Parity Asymmetry in the CMBR Temperature Power Spectrum

We study the power asymmetry between even and odd multipoles in the multipolar expansion of CMB temperature data from WMAP, recently reported in the literature. We introduce an alternate statistic which probes this effect more sensitively. We find that the data is highly anomalous and consistently outside $2\sigma$ significance level in the whole multipole range $l=[2,101]$. We examine the possibility that this asymmetry may be caused by the foreground cleaning procedure or by residual foregrounds. By direct simulations, using the Planck Sky Model for foregrounds we rule out this possibility. We also examine several possible sub-dominant foregrounds, which might lead to such an asymmetry. However in all cases we are unable to explain the signal seen in data. We next examine cleaned maps, using procedures other than the one followed by the WMAP Science team. Specifically we analysed the maps cleaned by the IPSE procedure, Needlets and the harmonic ILC procedure. In all these cases we do not find a statistically significant signal of power asymmetry. This is in contrast to the result obtained by the WMAP best fit power spectrum as well as the ILC map. Finally, we test for the contribution of low-$l$ multipoles to the observed power asymmetry. We find that if we eliminate the first six multipoles, $l=[2,7]$, the significance falls below $2\sigma$ CL. Hence we find that the signal gets dominant contribution from low-$l$ modes.
Comment: 19 pages, 15 figures, two new figures added with some updated text; Accepted for publication in MNRAS; References updated in accordance with the journal version