Pacman ( II ) : Application and Testing

We proposed a new method to calculate Faraday rotation measure (RM) maps from multifrequency polarisation angle data (Dolag et al.) in order to avoid the so-called nπ-ambiguity. Here we apply our Pacman algorithm to two polarisation data sets of extended radio sources, Abell 2255 and Hydra A, and compare the RM maps obtained using Pacman to RM maps obtained employing already existing methods. We demonstrate the reliability and the robustness of Pacman. In order to study the influence of map making artefacts, which are imprinted by wrong solutions to the nπ-ambiguities, and of the error treatment of the data, we calculated and compared magnetic field power spectra from various RM maps. The power spectra were derived using the method recently proposed by Enßlin & Vogt (2003). We demonstrate the sensitivity of statistical analysis to artefacts and noise in the RM maps and thus, we demonstrate the importance of an unambiguous determination of RM maps and an understanding of the nature of the noise in the data. Additionally, we introduce and perform statistical tests to estimate the quality of the derived RM maps, which demonstrate the quality improvements due to Pacman .