The effect of asymmetric beams in the Wilkinson Microwave Anisotropy Probe experiment

TL;DR

This paper simulates the WMAP CMB observations considering asymmetric beam shapes to accurately re-estimate beam transfer functions, finding minimal impact on the power spectrum and cosmological parameters.

Contribution

It introduces a simulation-based method to directly measure the effects of asymmetric beams on WMAP data, avoiding artificial symmetrization.

Findings

Beam transfer function differences are less than 1%.

Net effect on the power spectrum is under 0.6%.

Cosmological parameters are negligibly affected.

Abstract

We generate simulations of the CMB temperature field as observed by the WMAP satellite, taking into account the detailed shape of the asymmetric beams and scanning strategy of the experiment, and use these to re-estimate the WMAP beam transfer functions. This method avoids the need of artificially symmetrizing the beams, as done in the baseline WMAP approach, and instead measures the total convolution effect by direct simulation. We find only small differences with respect to the nominal transfer functions, typically less than 1% everywhere, and less than 0.5% at l<400. The net effect on the CMB power spectrum is less than 0.6%. The effect on all considered cosmological parameters are negligible. For instance, we find that the spectral index of scalar perturbations after taking into account the beam asymmetries is n_s = 0.964 +- 0.014, corresponding to a negative shift of -0.1 sigma compared to the previously released WMAP results. Our CMB sky simulations are made publicly available, and can be used for general studies of asymmetric beam effects in the WMAP data.