| Abstract: The neutral Hydrogen spin-flip transition at 21-cm wavelength is a critical EM-window into the evolution of the Universe at redshifts between the CMB, and late time astrophysics. This signal, especially its statistical properties, such as the power-spectrum from radio-interferometers, promises to be an invaluable tool to probe early astrophysics, such as the Epoch of Reionization (EoR), star formation efficiency and X-ray heating of the ISM. The Hydrogen Epoch of Reionization Array (HERA) is a specialized radio-interferometer designed to detect the EoR power-spectrum. While its unique, redundant, and precision calibration techniques coupled with foreground avoidance have produced state-of-the-art upper limits, HERA analysis efforts have also uncovered challenges in the form of newer systematics. HERA Phase II observation results consist of major updates and upgrades from the previous upper limits. Novel hardware upgrades, such as wideband Vivaldi feeds, extend our bandwidth from 47 ~ 234 MHz, covering redshifts from 5 to 25. These upgrades result in newer systematics, such as crosstalk arising from mutual coupling in the new feeds. Redundant calibration of such arrays may also result in a partial decoherence of the 21-cm signal due to non-idealities, termed as signal loss. We will discuss the new upper limits on the 21-cm power-spectra. We will present an in-depth analysis of the 21-cm signal loss estimates due to non-redundancy in HERA and how to disentangle these estimates from other effects, such as different foreground sources interacting with different sections of the primary-beam. In addition, we will also discuss a suite of statistical tests that are useful for identifying hidden systematics that deviate from pure noise statistics expected at current sensitivities that are above the levels of most fiducial EoR models. This will be pivotal in the near future to conclusively detect the 21-cm power-spectra, bordering on the edge of precision radio-astronomy and cosmology. |
