Abstract
Abstract 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 first time the highly sensitive Atacama Large Millimeter/submillimeter Array (ALMA); reaching an angular resolution of 25 μas, 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° in phase. The M87 data reveal the presence of two nulls in correlated flux density at ∼3.4 and ∼8.3 Gλ 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.
Highlights
The principle of very long baseline interferometry (VLBI) is to connect distant radio telescopes to create a single virtual telescope
Calibration, and data interpretation are: (1) a clock frequency instability at PV resulting in ∼50% amplitude loss to that station; (2) recorder configuration issues at Atacama Pathfinder Experiment (APEX) resulting in a significant number of data gaps and low data validity at correlation; (3) pointing errors at Large Millimeter Telescope Alfonso Serrano (LMT), large compared to the beam, resulting in unpredictable amplitude loss and inter- and intra-scan gain variability; and (4) a common local oscillator (LO) used at Submillimeter Array (SMA) and James Clerk Maxwell Telescope (JCMT) resulting in opposite sideband contamination at the level of ∼15% for short integration times, making the SMA–JCMT intra-site baseline less useful for calibration
The phased arrays combine the total collecting area of all their dishes into one virtual telescope. This depends on precise phase alignment of the signals, with an accuracy that is captured by the APEX JCMT LMT PV Submillimeter Telescope (SMT) South Pole Telescope (SPT) SMA6 ALMA37
Summary
The principle of very long baseline interferometry (VLBI) is to connect distant radio telescopes to create a single virtual telescope. EHT observations result in data spanning a wide range of signal-to-noise ratio (S/N) due to the heterogeneous nature of the array, and the high observing frequency produces data that are sensitive to systematics in the signal chain These factors, along with the typical challenges associated with VLBI, have motivated the development of specialized processing and calibration techniques. Calibration, and data interpretation are: (1) a clock frequency instability at PV resulting in ∼50% amplitude loss to that station; (2) recorder configuration issues at APEX resulting in a significant number of data gaps and low data validity at correlation; (3) pointing errors at LMT, large compared to the beam, resulting in unpredictable amplitude loss and inter- and intra-scan gain variability; and (4) a common local oscillator (LO) used at SMA and JCMT resulting in opposite sideband contamination at the level of ∼15% for short integration times, making the SMA–JCMT intra-site baseline less useful for calibration. All telescopes apart from ALMA observed in circular polarization with the installation of quarter-wave
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