First M87 Event Horizon Telescope Results. II. Array and Instrumentation (1906.11239v1)

Abstract: The Event Horizon Telescope (EHT) is a very long baseline interferometry (VLBI) array that comprises millimeter- and submillimeter-wavelength telescopes separated by distances comparable to the diameter of the Earth. At a nominal operating wavelength of ~1.3 mm, EHT angular resolution (lambda/D) is ~25 micro-as, which is sufficient to resolve nearby supermassive black hole candidates on spatial and temporal scales that correspond to their event horizons. With this capability, the EHT scientific goals are to probe general relativistic effects in the strong-field regime and to study accretion and relativistic jet formation near the black hole boundary. In this Letter we describe the system design of the EHT, detail the technology and instrumentation that enable observations, and provide measures of its performance. Meeting the EHT science objectives has required several key developments that have facilitated the robust extension of the VLBI technique to EHT observing wavelengths and the production of instrumentation that can be deployed on a heterogeneous array of existing telescopes and facilities. To meet sensitivity requirements, high-bandwidth digital systems were developed that process data at rates of 64 gigabit/s, exceeding those of currently operating cm-wavelength VLBI arrays by more than an order of magnitude. Associated improvements include the development of phasing systems at array facilities, new receiver installation at several sites, and the deployment of hydrogen maser frequency standards to ensure coherent data capture across the array. These efforts led to the coordination and execution of the first Global EHT observations in 2017 April, and to event-horizon-scale imaging of the supermassive black hole candidate in M87.

• The paper details the design and implementation of a global VLBI array that achieved 25 μas resolution for imaging M87’s supermassive black hole.
• It describes advanced instrumentation including high-bandwidth digital systems and hydrogen maser frequency standards for coherent data capture.
• The study demonstrates how enhanced phasing and atmospheric adaptations enable breakthrough tests of general relativity in strong-field regimes.

Overview of the Event Horizon Telescope Collaboration's Instrumentation and Array Design

The described academic paper from the Event Horizon Telescope (EHT) Collaboration, published in The Astrophysical Journal Letters, outlines instrumental and technical advancements that enabled the first event-horizon-scale imaging of a supermassive black hole in the galaxy M87. This document serves as a detailed account of the technical components and design considerations that facilitated such a scientific endeavor, emphasizing the successful integration and performance of a globally distributed very long baseline interferometry (VLBI) system.

The EHT array's unique capability to achieve an ultra-high angular resolution of about 25 micro-arcseconds (μas) at 1.3 mm wavelength is a standout feature. This is made possible by the array's Earth-sized collection of millimeter- and submillimeter-wavelength telescopes, which allows the resolution necessary to paper supermassive black holes (SMBHs) at event-horizon scales.

Several key technological developments were imperative for achieving the EHT's science goals, particularly the paper of general relativistic effects in the strong-field regime and processes near a black hole's boundary. Critically, the paper details the enhancements to VLBI techniques, adapting them for high-frequency observations and ensuring sufficient sensitivity. This required high-bandwidth digital systems capable of processing data at an impressive rate of 64 gigabits per second, significantly surpassing the capabilities of contemporary cm-wavelength VLBI arrays.

To ensure coherent data capture across its global array, the EHT employed hydrogen maser frequency standards. In addition, phasing systems and new receiver installations were deployed at several strategic sites. The 2017 observations marked the coordination of these efforts, culminating in the first imaging of M87's SMBH, contributing new observational evidence within the field of black hole physics and general relativity.

Technical Achievements and Scientific Implications

The EHT's ability to capture SMBH shadows with precision imaging is underpinned by its array's intricate structure and instrumentation. Several critical components feature prominently:

• Wideband VLBI Systems: The deployment of R2DBE-based systems, utilizing Xilinx's FPGA technology for rapid data sampling and transmission, was vital for achieving EHT's data rates. This setup involves the synchronization of clocks across the VLBI network, ensuring coherent integration across a globally distributed array.
• Phasing of Connected-element Arrays: The alignment of antenna signals at powerhouse facilities like ALMA and the SMA was essential to synthesize larger effective apertures from multiple smaller ones. This capability significantly bolstered the overall sensitivity of the array.
• High-altitude and Atmospheric Adaptations: Instruments and infrastructure across diverse, challenging environments required specific adjustments for performance assurance, including helium-filled hard drives for recording reliability at high altitudes and a meticulous management of atmospheric phase errors.

The scientific implications of these technical developments are profound. They span testing fundamental aspects of general relativity, such as verifying the expected "shadow" morphology of black holes dictated by their gravitational field. The ability to resolve the innermost structure of SMBHs also extends our understanding of accretion disks and jet formations, critical processes influencing galactic evolution.

Prospects for Future Developments

Looking forward, the expansion of EHT's array to include additional radio telescopes at novel locations and enhancements to existing systems will enrich (u, v) coverage, pwering further detailed imaging. Future work may include 0.87 mm wavelength observations, offering enhanced angular resolution critical for distinguishing finer details in black hole observations. Moreover, the prospect of incorporating satellite-based VLBI stations could dramatically extend baseline lengths beyond Earth, opening pathways to resolving even more distant or complex black hole environments.

The paper's advanced technical narrative showcases not only the state-of-the-art capabilities within the EHT but also a meticulous framework for future innovations in black hole imaging and general relativistic studies. These advancements will decisively shape successive explorations into the enigmatic behaviors surrounding black holes, promising to clarify long-standing theoretical constructs.