Public Data Release of the Event Horizon Telescope 2021 Observations

This data delivery contains what is defined by the Event Horizon Telescope Collaboration (EHTC) as L1 data for the Very Long Baseline Interferometry (VLBI) 1-mm 2021 observing campaign by the Event Horizon Telescope (EHT). The 2021 campaign was the third where the Atacama Large Millimeter Array (ALMA) participated as a phased array, a capability that was developed by the ALMA Phasing Project (APP; Doeleman 2010; Fish et al. 2013; Matthews et al. 2018). New telescopes joining the EHT in this campaign were the IRAM NOEMA Observatory and the Kitt Peak 12-meter Telescope. The EHT L1 data encompasses the VLBI correlator output, converted into circular polarization basis, as well as the calibration information from the participating telescopes. For ALMA this includes their quality assurance (QA2) output that is used for the polarization basis conversion. The processing carried out on this L1 data delivery is largely the same as for the EHT 2018 April observations as described in Event Horizon Telescope Collaboration et al. (2024). Aspects that are the same as the EHT 2017 April campaign are described in more detail in Event Horizon Telescope Collaboration et al. (2019). For scientific results derived from the EHT 2021 April Observation Campaign see https://eventhorizontelescope.org/publications. Additional data products derived from this L1 data can be found at https://eventhorizontelescope.org/for-astronomers/data.

This data delivery includes data from the following facilities: ALMA, the Atacama Pathfinder Experiment (APEX), the Submillimeter Telescope (SMT), the James Clerk Maxwell Telescope (JCMT), the IRAM 30m telescope, the phased Submillimeter Array (SMA), the South Pole Telescope (SPT), the Greenland Telescope (GLT), the IRAM NOEMA Observatory and the Kitt Peak 12-meter telescope. More information about the facilities used for these observations can be found in Event Horizon Telescope Collaboration et al. (2024).