Characterizing the accuracy of ALMA linear-polarization mosaics

International audienceWe characterize the accuracy of linear-polarization mosaics made using the Atacama Large Millimeter/submillimeter Array (ALMA). First, we observed the bright, highly linearly polarized blazar 3C 279 at Bands 3, 5, 6, and 7 (3 mm, 1.6 mm, 1.3 mm, and 0.87 mm, respectively). At each band, we measured the blazar’s polarization on an 11 × 11 grid of evenly spaced offset pointings covering the full-width at half-maximum (FWHM) area of the primary beam. After applying calibration solutions derived from the on-axis pointing of 3C 279 to all of the on- and off-axis data, we find that the residual polarization errors across the primary beam are similar at all frequencies: the residual errors in linear polarization fraction P frac and polarization position angle χ are ≲0.001 (≲0.1% of Stokes I) and ≲ 1° near the center of the primary beam; the errors increase to ∼0.003–0.005 (∼0.3%–0.5% of Stokes I) and ∼1°–5° near the FWHM as a result of the asymmetric beam patterns in the (linearly polarized) Q and U maps. We see the expected double-lobed “beam squint” pattern in the circular polarization (Stokes V) maps. Second, to test the polarization accuracy in a typical ALMA project, we performed observations of continuum linear polarization toward the Kleinmann–Low nebula in Orion (Orion-KL) using several mosaic patterns at Bands 3 and 6. We show that after mosaicking, the residual off-axis errors decrease as a result of overlapping multiple pointings. Finally, we compare the ALMA mosaics with an archival 1.3 mm Combined Array for Research in Millimeter-wave Astronomy polarization mosaic of Orion-KL and find good consistency in the polarization patterns