Unveiling the Structure of Sagittarius A*: First VLBI Image at 86 GHz with ALMA

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Issaoun et al. 2019, The Size, Shape, and Scattering of Sagittarius A* at 86 GHz: First VLBI with ALMA

The Galactic Center supermassive black hole Sagittarius A* (Sgr A*) is one of the most promising targets to study the dynamics of black hole accretion and outflow via direct imaging with very long baseline interferometry (VLBI). At 3.5 mm (86 GHz), the emission from Sgr A* is resolvable with the Global Millimeter VLBI Array (GMVA). We present the first observations of Sgr A* with the phased Atacama Large Millimeter/submillimeter Array (ALMA) joining the GMVA. Our observations achieve an angular resolution of ~87{\mu}as, improving upon previous experiments by a factor of two. We reconstruct a first image of the unscattered source structure of Sgr A* at 3.5 mm, mitigating effects of interstellar scattering. The unscattered source has a major axis size of 120 ± 34{\mu}as (12 ± 3.4 Schwarzschild radii), and a symmetrical morphology (axial ratio of 1.2+0.3−0.2), which is further supported by closure phases consistent with zero within 3{\sigma}. We show that multiple disk-dominated models of Sgr A* match our observational constraints, while the two jet-dominated models considered are constrained to small viewing angles. Our long-baseline detections to ALMA also provide new constraints on the scattering of Sgr A*, and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope. Our results provide the most stringent constraints to date for the intrinsic morphology and refractive scattering of Sgr A*, demonstrating the exceptional contribution of ALMA to millimeter VLBI.

The image wasn't in the paper, nor is this an image from the Event Horizon Telescope, as I expected it would be. In either case, the image (courtesy of phys.org) can be seen here:

Spoiler

Top left: simulation of Sgr A* at 86 GHz. Top right: simulation with added effects of scattering. Bottom right: scattered image from the observations, how Sgr A* appears in the sky. Bottom left: the unscattered image, after removing the effects of scattering in our line of sight, revealing how Sgr A* actually looks. Credit: S. Issaoun, M. Mościbrodzka, Radboud University/ M. D. Johnson, CfA

 

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Something went wrong with the link. I guess it should go there: https://iopscience.iop.org/article/10.3847/1538-4357/aaf732/meta

Promising outlook for the Event Horizon Telescope:

and we show that refractive scattering effects are likely to be weak for images of Sgr A* at 1.3 mm with the Event Horizon Telescope

 

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Not sure why my links keep dying, this is the third time now in the last few days; it only seems to occur when using the Rich Text Editor instead of typing out the BBcode.

In any case yeah, I linked to the arxiv version originally.

Having said that, is it just me or does the simulation differ quite a bit from the unscattered image?

 

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The actual data doesn't have the required resolution to look as nice as the simulation. The Event Horizon telescope picture should look better - but don't expect anything as good as the simulation picture.