First M87 Event Horizon Telescope Results. III. Data Processing and Calibration

TLDR

EHT 1.3 mm observations of M87 and 3C 279, the first to include ALMA, achieve 25 µas resolution but face challenges from rapid atmospheric phase fluctuations, wide bandwidth, and a heterogeneous array. The study presents the calibration and reduction of these observations, developing three independent pipelines for phase calibration and fringe detection. Three independent pipelines, each tailored to EHT requirements, produced calibrated total‑intensity amplitude and phase, validated by QA tests that limit baseline systematic errors to 2 % in amplitude and 1° in phase. The M87 data reveal two nulls in correlated flux at ~3.4 and ~8.3 Gλ and temporal evolution in closure quantities, indicating intrinsic variability on day timescales and providing the first opportunity to image horizon‑scale structure.

Abstract

We present the calibration and reduction of Event Horizon Telescope (EHT) 1.3mm radio wavelength observations of the supermassive black hole candidate at the center of the radio galaxy M87 and the quasar 3C 279, taken during the 2017 April 5-11 observing campaign. These global very long baseline interferometric observations include for the first time the highly sensitive Atacama Large Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25 micro-as, with characteristic sensitivity limits of ~1 mJy on baselines to ALMA and ~10 mJy on other baselines. The observations present challenges for existing data processing tools, arising from the rapid atmospheric phase fluctuations, wide recording bandwidth, and highly heterogeneous array. In response, we developed three independent pipelines for phase calibration and fringe detection, each tailored to the specific needs of the EHT. The final data products include calibrated total intensity amplitude and phase information. They are validated through a series of quality assurance tests that show consistency across pipelines and set limits on baseline systematic errors of 2% in amplitude and 1 degree in phase. The M87 data reveal the presence of two nulls in correlated flux density at ~3.4 and ~8.3 giga-lambda and temporal evolution in closure quantities, indicating intrinsic variability of compact structure on a timescale of days, or several light-crossing times for a few billion solar-mass black hole. These measurements provide the first opportunity to image horizon-scale structure in M87.