Analysis of the Data from Compton X-ray Polarimeters which Measure the Azimuthal and Polar Scattering Angles

X-ray polarimetry has the potential to make key-contributions to our understanding of galactic compact objects like binary black hole systems and neutron stars, and extragalactic objects like active galactic nuclei, blazars, and Gamma Ray Bursts. Furthermore, several particle astrophysics topics can be addressed including uniquely sensitive tests of Lorentz invariance. In the energy range from 10 keV to several MeV, Compton polarimeters achieve the best performance. In this paper we evaluate the benefit that comes from using the azimuthal and polar angles of the Compton scattered photons in the analysis, rather than using the azimuthal scattering angles alone. We study the case of an ideal Compton polarimeter and show that a Maximum Likelihood analysis which uses the two scattering angles lowers the Minimum Detectable Polarization (MDP) by ~20% compared to a standard analysis based on the azimuthal scattering angles alone. The accuracies with which the polarization fraction and the polarization direction can be measured improve by a similar amount. We conclude by discussing potential applications of Maximum Likelihood analysis methods for various polarimeter experiments.Comment: Accepted for publication in Astroparticle Physics (14 pages, 4 figures

Biodiversitätsmanagement mit Wasserbüffeln in renaturierten Feuchtgebieten

The use of water buffalos for landscape maintenance started ten years ago in Germany. Now, more than 2,100 buffalos are kept by about 90 breeders, and first results concerning their usefulness for landscape management are available. Buffalos are mainly used on particularly wet sites which cannot be grazed by cattle or other domestic animals. Although grazing of wetlands, river banks and water bodies is still controversial, early results from literature and our own research clearly indicate the beneficial impact of moderate grazing on such sites for birds, amphibians, vegetation and insects. This paper presents a short literature review and the first results of the BUBALUS project at Brandenburg University of Technology (BTU) and general experience from other projects.Seit etwa zehn Jahren gibt es in Deutschland den verstärkten Trend, Wasserbüffel zu halten. Inzwischen gibt es mehr als 2.100 Wasserbüffel bei mehr als 90 Haltern. Erste Ergebnisse ihrer Eignung als Landschaftspfleger in Feuchtgebieten sind nun verfügbar. Die Büffel werden überwiegend auf besonders nassen Standorten eingesetzt, die für die Haltung von Rindern oder anderen Haustieren nicht geeignet sind. Obwohl die Beweidung von Nassstandorten, Ufern und Gewässern noch immer umstritten ist, zeigen die verfügbaren Ergebnisse den Nutzen für Vögel, Amphibien, Insekten und Vegetation. Dieser Artikel gibt eine kurze Übersicht zu relevanter Literatur sowie erste Ergebnisse des BUBALUS Projektes der Brandenburgischen Technischen Universität Cottbus (BTU) sowie generelle Erfahrungen aus anderen Projekten

Analyzing the Data from X-ray Polarimeters with Stokes Parameters

X-ray polarimetry promises to deliver unique information about the geometry of the inner accretion flow of astrophysical black holes and the nature of matter and electromagnetism in and around neutron stars. In this paper, we discuss the possibility to use Stokes parameters - a commonly used tool in radio, infrared, and optical polarimetry - to analyze the data from X-ray polarimeters such as scattering polarimeters and photoelectric effect polarimeters, which measure the linear polarization of the detected X-rays. Based on the azimuthal scattering angle (in the case of a scattering polarimeter) or the azimuthal component of the angle of the electron ejection (in the case of a photoelectric effect polarimeter), the Stokes parameters can be calculated for each event recorded in the detector. Owing to the additive nature of Stokes parameters, the analysis reduces to adding the Stokes parameters of the individual events and subtracting the Stokes parameters characterizing the background (if present). The main strength of this kind of analysis is that the errors on the Stokes parameters can be computed easily and are well behaved - in stark contrast of the errors on the polarization fraction and polarization direction. We demonstrate the power of the Stokes analysis by deriving several useful formulae, e.g. the expected error on the polarization fraction and polarization direction for a detection of $N_S$ signal and $N_{BG}$ background events, the optimal observation times of the signal and background regions in the presence of non-negligible background contamination of the signal, and the minimum detectable polarization (MDP) that can be achieved when following this prescription.Comment: 9 pages, 2 figures, accepted for publication in Astropart. Phy