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Metrics and Motivations for Earth-Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope

Palumbo, Daniel C. M. and Doeleman, Sheperd S. and Johnson, Michael D. and Bouman, Katherine L. and Chael, Andrew A. (2019) Metrics and Motivations for Earth-Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope. Astrophysical Journal, 881 (1). Art. No. 62. ISSN 1538-4357.

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Very-long-baseline interferometry (VLBI) at frequencies above 230 GHz with Earth-diameter baselines gives spatial resolution finer than the ~50 μas "shadow" of the supermassive black hole at the Galactic Center, Sagittarius A* (Sgr A*). Imaging static and dynamical structure near the "shadow" provides a test of general relativity and may allow measurement of black hole parameters. However, traditional Earth-rotation synthesis is inapplicable for sources (such as Sgr A*) with intraday variability. Expansions of ground-based arrays to include space-VLBI stations may enable imaging capability on timescales comparable to the prograde innermost stable circular orbit (ISCO) of Sgr A*, which is predicted to be 4–30 minutes, depending on black hole spin. We examine the basic requirements for space VLBI, and we develop tools for simulating observations with orbiting stations. We also develop a metric to quantify the imaging capabilities of an array irrespective of detailed image morphology or reconstruction method. We validate this metric on example reconstructions of simulations of Sgr A* at 230 and 345 GHz, and use these results to motivate expanding the Event Horizon Telescope to include small dishes in Low Earth Orbit (LEO). We demonstrate that high-sensitivity sites such as the Atacama Large Millimeter/Submillimeter Array (ALMA) make it viable to add small orbiters to existing ground arrays, as space-ALMA baselines would have sensitivity comparable to ground-based non-ALMA baselines. We show that LEO-enhanced arrays sample half of the diffraction-limited Fourier plane of Sgr A* in less than 30 minutes, enabling reconstructions of near-horizon structure with a normalized root-mean-square error ≾0.3 on sub-ISCO timescales.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Palumbo, Daniel C. M.0000-0002-7179-3816
Doeleman, Sheperd S.0000-0002-9031-0904
Johnson, Michael D.0000-0002-4120-3029
Bouman, Katherine L.0000-0003-0077-4367
Chael, Andrew A.0000-0003-2966-6220
Alternate Title:Metrics and Motivations for Earth–Space VLBI: Time-resolving Sgr A* with the Event Horizon Telescope
Additional Information:© 2019 The American Astronomical Society. Received 2019 March 20; revised 2019 June 18; accepted 2019 June 20; published 2019 August 13. We thank Ramesh Narayan for suggesting the (u, v) filling fraction diagnostic, as well as Maura Shea and Vincent Fish for their discussions of space expansions to the EHT. We are grateful to Joseph Lazio for facilitating early discussions of the feasibility of high-frequency VLBI from space. Jonathan Weintroub, Alex Raymond, and Kari Hayworth were instrumental in understanding space-VLBI hardware requirements. We thank Freek Roelofs and Heino Falcke for their discussion of the science possibilities of space VLBI, for their hospitality during the Future of High-Resolution Radio Interferometry in Space workshop, and for fostering collaboration around space VLBI within the radio astronomy community at large. We are grateful to Avery Broderick for providing simulations of the orbiting "hot spot." Finally, we thank our referee for the careful and thoughtful feedback on our manuscript. We thank the National Science Foundation (AST-1440254, AST-1716536) and the Gordon and Betty Moore Foundation (GBMF-5278) for financial support of this work. This work was supported in part by the Black Hole Initiative at Harvard University, which is supported by a grant from the John Templeton Foundation.
Funding AgencyGrant Number
Gordon and Betty Moore FoundationGBMF-5278
John Templeton FoundationUNSPECIFIED
Subject Keywords:galaxies: individual (Sgr A*) – Galaxy: center – space vehicles – techniques: interferometric
Issue or Number:1
Record Number:CaltechAUTHORS:20190813-111615301
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Official Citation:Daniel C. M. Palumbo et al 2019 ApJ 881 62
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:97863
Deposited By: Tony Diaz
Deposited On:13 Aug 2019 18:27
Last Modified:03 Oct 2019 21:35

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