# The Atacama Cosmology Telescope: CMB Polarization at $200<\ell<9000$

**Authors:**Sigurd Naess, Matthew Hasselfield, Jeff McMahon, Michael D. Niemack, Graeme E. Addison, Peter A. R. Ade, Rupert Allison, Mandana Amiri, Nick Battaglia, James A. Beall, Francesco de Bernardis, J Richard Bond, Joe Britton, Erminia Calabrese, Hsiao-mei Cho, Kevin Coughlin, Devin Crichton, Sudeep Das, Rahul Datta, Mark J. Devlin, Simon R. Dicker, Joanna Dunkley, Rolando Dünner, Joseph W. Fowler, Anna E. Fox, Patricio Gallardo, Emily Grace, Megan Gralla, Amir Hajian, Mark Halpern, Shawn Henderson, J. Colin Hill, Gene C. Hilton, Matt Hilton, Adam D. Hincks, Renée Hlozek, Patty Ho, Johannes Hubmayr, Kevin M. Huffenberger, John P. Hughes, Leopoldo Infante, Kent Irwin, Rebecca Jackson, Simon Muya Kasanda, Jeff Klein, Brian Koopman, Arthur Kosowsky, Dale Li, Thibaut Louis, Marius Lungu, Mathew Madhavacheril, Tobias A. Marriage, Loïc Maurin, Felipe Menanteau, Kavilan Moodley, Charles Munson, Laura Newburgh, John Nibarger, Michael R. Nolta, Lyman A. Page, Christine Pappas, Bruce Partridge, Felipe Rojas, Benjamin Schmitt, Neelima Sehgal, Blake D. Sherwin, Jon Sievers, Sara Simon, David N. Spergel, Suzanne T. Staggs, Eric R. Switzer, Robert Thornton, Hy Trac, Carole Tucker, Masao Uehara, Alexander Van Engelen, Jon Ward, Edward J. Wollack

**Category:**Cosmology and Nongalactic Astrophysics

We report on measurements of the cosmic microwave background (CMB) and celestial polarization at 146 GHz made with the Atacama Cosmology Telescope Polarimeter (ACTPol) in its first three months of observing. Four regions of sky covering a total of 270 square degrees were mapped with an angular resolution of $1.3'$. The map noise levels in the four regions are between 11 and 17 $\mu$K-arcmin. We present TT, TE, EE, TB, EB, and BB power spectra from three of these regions. The observed E-mode polarization power spectrum, displaying six acoustic peaks in the range $200<\ell<3000$, is an excellent fit to the prediction of the best-fit cosmological models from WMAP9+ACT and Planck data. The polarization power spectrum, which mainly reflects primordial plasma velocity perturbations, provides an independent determination of cosmological parameters consistent with those based on the temperature power spectrum, which results mostly from primordial density perturbations. We find that without masking any point sources in the EE data at $\ell<9000$, the Poisson tail of the EE power spectrum due to polarized point sources has an amplitude less than $2.4$ $\mu$K$^2$ at $\ell = 3000$ at 95\% confidence. Finally, we report that the Crab Nebula, an important polarization calibration source at microwave frequencies, has 8.7\% polarization with an angle of $150.7^\circ \pm 0.6^\circ$ when smoothed with a $5'$ Gaussian beam.

**Comments:**16 pages, 15 figures, 5 tables

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