21cm Epoch of Reionisation Power Spectrum with Closure Phase using the Murchison Widefield Array

The radio interferometric closure phases can be a valuable tool for studying cosmological {H\scriptsize{I}}~from the early Universe. Closure phases have the advantage of being immune to element-based gains and associated calibration errors. Thus, calibration and errors therein, which are often sources of systematics limiting standard visibility-based approaches, can be avoided altogether in closure phase analysis. In this work, we present the first results of the closure phase power spectrum of {H\scriptsize{I}}~21-cm fluctuations using the Murchison Widefield Array (MWA), with $\sim 12$ hours of MWA-phase II observations centered around redshift, $z\approx 6.79$, during the Epoch of Reionisation. On analysing three redundant classes of baselines – 14~m, 24~m, and 28~m equilateral triads, our estimates of the $2\sigma$ ($95\mathrm{\%}$ confidence interval) 21-cm power spectra are $\lesssim (184{)}^{2}pseudo\mathrm{m}{\mathrm{K}}^2$ at $k_{\mathrm{\mid }\mathrm{\mid }}=0.36$ $pseudoh\mathrm{M}\mathrm{p}{\mathrm{c}}^{-1}$ in the EoR1 field for the 14~m baseline triads, and $\lesssim (188{)}^{2}pseudo\mathrm{m}{\mathrm{K}}^2$ at $k_{\mathrm{\mid }\mathrm{\mid }}=0.18$ $pseudoh\mathrm{M}\mathrm{p}{\mathrm{c}}^{-1}$ in the EoR0 field for the 24~m baseline triads. The ``$pseudo$’’ units denote that the length scale and brightness temperature should be interpreted as close approximations. Our best estimates are still 3-4 orders high compared to the fiducial 21-cm power spectrum; however, our approach provides promising estimates of the power spectra even with a small amount of data. These data-limited estimates can be further improved if more datasets are included into the analysis. The evidence for excess noise has a possible origin in baseline-dependent systematics in the MWA data that will require careful baseline-based strategies to mitigate, even in standard visibility-based approaches.