A comparative analysis of GNSS-inferred precipitable water vapour at the potential sites for the Africa Millimetre Telescope

In this study, GNSS station data from both the H.E.S.S. site and the Gamsberg Mountain, MERRA-2 data from both the H.E.S.S. site and Gamsberg Mountain were processed and converted into PWV for both sites. MERRA-2 data were used to find whether there is an agreement with the GNSS station data, and therefore validate and support the measurements from the GNSS station. Moreover, MERRA-2 data were used together with am to model and find the relationships between PWV and opacity at 86, 230, and 345 GHz for both sites. Furthermore, the PWV and atmospheric transmission at 86, 230, and 345 GHz was quantified during the EHT window of observations and the winter period which offered possibilities of single-dish and VLBI observations.

The Event Horizon Telescope (EHT) is a network of antennas across the globe currently used to image supermassive black holes (SMBHs) at a frequency of 230 GHz. Since the release of the image of M87 in 2019 and, subsequently, that of Sgr A in 2022 by the EHT collaboration, the focus has shifted to dynamically imaging SMBHs. This has led to a search for potential sites to extend and fill in the gaps within the EHT network. The Gamsberg Mountain and the High-Energy Stereoscopic System (H.E.S.S.) site are both located within the Khomas highlands and have been identified as potential sites for the Africa Millimetre Telescope (AMT). Precipitable water vapour (PWV) in the atmosphere is the main source of opacity and noise from atmospheric emissions when observing at millimetre to sub-millimetre wavelengths. This study aims to establish the PWV content and the atmospheric transmission at 86, 230, and 345 GHz at the AMT potential sites using Global Navigation Satellite System derived PWV data. Results show both sites have potential for observations at 86 and 230 GHz, with 345 GHz possible at the Gamsberg Mountain during winter. The overall median PWV of 14.27 and 9.25 mm was calculated at the H.E.S.S. site and the Gamsberg Mountain, respectively. The EHT window had PWV medians of 16.62 and 11.20 mm at the H.E.S.S. site and Gamsberg Mountain, respectively. Among the two sites, the Gamsberg Mountain had the lowest PWV conditions, therefore making it the most suitable site for the AMT