ar X iv : a st ro - p h / 99 03 05 1 v 1 2 M ar 1 99 9 Where is the COBE maps ’ non - Gaussianity ?

We review our recent claim that there is evidence of non-Gaussianity in the 4 Year COBE DMR data. We present some new results concerning the effect of the galactic cut upon the non-Gaussian signal. These findings imply a localization of the non-Gaussian signal on the Northern galactic hemisphere. I EVIDENCE FOR CMB NON-GAUSSIANITY In a recent paper [1] we showed that the 4 Year COBE DMR data exhibits evidence of non Gaussianity at a high confidence level. We made use of statistical tools described in more detail in [2,3]. Since then our result has been corroborated by two other groups [4,5]. In this review we revisit our analysis, and the tests to which we have subjected it, including some new results. In our analysis we propose, and work with, an estimator for the normalized bispectrum, denoted by I l . We refer the reader to [1] for its definition. We then applied this estimator to the inverse noise variance weighted, average maps of the 53A, 53B, 90A and 90B COBE-DMR channels, with monopole and dipole removed, at resolution 6, in ecliptic pixelization. We use the extended galactic cut of [6], and [7] to remove most of the emission from the plane of the Galaxy. We apply our statistics to the DMR maps before and after correction for the plausible diffuse foreground emission outside the galactic plane as described in [8], and [9]. By means of Monte Carlo simulations we also found the distributions Pl(I 3 l ) for what we should have seen assuming a Gaussian signal, which is then processed by the experimental set up associated with DMR. These P (I l ) were inferred from 25000 realizations (see Fig. 1). The observed I l and the distributions Pl(I 3 l ) are plotted in Fig. 1. One immediately notices the presence of a significant deviant. In order to quantify this deviant we define the goodness of fit statistic FIGURE 1. The vertical thick dashed line represents the value of the observed I l . The solid line is the probability distribution function of I l for a Gaussian sky with extended galactic cut and DMR noise.