UvA-DARE (Digital Academic Repository) First Event Horizon Telescope

We present the calibration and reduction of Event Horizon Telescope ( EHT ) 1.3 mm 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 fi rst time the highly sensitive Atacama Large Millimeter / submillimeter Array ( ALMA ) ; reaching an angular resolution of 25 μ 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 fl uctuations, wide recording bandwidth, and highly heterogeneous array. In response, we developed three independent pipelines for phase calibration and fringe detection, each tailored to the speci fi c needs of the EHT. The fi nal 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 ° in phase. The M87 data reveal the presence of two nulls in correlated fl ux density at ∼ 3.4 and ∼ 8.3 G λ 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 fi rst opportunity to image horizon-scale structure