Deep multi-redshift limits on Epoch of Reionisation 21cm Power Spectra
  from Four Seasons of Murchison Widefield Array Observations
  We compute the spherically-averaged power spectrum from four seasons of data
obtained for the Epoch of Reionisation (EoR) project observed with the
Murchison Widefield Array (MWA). We measure the EoR power spectrum over $k=
0.07-3.0~h$Mpc$^{-1}$ at redshifts $z=6.5-8.7$. The largest aggregation of 110
hours on EoR0 high-band (3,340 observations), yields a lowest measurement of
(43~mK)$^2$ = 1.8$\times$10$^3$ mK$^2$ at $k$=0.14~$h$Mpc$^{-1}$ and $z=6.5$
(2$\sigma$ thermal noise plus sample variance). Using the Real-Time System to
calibrate and the CHIPS pipeline to estimate power spectra, we select the best
observations from the central five pointings within the 2013--2016 observing
seasons, observing three independent fields and in two frequency bands. This
yields 13,591 2-minute snapshots (453 hours), based on a quality assurance
metric that measures ionospheric activity. We perform another cut to remove
poorly-calibrated data, based on power in the foreground-dominated and
EoR-dominated regions of the two-dimensional power spectrum, reducing the set
to 12,569 observations (419 hours). These data are processed in groups of 20
observations, to retain the capacity to identify poor data, and used to analyse
the evolution and structure of the data over field, frequency, and data
quality. We subsequently choose the cleanest 8,935 observations (298 hours of
data) to form integrated power spectra over the different fields, pointings and
redshift ranges.
