Leakage of power from dipole to higher multipoles due to non-symmetric WMAP beam

Abstract: A number of studies of WMAP and Planck have highlighted that the power at the low multipoles in CMB power spectrum are lower than their theoretically predicted values. Possible angular correlation between the orientations of these low multipoles have also been claimed. It is important to investigate the possibility that the power deficiency at low multipoles may not be of primordial origin and is only an observation artifact coming from the scan procedure adapted in the WMAP or Planck satellites. Therefore, its always important to investigate all the observational artifacts that can mimic them. The CMB dipole which is almost 550 times higher than the quadrupole can leak to the higher multipoles due to the non-symmetric beam shape of the WMAP. In this paper a formalism has been developed and simulations are carried out to study the effect of the non-symmetric beam on this power transfer. It is interesting to observed that a small but non-negligible amount of power from the dipole can get transferred to the quadrupole and the higher multipoles due to the non-symmetric beam. It is shown that in case of WMAP scan strategy the shape of the quadrupole coming due to this power leakage is very much similar to the observed quadrupole from WMAP data. Simulations have also been carried out for Planck scan strategy. It is seen that for Planck scan strategy the power transfer is not only limited to the quadrupole but also to a few higher low multipoles. Since the actual beam shapes of Planck are not publicly available we present results in terms of upper limits on asymmetric beam parameters that would contaminate of the quadrupole power at the level of $1\mu K$.