Studying large-scale structures and polarization of the Northern sky facilitating single-station data of the Low Frequency Array (LOFAR)

Abstract

All-sky surveys help to gather a deep knowledge about the nature of large-scale structures in the Universe, the Milky Way and extragalactic galaxies. In the context of that, observations in the radio domain are important, as they provide information which cannot be archived from any other observing frequency. This thesis will present a new northern sky survey at radio frequencies below 200 MHz, made with one of the new generation of radio telescopes the Low Frequency Array (LOFAR; van Haarlem et al., 2013). Using data of a single station of this array a novel approach has developed to produce a northern sky survey. The necessary steps are described on how to generate a scientific usable sky map from the raw voltages. The scientific analysis of these maps will be discussed and used for further studies. <br /> An overview of the general properties of the LOFAR array will be given in Chapter 2. In this array a single station will have a physical diameter of around 60 m and therefore it allows to study large-scale emission in the northern sky at 30 to 240 MHz. Even more, with its wide frequency range and the ability to observe in polarization, LOFAR allows polarization studies at frequencies below 240 MHz. In this way it is an important instrument to study the Milky Way at low frequencies. In case of Galactic and extra-galactic emission this was never possible before. Unfortunately with existing LOFAR software it was not possible to perform all-sky imaging with a frequency resolution better than 200 kHz. Due to physical limitation, in particular the strong Faraday Rotation at low frequencies, this spectral resolution is not sufficient enough. To solve this problem, this thesis will present a way how to use raw voltages from the Transient Buffer Boards (TBBs) for all-sky imaging at sufficiently high frequency resolution. In particular for all necessary steps new software tools and algorithm had to be developed. The correlation of the data will be described in Chapter 3, as well as all the technical challenges of the further data reduction. This includes the data inspection, the calibration and dealing with the antenna pattern and ionospheric effects. Especially the self-calibration will become an important method, to calibrate strong unknown structures and individual sources, to improve the image quality. Some of the strong unknown source can be an active Sun, a bursting Jupiter or even just strong men made radio frequency interference (RFI). As all observations presented in this thesis were done with the LOFAR station in Effelsberg dealing with the strong RFI is one of the main challenge in the calibration process. <br /> The developed software is afterwards used to produce single all-sky images of a LOFAR station. In Chapter 4 the procedure is explained how a high number of single all-sky observations were combined to map out the entire northern sky down to a declination of 0 degrees. The detected large-scale structures in this new map will be studied and compared to earliersurveys. In Chapter 5 a spectral analysis using the LOFAR survey at various frequencies is discussed. As part of this analysis the spectral index maps between frequencies below 50 MHz and the high frequency surveys at 408 MHz (Haslam et al., 1982), 820 MHz (Berkhuijsen, 1972) and 1400 MHz (Reich et al., 1981) will be investigated. This allows to study physical nature of the continuum emission and a possible flattening of the spectrum toward lower frequency due to thermal absorption. Furthermore the spectral index data will be used to estimate turn-overs at low frequencies and high frequency breaks and indicate a possible transition from thermal to non-thermal emission. In Chapter 6 a short analysis of the spectral properties of the strong radio source Cas A is investigated. In general measurements of this source during the last years show that the flux density is decreasing. The LOFAR measurements do confirm the predicted intensities of the Cas A model and also indicates similar flux densities for the strong radio source Cyg A. <br /> In order to determine the influence of the ionosphere on the polarization measurements pulsar observation are used. Chapter 7 will discuss the variations of the Rotation Measure (RM) values of the pulsars with time due to ionospheric fluctuations. Taking all corrections into account the polarization properties of Jupiter bursts can now be studied. The measurements show that the burst are highly linear- and circular-polarized and that the RM values per burst are frequency depended (Chapter 8). In Chapter 9 the polarization properties of the large-scale emission from the Milky Way will be discussed. For this linear polarization intensity maps are created and RM Synthesis will be used to determine the generally weak polarization emission.