# Tag Info

83

I was surprised too when I first heard they were trying to image M87's black hole. The short answer is because it's really, really big. It is 1500 times bigger (diameter) than our Sagittarius A*, and 2100 times farther away. This makes its apparent size about 70% of that of Sgr A*, which they are also attempting to image. A cursory search of wikipedia's ...

41

There was a mention of Sagittarius A* during the Q+A portion of the press conference; the team indicated that they hope to produce an image sometime in the future (although they were careful to make no promises, and they're not assuming they'll be successful). That said, I'm not wholly surprised that we ended up seeing M87, rather than Sgr A*, for a couple ...

37

No, you aren't seeing the shape of the accretion disk. Although its plane is almost that of the picture it is far larger and fainter than the ring that is seen. The reason for this asymmetry is almost entirely due to Doppler beaming and boosting of radiation arising in matter travelling at relativistic speeds very close to the black hole. This in turn is ...

34

There are a few criteria necessary to see a black hole with the Event Horizon Telescope. They are, in importance: Active Feeding: you need a thick accretion disk with lots of matter accreting onto the black hole. M87 fits this criteria, and is a glut, consuming about 90 Earth masses a day. Apparent size. Even though it is 53 million light-years away, M87 ...

21

I've found an explanation in Dutch here by Heino Falcke, one of the EHT founders. Translation: Hard to photograph It was easiest to take a picture of M87. "It is very difficult to photograph the black hole in our Milky Way, because the material around it moves very fast: the vortex rotates around its axis in 20 minutes. Compare it to a toddler who has ...

20

As Ingolifs says, Sgr A* and M87* are the obvious candidates. At the press conference, Heino Falcke explained why they got a picture of M87* first: But it would take some more time because Sagittarius A Star is 1000 times faster and smaller. Its like a toddler who is moving constantly. In comparison, M87 is much slower, like a big bear. — The ...

16

No, it would not, because it operates in the visible spectrum and the EHT is an array of radio telescopes. For the "very long baseline interferometry" technique to work, all the telescopes have to be operating at the same wavelength, because combining the signals involves measuring exactly how well the peaks and troughs of the radio waves from the ...

15

Another quick note - They are trying to get a photo of Sag. A*: From Space.com The project has been scrutinizing two black holes — the M87 behemoth, which harbors about 6.5 billion times the mass of Earth's sun, and our own Milky Way galaxy's central black hole, known as Sagittarius A*. This latter object, while still a supermassive black hole, is a runt ...

10

There's some recent information which is worthy of an update to the answer (despite the difficulty of typing MathJax on my phone). I've quoted minimally as I wouldn't have improved upon what these scientists have published. Previous edits remain beneath this addition. In the paper "Measurement of the spin of the M87 black hole from its observed twisted ...

9

I believe we are seeing one of the effects of the accretion disk rotating at very high speeds. This is called relativistic beaming, and it occurs because particles (in this case matter in the accretion disk) that are travelling at relativistic speeds (say, upwards of .2c), tend to preferentially emit their radiation in a cone towards the direction of motion. ...

8

As already explained in John Rennie's answer, the effective potential of the photon has a maximum at the photon sphere ($r = 1.5 r_s$). It can be shown that, in Newtonian mechanics, the effective potential of a spherical distribution of matter presents always one minimum and cannot have any maximum. This may be the reason why the orbits of the photon look so ...

7

One of the things that can be confusing for beginners to GR is that there is no physical significance to the choice of coordinates we make. For example when studying static black holes we can use Schwarzschild coordinates, isotropic coordinates, Gullstrand-Painlevé coordinates, Eddington-Finkelstein coordinates, Kruskal-Szekeres coordinates and lots of ...

6

The picture is of the central region of M87, taken at a wavelength where the gas is "optically thin". The ring of bright light is pretty much exactly where it is expected to be for the synchrotron radiation emitted by the hot gas to have been gravitationally lensed by a black hole with the same mass as deduced previously from the motion of stars close to ...

