The HERA-19 Commissioning Array: Direction-dependent Effects

Abstract

2 Kohn et al.Foreground power dominates the measurements of interferometers that seek a statistical detection of highly-redshifted Hi emission from the Epoch of Reionization (EoR). The chromaticity of the instrument creates a boundary in the Fourier transform of frequency (proportional to k ) between spectrally smooth emission, characteristic of the strong synchrotron foreground (the "wedge"), and the spectrally structured emission from Hi in the EoR (the "EoR window"). Faraday rotation can inject spectral structure into otherwise smooth polarized foreground emission, which through instrument effects or miscalibration could possibly pollute the EoR window. For instruments pursuing a "foreground avoidance" strategy of simply measuring in the EoR window, and not attempting to model and remove foregrounds, as is the plan for the first stage of the Hydrogen Epoch of Reionization Array (HERA), characterizing the intrinsic instrument polarization response is particularly important. Using data from the HERA 19-element commissioning array, we investigate the polarization response of this new instrument in the power spectrum domain. We perform a simple image-based calibration based on the unpolarized diffuse emission of the Global Sky Model, and show that it achieves qualitative redundancy between the nominally-redundant baselines of the array and reasonable amplitude accuracy. We construct power spectra of all fully polarized coherencies in all pseudo-Stokes parameters, and discuss the achieved isolation of foreground power due to the intrinsic spectral smoothness of the foregrounds, the instrument chromaticity, and the calibration. We compare to simulations based on an unpolarized diffuse sky model and detailed electromagnetic simulations of the dish and feed, confirming that in Stokes I, the calibration does not add significant spectral structure beyond that expected from the interferometer array configuration and the modeled primary beam response. Further, this calibration is stable over the 8 days of observations considered. Excess power is seen in the power spectra of the linear polarization Stokes parameters which is not easily attributable to leakage via the primary beam, and results from some combination of residual calibration errors and actual polarized emission. Stokes V is found to be highly discrepant from the expectation of zero power, strongly pointing to the need for more accurate polarized calibration.

Patterns of primary beam non-redundancy in close-packed 21 cm array observations

“…However, due to a wide variety of instrumental, atmospheric, and modelling effects, the recovered instrumental calibration always deviates from its true values in real observations (e.g. Datta et al 2009Datta et al , 2010Barry et al 2016;Patil et al 2016;Ewall-Wice et al 2017;Gehlot et al 2018;de Gasperin et al 2019;Kohn et al 2019;Mouri Sar-darabadi & Koopmans 2019;Dillon et al 2020;Kern et al 2020;Kumar et al 2020).…”

Mapping discrete galaxies at cosmic dawn with 21-centimeter observations

“…Meanwhile, on large scales the power spectrum is boosted compared to previous predictions by a factor that is between 2 and 7 depending on the astrophysical parameters. Thus, the signature of discrete galaxies is also a promising goal for radio arrays targeting the 21-cm power spectrum at cosmic dawn, such as the Hydrogen Epoch of Reionization Array (Kohn et al 2019) (HERA) and the New Extension in Nançay Upgrading LOFAR (Zarka et al 2012) (NenuFAR).…”

Methods of Error Estimation for Delay Power Spectra in $21\,\textrm{cm}$ Cosmology

“…For example, LOFAR has used the Stokes V parameter as an estimator of noise level (Patil et al 2017;Gehlot et al 2019;Mertens et al 2020) since the astrophysical sky is expected to exhibit only extremely weak circular polarization. However, reliably estimating Stokes V power requires more accurate polarization calibration solutions than that are currently available for HERA (Kohn et al 2019). Since one of our goals is to test our error estimation methods on HERA data, we will omit discussion of Stokes V techniques in this paper.…”