In situ measurement of MWA primary beam variation using ORBCOMM

Abstract

We provide the first in situ measurements of antenna element (tile) beam shapes of the Murchison Widefield Array (MWA), a low radio-frequency interferometer and an SKA precursor. Most current MWA processing pipelines use an assumed beam shape, errors in which can cause absolute and relative flux density errors, as well as polarisation 'leakage'. This makes understanding the primary beam of paramount importance, especially for sensitive experiments such as a measurement of the 21 cm line from the epoch of reionisation (EoR). The calibration requirements for measuring the EoR 21 cm line are so extreme that tile to tile beam variations may affect our ability to make a detection. Measuring the primary beam shape from visibilities alone is challenging, as multiple instrumental, atmospheric, and astrophysical factors contribute to uncertainties in the data. Building on the methods of Neben et al. (2015), we tap directly into the receiving elements of the MWA before any digitisation or correlation of the signal. Using ORBCOMM satellite passes we are able to produce all-sky maps for 4 separate tiles in the XX polarisation. We find good agreement with the cutting-edge 'fully' embedded element (FEE) model of Sokolowski et al. (2017), and observe that the MWA beamformers consistently recreate beam shapes to within ~1dB in the reliable areas of our beam maps. We also clearly observe the effects of a missing dipole from a tile in one of our beam maps, and show that the FEE model is able to reproduce this modified beam shape. We end by motivating and outlining additional onsite experiments to further constrain the primary beam behaviour.

arXiv comment13 pages, 7 figures Accepted to PASA

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• Using negative-latency gravitational wave alerts to detect prompt radio bursts from binary neutron star mergers with the Murchison Widefield Array
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• Fundamental uncertainty levels of 21cm power spectra from a delay analysis
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• Impact of instrument and data characteristics in the interferometric reconstruction of the 21 cm power spectrum
• All Sky Modelling Requirements for Bayesian 21 cm Power Spectrum Estimation with BayesEoR
• Working Principle of the Calibration Algorithm for High Dynamic Range Solar Imaging with Square Kilometre Array Precursor
• Tackling the Unique Challenges of Low-frequency Solar Polarimetry with the Square Kilometre Array Low Precursor: Pipeline Implementation
• Effects of model incompleteness on the drift-scan calibration of radio telescopes
• Holographic Calibration of Phased Array Telescopes
• A Differentiable, End-to-End Forward Model for 21 cm Cosmology: Estimating the Foreground, Instrument, and Signal Joint Posterior
• Characterizing Beam Errors for Radio Interferometric Observations of Reionization
• Robust direction-dependent gain-calibration of beam-modelling errors far from the target field