anderer Publikationen. Der Mangel an Hot Jupiters um M-Zwerge spielt eine wichtige Rolle für die Theorien von Planetenentstehung von Exoplaneten um Sterne mit niedriger Masse. Das Fehlen von Gasriesen in kurzperiodischen Orbits um M-Sterne zeigt, dass der Disk-Instabilitätsmechanismus in Kombination mit einer Migration des Orbits nicht benötigt wird,um die Planetenbildung zu beschreiben. Die deutlich geringere Effizienz des ``Core Accretion'' Modells, Jupiter um kühle Sterne zu erzeugen, wird durch die Nullhypothese bestätigt. Unser oberes Limit ist jedoch immer noch höher als die Detektionsraten von kurzperiodigen Gasriesen um heißere Sterne. Wir können also keinen abschließendes Fazit über die Entstehungsmodelle von jupiterähnlichen Planeten um kühle, massearme Hauptreihensterne ziehen. Momentan gibt es noch nicht ausreichende Beobachtungen, um die Vorhersage, dass Hot Jupiters weniger häufig um M-Sterne als um sonnenähnliche Sterne sind, zu untermauern. Um dies zu erreichen, muss eine noch grössere Anzahl an M-Sternen beobachtet werden. Dies könnte durch eine umfassende Analyse aller vier WTS Felder erreicht werden oder aber durch andere momentan durchgeführte M-Zwerg Transit Surveys wie etwa Pan-Planets oder PTF/M-Dwarfs, welche nahezu 100 000 M-Sterne beobachten. Aktuelle und zukünftige Weltraummissionen wie Kepler und GAIA könnten ebenfalls weitere Erkenntnise über die Häufigkeit von Hot Jupiters um massearme Sterne liefern. Im letzten Teil dieser Arbeit präsentieren wir weitere Anwendungen der difference imaging Lichtkurven. Wir haben insgesamt fünf lichtschwache, bedeckungsveränderliche Doppelsternsysteme mit extrem kurzen Perioden unterhalb von 0.23 Tagen detektiert. Ausserdem konnten wir zwei weitere unbestätigte und ein bestätigtes bedeckungsveränderliches M-Zwerg/M-Zwerg Doppelstern-System finden. Alle in dieser Arbeit präsentierten Detektionen demonstrieren die Stärken der difference imaging Methode, welche vor allem bei lichtschwächeren Magnituden entscheidende Vorteile bringt.

No description available.

Two analyses searching for squarks and gluinos which decay into final states with multiple jets, an isolated electron or muon and a large missing transverse energy are presented. Both rely on data taken by the ATLAS detector in pp collisions at a center-of-mass energy of 8 TeV at the LHC during 2012. The first analysis uses a subset of 5.8 fb-1 of this dataset, the other analysis uses the full statistics of 20.3 fb-1. Both analysis share the same methods regarding the triggers and the background estimation techniques. The two dominant backgrounds are ttbar and W+jets production. The ttbar and the W+jets backgrounds are estimated in a semi-data-driven method. The minor QCD multi-jet background is estimated in an entirely data-driven method. The final background estimates in the analyses are derived in a profile-log-likelihood fit. None of the analyses sees an excess beyond Standard Model expectations. The analysis of the partial dataset derives limits in a MSUGRA/CMSSM model with parameters A_0=0, tan(beta) = 10 and mu > 0 and excludes squarks and gluinos with masses below 1.2 TeV for equal squark and gluino masses. The analysis of the full dataset derives limits in simplified models and in a MSUGRA/CMSSM model with parameters A_0=-2 m_0, tan(beta) = 30 and mu > 0. Gluinos (squarks) with masses below 1.2 TeV (750 GeV) can be excluded for vanishing LSP masses in simplified models. Gluino masses below 1.2 TeV can be excluded for every m_0 value in the MSUGRA/CMSSM model.

Tropospheric water vapour plays an important role in thermodynamic and radiative processes which have an immediate impact on the weather and climate system. However, the processes that determine the distribution of water vapour remain poorly understood. The complexity arises out of a range of source and sink processes from convective clouds on the kilometre scale to cloud systems associated with motions on scales of a thousand or more kilometres, as well as advection of water vapour as a passive tracer outside of clouds. While large-scale advection of water vapour is well represented in general circulation models, the simulation of small-scale moist processes that are of central importance to the representation of clouds are heavily dependent on parameterisations. However, observations as well as processes that determine the distribution of the water vapour field are insufficiently explored, leading to constrained parameterisations and therefore contributing significantly to the uncertainty of numerical weather and climate predictions. Hence, a more accurate description of the inhomogeneous water vapour field based on high-resolution observations is required. This thesis investigates a comprehensive data set of two-dimensional airborne water vapour observations in the free troposphere collected by a Differential Absorption Lidar (DIAL) in order to gain a height-resolved statistical characterisation of the inhomogeneous water vapour field. Structure functions, i.e., statistical moments up to the fifth order of absolute increments over a range of scales, are investigated and power-law behaviour or scale dependence is identified over horizontal distances from about 5~km to 100~km. The slope of the power-law fit, the so-called scaling exponent, is found to take different values, depending on whether or not the observations were taken in an air mass where convective clouds were present. These results are consistent with a non-convective regime that is dominated by large-scale advective processes, leading to monofractal scaling, but strong localised input of small-scale variability by convective circulations leading to intermittent fields. Further, the observed power-law statistics are used to evaluate the high-resolution numerical weather prediction model COSMO-DE of the German weather service with regard to the small-scale water vapour variability. The results of the scaling exponent analysis of cloud-free and partly cloudy scenes suggest, that the small-scale variance is modeled quite well in comparison with the lidar observations. By using the advantage of the model simulation where data is not limited to a specific flight path, the influence of sampling limitation is estimated and is found to be not significant. Further, the simulation provides humidity data in and beneath clouds which allows for an estimation of the uncertainty of data gaps in the lidar observations due to optically thick clouds. The error is identified to be in a range of only few percents. This thesis demonstrates that airborne DIAL observations are useful to build up a height-resolved statistical characterisation of tropospheric water vapour variability that allows to distinguish physical mechansims that are responsible for the water vapour distribution, to get new insights into stochastic parameterisations and further to use the structure function method as a suitable reality check of the numerical weather model COSMO-DE.

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