1. They are ganging multiple radio telescopes to synthetically create a huge telescope with an aperture of 10,000 km. That gives them enormously good resolution (for a radio telescope). And using that to look at the giant black hole at the center of our galaxy. (Sagittarius A)
2. There are limitations because the number of telescopes are small. That makes the data they've collected somewhat ambiguous.
3. As they add more radio telescopes they should get a clearer picture.
4. Currently they can resolve to about 3 times the predicted 'radius' of Sagittarius A
5. Ultimately I think they want to be able to actually resolve Sagittarius A*. And they are getting close.
Resolution for telescopes is ~size/wavelength. But there's lots of dust between us and the center of our galaxy, so you can't see it with visible light. Typical options are x-ray to gamma radiation, and long IR to microwave. To get decent resolution with microwave, you need ~10^4 km telescopes. This array approximates that.
Anyway, they're fitting their data to models for source shape. They argue that it's neither a point source, nor a uniform source of size expected for the accretion disk.
Bottom line, I think, it's probably hot spots. Our black hole is fortunately rather quiet. We've observed pulses of x-ray radiation from accreting stuff, scattered by gas clouds near the galactic center.[0] Also fortunately, the black hole's axis doesn't point near us, so we're relatively safe from getting fried.
"Last year [2015-2016 (2)] researchers "heard" black holes for the first time, when they detected the gravitational waves unleashed as two of them crashed together and merged. Now, they want to see a black hole, or at least its silhouette. Next month, astronomers will harness radio telescopes across the globe to create the equivalent of a single Earth-spanning dish—an instrument powerful enough, they hope, to image black holes backlit by the incandescent gas swirling around them. Their targets are the supermassive black hole at the heart of our Milky Way galaxy, known as Sagittarius A* (Sgr A*), and an even bigger one in the neighboring galaxy M87."
(Also see (1))
Now the article in the current main HN link talks about and links to the work just published (2018 May 24, and open access):
"The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data."
Translated, approximately: "what we see when we analyze the data is something that probably isn't symmetric in all directions and can be either disk-like or has jets."
The humanity is "seeing" (using the black hole in the center of our galaxy for the first time! We are looking (combining the radio telescopes across the world as a single telescope and using complex calculations to calculate the results) at the "event horizon" of the black hole in the center of our galaxy, the place from where nothing escapes!
And the first picture is blurry but hints on "not point-like." (1)
And the current article ends:
“The results are an important step to ongoing development of the Event Horizon Telescope”, says Sheperd Doeleman from the Harvard-Smithsonian Center for Astrophysics and director of the EHT project. “The analysis of new observations, which since 2017 also include ALMA, will bring us another step closer to imaging the black hole in the centre of our Galaxy.”
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1) Another approachable introduction to what is being done, with a lot of illustrations, is here:
