A new MWA limit on the 21 cm Power Spectrum at Redshifts $\sim$ 13 $-$ 17

TL;DR

This study presents new upper limits on the 21 cm power spectrum at redshifts 13-17 using 15 hours of MWA data, addressing systematics and calibration challenges in the lowest frequency band.

Contribution

It introduces an optimized calibration and data selection strategy for low-frequency MWA observations to improve constraints on the early universe's neutral hydrogen distribution.

Findings

Two sigma upper limits on the 21 cm power spectrum were established.

The lowest limit achieved is 6.3×10^6 mK^2 at k=0.14 h/Mpc and z=15.2.

Approximately 5.5 hours of data were used after removing contaminated observations.

Abstract

Observations in the lowest MWA band between $75-100$ MHz have the potential to constrain the distribution of neutral hydrogen in the intergalactic medium at redshift $\sim 13-17$. Using 15 hours of MWA data, we analyse systematics in this band such as radio-frequency interference (RFI), ionospheric and wide field effects. By updating the position of point sources, we mitigate the direction independent calibration error due to ionospheric offsets. Our calibration strategy is optimized for the lowest frequency bands by reducing the number of direction dependent calibrators and taking into account radio sources within a wider field of view. We remove data polluted by systematics based on the RFI occupancy and ionospheric conditions, finally selecting 5.5 hours of the cleanest data. Using these data, we obtain two sigma upper limits on the 21 cm power spectrum in the range of $0.1\lessapprox k \lessapprox 1 ~\rm ~h~Mpc^{-1}$ and at $z$=14.2, 15.2 and 16.5, with the lowest limit being $6.3\times 10^6 ~\rm mK^2$ at $\rm k=0.14 \rm ~h~Mpc^{-1}$ and at $z=15.2$ with a possibility of a few \% of signal loss due to direction independent calibration.