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(Abridged)Motivated by the recent results of Hansen et al. (2008) concerninga noticeable hemispherical power asymmetry in the WMAP data on small angularscales, we revisit the dipole modulated signal model introduced by Gordon etal. (2005). This model assumes that the true CMB signal consists of a Gaussianisotropic random field modulated by a dipole, and is characterized by anoverall modulation amplitude, A, and a preferred direction, p. Previousanalyses of this model has been restricted to very low resolution due tocomputational cost. In this paper, we double the angular resolution, andcompute the full corresponding posterior distribution for the 5-year WMAP data.The results from our analysis are the following: The best-fit modulationamplitude for l <= 64 and the ILC data with the WMAP KQ85 sky cut is A=0.072+/- 0.022, non-zero at 3.3sigma, and the preferred direction points towardGalactic coordinates (l,b) = (224 degree, -22 degree) +/- 24 degree. Thecorresponding results for l <~ 40 from earlier analyses was A = 0.11 +/- 0.04and (l,b) = (225 degree,-27 degree). The statistical significance of a non-zeroamplitude thus increases from 2.8sigma to 3.3sigma when increasing l_max from40 to 64, and all results are consistent to within 1sigma. Similarly, theBayesian log-evidence difference with respect to the isotropic model increasesfrom Delta ln E = 1.8 to Delta ln E = 2.6, ranking as "strong evidence" on theJeffreys' scale. The raw best-fit log-likelihood difference increases fromDelta ln L = 6.1 to Delta ln L = 7.3. Similar, and often slightly stronger,results are found for other data combinations. Thus, we find that the evidencefor a dipole power distribution in the WMAP data increases with l in the 5-yearWMAP data set, in agreement with the reports of Hansen et al. (2008).

Increasing evidence for hemispherical power asymmetry in the five-year WMAP data