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

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 mu as, with characteristic sensitivity limits of similar to 1 mJy on baselines to ALMA and similar to 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 degrees in phase. The M87 data reveal the presence of two nulls in correlated flux density at similar to 3.4 and similar to 8.3 G 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.Academy of Finland [274477, 284495, 312496]; European Commission Framework Programme Horizon 2020 Research and Innovation action [731016]; Black Hole Initiative at Harvard University through John Templeton Foundation [60477]; Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT, Chile) [PIA ACT172033, Fondecyt 1171506, BASAL AFB-170002, ALMA-conicyt 31140007]; Consejo Nacional de Ciencia y Tecnologia (CONACYT, Mexico) [104497, 275201, 279006, 281692]; Direccion General de Asuntos del Personal Academico-Universidad Nacional Autonoma de Mexico (DGAPA-UNAM) [IN112417]; European Research Council Synergy Grant "BlackHoleCam: Imaging the Event Horizon of Black Holes" [610058]; Generalitat Valenciana postdoctoral grant [APOSTD/2018/177]; Gordon and Betty Moore Foundation [GBMF 947, GBMF-3561, GBMF-5278]; Japanese Government (Monbukagakusho: MEXT) Scholarship; Japan Society for the Promotion of Science (JSPS) [JP17J08829]; JSPS Overseas Research Fellowships; Key Research Program of Frontier Sciences, Chinese Academy of Sciences (CAS) [QYZDJ-SSW-SLH057, QYZDJ-SSW-SYS008]; Leverhulme Trust Early Career Research Fellowship; MEXT/JSPS KAKENHI [18KK0090, JP18K13594, JP18K03656, JP18H03721, 18K03709, 18H01245, 25120007]; MIT International Science and Technology Initiatives (MISTI) Funds; Ministry of Science and Technology (MOST) of Taiwan [105-2112-M-001-025-MY3, 106-2112-M-001-011, 106-2119-M-001-027, 107-2119-M-001-017, 107-2119-M-001-020, 107-2119-M-110-005]; National Aeronautics and Space Administration (NASA) [80NSSC17K0649]; National Key Research and Development Program of China [2016YFA0400704, 2016YFA0 400702]; National Science Foundation (NSF) [AST-0096454, AST-0352953, AST-0521233, AST-0705062, AST-0905844, AST-0922984, AST-1126433, AST-1140030, DGE-1144085, AST-1207704, AST-1207730]; Natural Science Foundation of China [11573051, 11633006, 11650110427, 10625314, 11721303, 11725312, 11873028, 11873073, U1531245, 11473010]; Natural Sciences and Engineering Research Council of Canada (NSERC); National Research Foundation of Korea [2015-R1D1A1A01056807, NRF-2015H1A2A1033752, NRF-2015H1D3A1066561]; Netherlands Organization for Scientific Research (NWO) VICI award [639.043.513]; Spinoza Prize [SPI 78-409]; Swedish Research Council [2017-00648]; Government of Canada through the Department of Innovation, Science and Economic Development Canada; Province of Ontario through the Ministry of Economic Development, Job Creation and Trade; Russian Science Foundation [17-12-01029]; Spanish Ministerio de Economia y Competitividad [AYA2015-63939-C2-1-P, AYA2016-80889-P]; US Department of Energy (USDOE) through the Los Alamos National Laboratory [89233218CNA000001]; Italian Ministero dell'Istruzione Universita e Ricerca through the grant Progetti Premiali 2012-iALMA [CUP C52I13000140001]; ALMA North America Development Fund; NSF [ACI-1548562, DBI-0735191, DBI-1265383, DBI-1743442]; Smithsonian Institution; Academia Sinica; National Key R&D Program of China [2017YFA0402700]; Science and Technologies Facility Council (UK); CNRS (Centre National de la Recherche Scientifique, France); MPG(Max-Planck-Gesellschaft, Germany); IGN (Instituto Geografico Nacional, Spain); State of Arizona; NSF Physics Frontier Center award [PHY-0114422]; Kavli Foundation; National Science Foundation [PLR-1248097]; NSF Physics Frontier Center [PHY-1125897]; Jansky Fellowship program of the National Radio Astronomy Observatory (NRAO); South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation (NRF), an agency of the Department of Science and Technology (DST) of South Africa; State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award [SEV-2017-0709]; European Union' s Horizon 2020 research and innovation programme [730562 RadioNet]; GBMF [GBMF-947]; Compute Ontario; Calcul Quebec; Compute Canada; NSF; CyVerse; [Chandra TM6-17006X]; [AST-1207752]; [MRI-1228509]; [OPP-1248097]; [AST-1310896]; [AST-1312651]; [AST-1337663]; [AST-1440254]; [AST-1555365]; [AST-1715061]; [AST-1615796]; [AST-1716327]; [OISE-1743747]; [AST-1816420]; [AST-1614868]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. 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First M87 Event Horizon Telescope Results. VII. Polarization of the Ring

In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of similar to 15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication

Constraints on black-hole charges with the 2017 EHT observations of M87*

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes

Constraints on black-hole charges with the 2017 EHT observations of M87*

Our understanding of strong gravity near supermassive compact objects has recently improved thanks to the measurements made by the Event Horizon Telescope (EHT). We use here the M87* shadow size to infer constraints on the physical charges of a large variety of nonrotating or rotating black holes. For example, we show that the quality of the measurements is already sufficient to rule out that M87* is a highly charged dilaton black hole. Similarly, when considering black holes with two physical and independent charges, we are able to exclude considerable regions of the space of parameters for the doubly-charged dilaton and the Sen black holes

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

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

SYMBA: An end-to-end VLBI synthetic data generation pipeline. Simulating Event Horizon Telescope observations of M 87

Context. Realistic synthetic observations of theoretical source models are essential for our understanding of real observational data. In using synthetic data, one can verify the extent to which source parameters can be recovered and evaluate how various data corruption effects can be calibrated. These studies are the most important when proposing observations of new sources, in the characterization of the capabilities of new or upgraded instruments, and when verifying model-based theoretical predictions in a direct comparison with observational data. Aims. We present the SYnthetic Measurement creator for long Baseline Arrays (SYMBA), a novel synthetic data generation pipeline for Very Long Baseline Interferometry (VLBI) observations. SYMBA takes into account several realistic atmospheric, instrumental, and calibration effects. Methods. We used SYMBA to create synthetic observations for the Event Horizon Telescope (EHT), a millimetre VLBI array, which has recently captured the first image of a black hole shadow. After testing SYMBA with simple source and corruption models, we study the importance of including all corruption and calibration effects, compared to the addition of thermal noise only. Using synthetic data based on two example general relativistic magnetohydrodynamics (GRMHD) model images of M 87, we performed case studies to assess the image quality that can be obtained with the current and future EHT array for different weather conditions. Results. Our synthetic observations show that the effects of atmospheric and instrumental corruptions on the measured visibilities are significant. Despite these effects, we demonstrate how the overall structure of our GRMHD source models can be recovered robustly with the EHT2017 array after performing calibration steps, which include fringe fitting, a priori amplitude and network calibration, and self-calibration. With the planned addition of new stations to the EHT array in the coming years, images could be reconstructed with higher angular resolution and dynamic range. In our case study, these improvements allowed for a distinction between a thermal and a non-thermal GRMHD model based on salient features in reconstructed images

First M87 Event Horizon Telescope Results. VII. Polarization of the Ring

In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ~15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication