Characterizing the accuracy of ALMA linear-polarization mosaics

TL;DR

This study assesses the accuracy of ALMA linear-polarization mosaics across multiple bands, demonstrating residual errors are minimal near the center and increase towards the edges, with improved accuracy through mosaicking and validation against archival data.

Contribution

The paper provides a detailed characterization of polarization accuracy in ALMA mosaics, including residual error quantification and validation with other observations, which was previously lacking.

Findings

Residual polarization errors are less than 0.1% near the center of the primary beam.

Errors increase to about 0.3-0.5% and 1-5 degrees near the FWHM due to beam asymmetries.

Mosaicking reduces off-axis errors and improves polarization accuracy.

Abstract

We 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$\times$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 $P_\textrm{frac}$ and polarization position angle $\chi$ are $\lesssim$0.001 ($\lesssim$0.1% of Stokes $I$) and $\lesssim$1$^\circ$ near the center of the primary beam; the errors increase to $\sim$0.003-0.005 ($\sim$0.3-0.5% of Stokes $I$) and $\sim$1-5$^\circ$ 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 CARMA polarization mosaic of Orion-KL and find good consistency in the polarization patterns.