Abstract

We present the deepest upper limits achieved by the Murchison Widefield Array to date at redshifts z=6.5, z=6.8, and z=7. This study is based on observations centred at (RA = 0h, DEC = −27°), collected between 2013 and 2021. The analysis builds upon the systematic framework developed in Nunhokee et al. (2024), which employs intermediate data products for data quality assessment to minimise contamination in the targeted power spectrum region. The final power spectra are constructed from 221 hours of observations for z=6.5 and 226 hours for z=6.8 and z=7.0, using the Cosmological HI Power Spectrum Estimator. Our results provide the first evidence of a heated intergalactic medium (IGM) at redshifts z=6.5 to z=7.0. Additionally, I will discuss how these results can be improved by refining the separation of thermal noise and systematics from foregrounds, leveraging fluctuations in the probability distributions of the power spectra (Trott et al.,submitted).