EHT 2024

April 2024 and November 2024 Band 9 Test Data

These observations were conducted to test the viability of the ALMA Phasing System (APS) in Band 9 and evaluate its potential use for future VLBI science observations. Unlike all ALMA receivers at lower frequencies, the Band 9 receiver is a double side-band receiver and thus it was not immediately clear that the APS would operate in that band, nor that the VLBI recordings would be viable. The test was approved on the condition that it take no more than five hours in total.

Initial phasing tests in Band 9 were conducted during the normal Event Horizon Telescope (EHT) campaign in April 2024 during windows when observing conditions were suited to Band 9 or 10 observations. As a result of these tests, accommodations were made to ALMA VLBI software to allow use of Band 9 for a short VLBI test session later in 2024.

Conditions were favorable for execution of the subsequent VLBI test on 2024 November 21 and the experiment proceeded with ALMA, the Atacama Pathfinder EXperiment telescope (APEX), and the James Clerk Maxwell Telescope (JCMT). Fringes on one of the test sources (J0423-012) were found between ALMA and APEX. The tests included a frequency check at JCMT using an observation of the 12CO(6-5) line at 691.435 GHz toward the Orion Becklin-Neugebauer (BN) object, as well as a few scans in Band 6 at ALMA and JCMT to verify system timing. (APEX timing was verified in a time-adjacent Band 3 session.)

The phasing tests and the VLBI session are described in detail in "First Very Long Baseline Interferometry Fringe Detection at 690 GHz" by M.-T. Chen, G. B. Crew, D. Bintley et al., PASP, 138:084104, 2026; https://doi.org/10.1088/1538-3873/ae92b6.

Prospective users of these data should be aware that very little of the operation of the system was routine and that to minimize on-sky time, standard ALMA calibrations were not performed. Additionally, the data have not undergone Quality Assurance Level 2 (QA2).

Some short snippets of the raw VLBI recordings are provided to allow verification of the correlated results.

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