  A non-Gaussian detection in the WMAP 1-year data is reported. The detection
has been found in the combined Q-V-W map proposed by the WMAP team (Komatsu et
al. 2003) after applying a wavelet technique based on the Spherical Mexican Hat
Wavelet (SMHW). The skewness and the kurtosis of the SMHW coefficients are
calculated at different scales. A non-Gaussian signal is detected at scales of
the SMHW around 4 deg (size in the sky of around 10 deg). The right tail
probability of the detection is approx. 0.4%. In addition, a study of
Gaussianity is performed in each hemisphere. The northern hemisphere is
compatible with Gaussianity, whereas the southern one deviates from Gaussianity
with a right tail probability of approx. 0.1%. Systematics, foregrounds and
uncertainties in the estimation of the cosmological parameters are carefully
studied in order to identify the possible source of non-Gaussianity. The
detected deviation from Gaussianity is not found to be caused by systematic
effects: 1) each one of the Q, V and W receivers shows the same non-Gaussianity
pattern, and 2) several combinations of the different receivers at each
frequency band do not show this non-Gaussian pattern. Similarly, galactic
foregrounds show a negligible contribution to the non-Gaussian detection:
non-Gaussianity is detected in all the WMAP maps and no frequency dependence is
observed. Moreover, the expected foreground contribution to the combined WMAP
map was added to CMB Gaussian simulations showing a behaviour compatible with
the Gaussian model. Influence of uncertainties in the CMB power spectrum
estimation are also quantified. Hence, possible intrinsic temperature
fluctuations (like secondary anisotropies and primordial features) can not be
rejected as the source of this non-Gaussian detection.
