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Published April 1, 2016 | Submitted + Published
Journal Article Open

Persistent Asymmetric Structure of Sagittarius A^* on Event Horizon Scales

Abstract

The Galactic Center black hole Sagittarius A^* (Sgr A^*) is a prime observing target for the Event Horizon Telescope (EHT), which can resolve the 1.3 mm emission from this source on angular scales comparable to that of the general relativistic shadow. Previous EHT observations have used visibility amplitudes to infer the morphology of the millimeter-wavelength emission. Potentially much richer source information is contained in the phases. We report on 1.3 mm phase information on Sgr A^* obtained with the EHT on a total of 13 observing nights over four years. Closure phases, which are the sum of visibility phases along a closed triangle of interferometer baselines, are used because they are robust against phase corruptions introduced by instrumentation and the rapidly variable atmosphere. The median closure phase on a triangle including telescopes in California, Hawaii, and Arizona is nonzero. This result conclusively demonstrates that the millimeter emission is asymmetric on scales of a few Schwarzschild radii and can be used to break 180° rotational ambiguities inherent from amplitude data alone. The stability of the sign of the closure phase over most observing nights indicates persistent asymmetry in the image of Sgr A^* that is not obscured by refraction due to interstellar electrons along the line of sight.

Additional Information

© 2016 The American Astronomical Society. Received 2015 October 1; accepted 2016 February 17; published 2016 March 22. The EHT is supported by multiple grants from the National Science Foundation (NSF) and a grant from the Gordon & Betty Moore Foundation (GBMF-3561) to SSD. The SMA is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics. The ARO is partially supported through the NSF University Radio Observatories program. The JCMT was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the UK, the Netherlands Organisation for Scientific Research, and the National Research Council of Canada. Funding for ongoing CARMA development and operations was supported by the NSF and CARMA partner universities. A.E.B. receives financial support from the Perimeter Institute for Theoretical Physics and the Natural Sciences and Engineering Research Council of Canada through a Discovery Grant. Research at Perimeter Institute is supported by the Government of Canada through Industry Canada and by the Province of Ontario through the Ministry of Research and Innovation. M.H. acknowledges support from MEXT/JSPS KAKENHI. L.L. and G.N.O.L. acknowledge the financial support of DGAPA, UNAM, and CONACyT, Mexico. The EHT gratefully acknowledges equipment donations from Xilinx Inc. Facility: EHT - .

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Published - apj_820_2_90.pdf

Submitted - 1602.05527v1.pdf

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Additional details

Created:
August 22, 2023
Modified:
October 18, 2023