On the non-Gaussianity observed in the COBE differential microwave radiometer sky maps

Tn this paper we pursue the origin of the non-Gaussianity determined by a b ispectrum analysis of the COBE Differential Microwave Radiometer (DMR) 4 yr sky maps. The robustness of the statistic is demonstrated by the rebinning of the data into 12 coordinate systems. By computing the bispectrum statis tic as a function of various data partitions (by channel, frequency, and ti me interval), we show that the observed non-Gaussian signal is driven by th e 53 GHz data. This frequency dependence strongly rejects the hypothesis th at the signal is cosmological in origin. A jackknife analysis of the coadde d 53 and 90 GHz sky maps reveals those sky pixels to which the bispectrum s tatistic is particularly sensitive. We find that by removing data from the 53 GHz sky maps for periods of time during which a known systematic effect perturbs the 31 GHz channels, the amplitudes of the bispectrum coefficients become completely consistent with that expected for a Gaussian sky. We con clude that the non-Gaussian signal detected by the normalized bispectrum st atistic in the publicly available DMR sky maps is due to a systematic artif act. The impact of removing the affected data on estimates of the normaliza tion of simple models of cosmological anisotropy is negligible.