Characterizing the accuracy of ALMA linear-polarization mosaics

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 $P_{\text{frac}}$ 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.

19 pages, 13 figures, 5 tables, 1 appendix. Accepted for publication in PASP. The data used in this paper were taken as part of the ALMA Extension and Optimization of Capabilities (EOC) program, and will be available (along with all calibration and imaging scripts) via the ALMA Science Archive on 1 Oct 2020

References in corpus (9)

• Tracing Magnetic Fields with Aligned Grains
• The Gould's Belt Distances Survey (GOBELINS) II. Distances and structure towards the Orion Molecular Clouds
• Correcting direction-dependent gains in the deconvolution of radio interferometric images
• The JCMT BISTRO Survey: The magnetic field strength in the Orion A filament
• ALMA observations of dust polarization and molecular line emission from the Class 0 protostellar source Serpens SMM1
• POLAMI: Polarimetric Monitoring of Active Galactic Nuclei at Millimetre Wavelengths. II. Widespread circular polarisation
• The Proper Motions of the Double Radio Source n in the Orion BN/KL Region
• Polarimetry at millimeter wavelengths with the NIKA camera: calibration and performance
• Direction Dependent Corrections in Polarimetric Radio Imaging I : Characterizing the effects of the primary beam on full Stokes imaging

Cited by in corpus (8)

• Gravity Driven Magnetic Field at ~1000 au Scales in High-mass Star Formation
• MeerKAT Primary Beam Measurements in the L Band
• The Explosion in Orion-KL as Seen by Mosaicking the Magnetic Field with ALMA
• Polarization from Aligned Dust Grains in the $\beta$ Pic Debris Disk
• The magnetic fields of the dusty nuclei and molecular outflows of Arp 220
• Performance of the polarization leakage correction in the PILOT data
• Observing the Sun with the Atacama Large Millimeter/submillimeter Array (ALMA): Polarization Observations at 3 mm
• Molecular line polarisation from the circumstellar envelopes of Asymptotic Giant Branch stars