EHT 2021

April 2021 VLBI Campaign Data

The Event Horizon Telescope Collaboration (EHTC) and Joint ALMA Observatory (JAO) announce the public data release of the Very Long Baseline Interferometry (VLBI) 1-mm 2021 observations by the Event Horizon Telescope (EHT). The overall goal of these observations were continued imaging of the supermassive black holes M87* and Sagittarius A* at event horizon scales and to image the quasar 3C 273 at high angular resolution.

Public release data packages are available from CyVerse Data Common (doi TO COME) and the ALMA Science Portal. For the each target, just click on the link in the table below to be taken directly to the corresponding download directory.

Project Codes:

This data delivery contains what is defined by the EHTC as L1 data for the 2021 observing campaign. The 2021 campaign was the third where the Atacama Large Millimeter/submillimeter Array (ALMA) participated as a phased array, a capability that was developed by the ALMA Phasing Project. The EHT L1 data encompass the VLBI correlator output, converted to a circular polarization basis, as well as the calibration information from the participating telescopes. For ALMA this includes the quality assurance (QA2) output that was used for the polarization basis conversion. A primary reference for the processing of EHT data is Event Horizon Telescope Collaboration et al. 2019, ApJL, 875, L3 (doi: 10.3847/2041-8213/ab0c57). For scientific results derived from the EHT 2021 April Observation Campaign see eventhorizontelescope.org/publications. Additional data products derived from these L1 data can be found at eventhorizontelescope.org/for-astronomers/data.

This data delivery includes observations from the following facilities: ALMA, the Atacama Pathfinder Experiment (APEX), the Submillimeter Telescope (SMT), the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the IRAM 30m telescope, the Submillimeter Array (SMA), the South Pole Telescope (SPT), the Greenland Telescope (GLT), the Northern Extended Millimeter Array (NOEMA) and the Kitt Peak 12m (KP). More information about these facilities can be found in Event Horizon Telescope Collaboration et al. 2019, ApJL, 875, L2 (doi: 10.3847/2041-8213/ab0c96).

April 2021 Band 7 and VLBI Test Data

VLBI Commissioning and Science Verification observations were obtained at 345 GHz (Band 7) on April 19, 2021 during a global campaign that included phased ALMA and the EHT. Participating EHT sites included APEX, GLT, JCMT, NOEMA, IRAM 30m, SMA, and SMT.

The observations spanned a total of 5 hours and targeted several bright quasars, as well as M87*. A full suite of calibrator measurements (including polarization calibrators) was also included in the schedule.

Weather conditions at ALMA were excellent, and VLBI fringes were detected on a number of baselines. Further details about the observations are provided in "Enabling New Science with the ALMA Phasing System - Phase 2 (APP2)" by Matthews & Crew (2024), available here  (see especially Section 4.2.5).

The ALMA interferometric data from this experiment are described in detail in "First polarization study of the M87 jet and active galactic
nuclei at submillimeter wavelengths with ALMA" by Goddi et al. (2025), Astronomy & Astrophysics, Volume 699, id.A265 (DOI: 0.1051/0004-6361/202554140).

NOTE: Prospective users of the 345 GHz VLBI data products should be aware that preliminary analysis has revealed possible data artifacts introduced by the PolConvert process used to convert ALMA's linearly polarized VLBI data products to a circular basis. This may impact any
quantitative analysis performed with these data and users should proceed with caution.

To obtain this data, just click on the link in the table below to be taken directly to the download directory.

Project Codes: