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A non-Gaussian detection in the WMAP 1-year data is reported. The detectionhas been found in the combined Q-V-W map proposed by the WMAP team (Komatsu etal. 2003) after applying a wavelet technique based on the Spherical Mexican HatWavelet (SMHW). The skewness and the kurtosis of the SMHW coefficients arecalculated at different scales. A non-Gaussian signal is detected at scales ofthe SMHW around 4 deg (size in the sky of around 10 deg). The right tailprobability of the detection is approx. 0.4%. In addition, a study ofGaussianity is performed in each hemisphere. The northern hemisphere iscompatible with Gaussianity, whereas the southern one deviates from Gaussianitywith a right tail probability of approx. 0.1%. Systematics, foregrounds anduncertainties in the estimation of the cosmological parameters are carefullystudied in order to identify the possible source of non-Gaussianity. Thedetected deviation from Gaussianity is not found to be caused by systematiceffects: 1) each one of the Q, V and W receivers shows the same non-Gaussianitypattern, and 2) several combinations of the different receivers at eachfrequency band do not show this non-Gaussian pattern. Similarly, galacticforegrounds show a negligible contribution to the non-Gaussian detection:non-Gaussianity is detected in all the WMAP maps and no frequency dependence isobserved. Moreover, the expected foreground contribution to the combined WMAPmap was added to CMB Gaussian simulations showing a behaviour compatible withthe Gaussian model. Influence of uncertainties in the CMB power spectrumestimation are also quantified. Hence, possible intrinsic temperaturefluctuations (like secondary anisotropies and primordial features) can not berejected as the source of this non-Gaussian detection.Comment: 33 pages, 14 figures. Revised to match version accepted for  publication in Ap

Detection of Non‐Gaussianity in theWilkinson Microwave Anisotropy ProbeFirst‐Year Data Using Spherical Wavelets