Non-Gaussian signatures in the five-year WMAP data as identified with isotropic scaling indices

We continue the analysis of non-Gaussianities in the cosmic microwave background by means of the scaling index method by applying this method on the single Q, V, W bands and the co-added VW band of the five-year data of the Wilkinson Microwave Anisotropy Probe. We compare each of the results with 1000 Monte Carlo simulations mimicking the Gaussian properties of the best fitting Lambda cold dark matter model. Based on the scaling indices, scale-dependent empirical probability distributions, moments of these distributions and chi(2) combinations of them are calculated, obtaining similar results as in the former analysis of the three-year data: we derive evidence for non-Gaussianity with a probability of up to 97.3 per cent for the mean when regarding the KQ75-masked full sky and summing up over all considered length-scales by means of a diagonal chi(2) statistics. Looking at only the northern or southern hemisphere of the galactic coordinate system, we obtain up to 98.5 or 96.6 per cent, respectively. For the standard deviation, these results appear as 95.6 per cent for the full sky (99.7 per cent north, 89.4 per cent south) and for a chi(2) combination of both measurements as 97.4 per cent (99.1 per cent north, 95.5 per cent south). We obtain larger deviations from Gaussianity when looking at separate scale lengths. By performing an analysis of rotated hemispheres, we detect an obvious asymmetry in the data. In addition to these investigations, we present a method of filling the mask with Gaussian noise to eliminate boundary effects caused by the mask. With the help of this technique, we identify several local features on the map, of which the most significant one turns out to be the well-known cold spot. When excluding all these spots from the analysis, the deviation from Gaussianity increases, which shows that the discovered local anomalies are not the reason of the global detection of non-Gaussianity, but actually were damping the deviations on average. Our analyses per band and per year suggest, however, that it is very unlikely that the detected anomalies are due to foreground effects.