Non-cosmological Signal Contributions to the COBE-DMR Four-Year Sky Maps
  We limit the possible contributions from non-cosmological sources to the COBE-DMR four-year sky maps. The DMR data are cross-correlated with maps of rich clusters, extragalactic IRAS sources, HEAO-1 A-2 X-ray emission and 5 GHz radio sources using a Fourier space technique. There is no evidence of significant contamination by such sources at an rms level of ~8 uK (95% confidence level at 7 degree resolution) in the most sensitive 53 GHz sky map. This level is consistent with previous limits set by analysis of earlier DMR data and by simple extrapolations from existing source models. We place a 95% confidence level rms limit on the Comptonization parameter averaged over the high-latitude sky of $\delta y\ <\ 1\ \times\ 10^{-6}$. Extragalactic sources have an insignificant effect on CMB power spectrum parameterizations determined from the DMR data. 
A.J. Banday, K.M. Gorski, C.L. Bennett, G. Hinshaw, A. Kogut, and G.F.   Smoot