A global array of telescopes reveals details on event horizon scales in Sgr A* | Hacker News Reader

A global array of telescopes reveals details on event horizon scales in Sgr A*

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132 pointsokket24 comments

The angular resolution is mind-blowing. Combining these telescopes gives us resolutions on the order of the orbit of Mercury around the sun, at a range of our distance from the center of the galaxy. If the Earth were the size of our eyes, it would be like seeing objects of diameter 11.5 meters (37.7 feet) from about 7622 km (4736 mi) away. Almost like seeing a small apartment building in London from Los Angeles, with the naked eye.

> The increased angular resolution provided by the APEX telescope now reveals details in the asymmetric and not point-like source structure, which are as small as 36 million km. This corresponds to dimensions that are only 3 times larger than the hypothetical size of the black hole (3 Schwarzschild radii).

> “It reveals details in the central radio source which are smaller than the expected size of the accretion disk”, adds Thomas Krichbaum, initiator of the mm-VLBI observations with APEX.

Synthesizing with Wikipedia:

> The current highest-resolution measurement, made at a wavelength of 1.3 mm, indicated an angular diameter for the source of 37 μas.[12] At a distance of 26,000 light-years, this yields a diameter of 44 million kilometers. For comparison, Earth is 150 million kilometers from the Sun, and Mercury is 46 million kilometers from the Sun at perihelion. The proper motion of Sgr A* is approximately −2.70 mas per year for the right ascension and −5.6 mas per year for the declination.[13][0]

[0] https://en.wikipedia.org/wiki/Sagittarius_A*

Diameter

  Earth 12,756,000 m
  Eye   0.025 m
  Ratio 510240000x

The best part is that they have to know the alignment the telescopes within .5 mm of each other, across thousands of miles.

  Distances between locations:
  Atacama, Argentina - Mauna Kea, HI  - 6544 mi - 10532 km
  Atacama, Argentina - Bishop, CA     - 5305 mi -  8358 km
  Atacama, Argentina - Mt. Graham, Az - 4742 mi -  7632 km
  Mauna Kea, HI      - Mt. Graham, Az - 2936 mi -  4725 km
  Mauna Kea, HI      - Bishop, CA     - 2531 mi -  4073 km
  Mt. Graham, Az     - Bishop, CA     -  581 mi -   935 km

Astounding. How can they do that?

Maths and computers.

More precisely, motors and precision position encoders.

Imagine if we put some radio telescopes on Mars and ran a baseline between Mars & Earth.

We'd get the largest baseline when the Sun is between the planets. This of course blocks interplanetary communication.

Another commenter mentions the 0.5 mm precision needed[0] - how would we overcome the communications issue to get an accurate measurement?

[0]https://news.ycombinator.com/item?id=17171476

I imagine the same way the article's array does: with an (interplanetary) version of sneakernet [0], taking independent measurements which are then delivered to be correlated in one place.

I think you could keep atomic clocks on Mars and Earth in sync with extremely accurate calculation of the Earth-Mars distance at any time, and thus the propagation delay between them.

EDIT: only one star!!

[0] https://en.wikipedia.org/wiki/Event_Horizon_Telescope#/media...

How long of a time would the Sun be blocking the Earth from Mars?

Nothing new here, the data is from 2013 and this paper is probably just APEX getting some ”longest baseline” fame in before the real results from the 2017 run, with a telescope on the south pole, relegate them to the ungloriuos support role.

For an actual update about the event horizon telescope, see here: https://eventhorizontelescope.org/blog/eht-status-update-may...

Can someone please explain simply?

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.

Data able to resolve the event horizon was taken last year, and analysis of it has already started.

Probably that will be done before the end of the year, so realizing this pressrelease is about 2013 data was pretty disappointing.

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.

0) http://chandra.harvard.edu/photo/2007/gcle/

An introduction from Science, March 2017:

http://www.sciencemag.org/news/2017/03/global-telescope-may-...

"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):

http://iopscience.iop.org/article/10.3847/1538-4357/aabe2e

From the abstract:

"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.”

---------

1) Another approachable introduction to what is being done, with a lot of illustrations, is here:

https://medium.com/starts-with-a-bang/2018-will-be-the-year-...

2) 2015 collected data, 2016 presented

journal paper: http://iopscience.iop.org/article/10.3847/1538-4357/aabe2e/m...

And the preprint: https://arxiv.org/abs/1805.09223

Thanks!

Man, what a dissapointment! I thought the EHT was done with their analysis of the 2017 run, but its just apax getting another publication out of old data.

The real beauty of Very Long Baseline Interferometry is that different sources have different noise thus a common signal of large order of magnitude smaller than the noise can be identified.

The original link is HTTPS with a self-signed certificate. Here's the same story from another source: https://phys.org/news/2018-05-apex-glimpse-heart-darkness.ht...

It's not a self signed cert.

Whoops, my bad. I had disabled that CA (T-TeleSec GlobalRoot Class 2) in Firefox at one point, along with all of the others, as an experiment to see how many are really needed. I haven't had to re-enable one in ages.

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