Revealing Non-circular beam effect in WMAP-7 CMB maps with BipoSH measures of Statistical Isotropy

Understand

Mild, unavoidable deviations from circular-symmetry of instrumental beams in current Cosmic Microwave Background (CMB) experiments now pose a significant challenge to deriving high precision inferences from the high sensitivity and resolution of CMB measurements.

• We present analytic results, verified by numerical simulations, that CMB maps of cosmological signal that respect underlying statistical isotropy (SI) symmetry, measured with an instrument that has mildly non-circular (NC) beam would, nevertheless, exhibit SI violation.
• Further, we show that appropriate observable measures constructed within the Bipolar spherical harmonic (BipoSH) representation of SI violation capture subtle NC-beam effects coupled with the scan strategy of the instrument.
• Accompanying their latest 7-year data release, the WMAP team published very high significance measurements of non-zero BipoSH spectra, A^{20}_{l l} and A^{20}_{l-2 l}, in the "W" and "V" band of the experiment.