Characterizing the Accuracy of ALMA Linear-polarization Mosaics

Charles L H Hull,Kouichiro Nakanishi,Eric Villard,Seiji Kameno,Rosita Paladino,Hiroshi Nagai,Valentin J M Le Gouellec,Edward B Fomalont,Crystal L Brogan,Josep M Girart,Paulo C Cortes,George A Moellenbrock

doi:10.1088/1538-3873/ab99cd

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 × 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 Pfrac 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.

Highlights

When an astronomical source is not observed at the pointing center, off-axis errors in linear and circular polarization will affect the resulting observations of polarized emission
Error in Q, U, Pfrac, and χ In Figure 3 (Band 3), and in Appendix A (Bands 5, 6, and 7), we show the errors across the primary beam in Stokes Q, Stokes U, χ, and Pfrac that we derived from the observations toward the highly linearly polarized blazar 3C 279, which had Pfrac ≈ 12% at the time of observation
After analyzing the observations of the highly linearly polarized blazar 3C 279, we find that the systematic errors in the polarization fraction δPfrac and in the position angle δχ are similar for all observing frequencies

Summary

INTRODUCTION

5 AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, F-91191 Gif-sur-Yvette. This emission from off-axis regions of multiple neighboring pointings could corrupt the final image, and here we characterize the error in a mosaicked image by analyzing linear-polarization observations of an extended, highly linearly polarized source with ALMA at. 14 At the time of publication, proposals for single-pointing polarization observations with ALMA may only target sources whose emission falls within at the requested band, which is where the off-axis polarization errors are minimal. With the exception of the selfcal phase solutions, all calibration tables that we apply to the off-axis data are solved for using on-axis data only—this includes D-terms and XY -phase We do this to mimic the standard calibration method of ALMA polarization data. These changes will most likely improve the errors that we report here, but future tests will be needed to confirm this

RESULTS

ORION-KL MOSAIC OBSERVATIONS

Comparison with CARMA

CONCLUSIONS