Searching for Faraday rotation in cosmic microwave background polarization

We use the Wilkinson Microwave Anisotropy Probe (WMAP) 9th-year foreground reduced data at 33, 41 and 61 GHz to derive a Faraday rotation at map and at angular power spectrum levels taking into account their observational errors. A processing mask provided by WMAP is used to avoid contamination from the disc of our Galaxy and local spurs. We have found a Faraday rotation component at both, map and power spectrum levels. The lack of correlation of the Faraday rotation with Galactic Faraday rotation, synchrotron and dust polarization from our Galaxy or with cosmic microwave background anisotropies or lensing suggests that it could be originated at reionization (ℓ ≲ 12). Even if the detected Faraday rotation signal is weak, the present study could contribute to establish magnetic fields strengths of B ~ 10 G at reionization.This work was partially supported by projects CSD2010-00064 and AYA2011-24728 of the Spanish Ministry of Economy and Competitiveness (MINECO) and by Junta de Andalucía Grant FQM-108.Peer Reviewe