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

TL;DR

This paper details the calibration and data processing methods for EHT observations of M87 and 3C 279, including the integration of ALMA, to achieve high-resolution imaging of black hole horizons.

Contribution

It introduces three independent pipelines for phase calibration and fringe detection tailored to EHT's challenging data, enabling reliable imaging of black hole event horizons.

Findings

Detection of nulls in correlated flux density at specific baselines.

Observation of intrinsic variability in the source structure over days.

Validation of calibration accuracy with low systematic errors.

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.