Four-year COBE DMR cosmic microwave background observations: Maps and basic results

In this Letter we present a summary of the spatial properties of the cosmic microwave background radiation based on the full 4 yr of COBE Differential Microwave Radiometer (DMR) observations, with additional details in a set of companion Letters. The anisotropy is consistent with a scale-invariant power-law model and Gaussian statistics. With full use of the multifrequency 4 yr DMR data, including our estimate of the effects of Galactic emission, we find a power-law spectral index of n = 1.2 +/- 0.3 and a quadrupole normalization Q(rms-PS) = 15.31(-2.8)(+3.8) mu K. For n = 1 the best-fit normalization is Q(rms-PS/n=1) = 18 +/- 1.6 mu K. These values are consistent with both our previous 1 yr and 2 yr results. The results include use of the l = 2 quadrupole term; exclusion of this term gives consistent results, but with larger uncertainties. The final DMR 4 yr sky maps, presented in this Letter, portray an accurate overall visual impression of the anisotropy since the signal-to-noise ratio is similar to 2 per 10 degrees sky map patch. The improved signal-to-noise ratio of the 4 yr maps also allows for improvements in Galactic modeling and limits on non-Gaussian statistics.