5

The jet is projected onto the image roughly in the E-W direction with the (main) jet coming towards us to the right of the black hole. The brightening in the ring is perpendicular to this. It is probably caused by the spin of the black hole (aligned with the jet), which drags emitting material around and Doppler boosts the emission seen to the south. This ...

5

How about Betelgeuse? Betelgeuse is about 640 light years away compared to 54 million light years for M87. It has an angular diameter of 0.042 to 0.056 arc seconds while the quoted resolution of the EHT was 0.000025 arc seconds so you'd expect some detail on its surface. Betelgeuse seems to be undergoing a number of rapid changes right now. It is a young ...

5

Presumably, yes, EHT observations could improve on existing radio observations of Betelgeuse (e.g. recent ALMA images and comparatively ancient VLA images). Any observations would likely be targeted at known photospheric mm or super-mm emission from the star, mapping radius and temperature changes. The ALMA data showed the existence of a spot $\sim1000$ K ...

4

How, precisely, do radio astronomers detect (and record) the phases of waves for interferometry? The popular press always talks about directly 'interfering' two waves as they come in, but can they tell the exact phase of a single wave? tl;dr: It's a good question. There is no such thing as "the exact phase of a single wave", it's only the phase ...

4

I cant see a definitive statement, but I can see some infiormation and make some reasonable suggestions: The two colours represent the two different collaborations. Blue for the EHT, yellow for the GMVA. We can see this by following the links on the source page of the image. The GMVA page says: Participating Stations: in Europe: Effelsberg (100m (Ef)), ...

4

If we could assume that most of the mm-wave emission from an ordinary star is photospheric, then the EHT could make a massive contribution to measuring the radii of stars. At the moment, this fundamental property can only be measured for stars in short-period eclipsing binaries or for a small set of nearby stars and more distant giant stars using infrared ...

4

Today, April 10th 2019, there was a press conference where finally an image of M87 was released: Scientists have obtained the first image of a black hole, using Event Horizon Telescope observations of the center of the galaxy M87. The image shows a bright ring formed as light bends in the intense gravity around a black hole that is 6.5 billion times more ...

3

The first error is due to the precision of the measurement, the second is an uncertainty associated with methodology used.

3

The question asks about "targets" but doesn't specify black holes. However the other "targets" are related to infall of black holes. Nonetheless, I think the answer is a solid "yes" either way. Data is recorded separately at each site. Hours and hours of digitization of one or more 2 GHz wide data channels around 230 GHz, down converted to baseband and ...

3

The Milky Way's central supermassive black hole (SMBH) is feeding, albeit at a very low level. Radio emission from the accretion disk (and/or weak jets) is responsible for the long-lived "Sgr A*" radio source. Here is a paper from 2000 (Falcke et al.) arguing that VLBI (as used by the Event Horizon Telescope) should be able to image the "black ...

3

Consider an extremely transparent lens. If you photograph the lens, what the camera really picks up where the lens covers is the distorted image of what is behind the lens. Would you say that is still a photograph of the lens? I would say yes. If you take a picture of an object coated with Vantablack, the amount of light entering the camera from that object ...

2

From whence is the Event Horizon Telescope black hole data available for amateur reconstruction? So I'm wondering what kind of data is available that would allow someone to reconstruct their own EHT image? In the CalTech video Imaging the Unseen: Taking the First Picture of a Black Hole - Katie Bouman - 6/7/2019 which I just found in this question, after ...

2

The resolution of an interferometer at a wavelength $\lambda$ is $\theta\sim\lambda/b$, where $b$ is the longest baseline (distance between two telescopes) in the array. According to the first of this week's six papers, with observations at $\lambda=1.3\text{ mm}$, the longest baseline was $b\approx10700\text{ km}$, giving us $\theta\sim25.1\mu\text{as}$. ...

2

Your top image is this from Dr Jean Lorre stock science images. Just a photo showing stars and the optical jet with a 22 MB CCD camera through a small telescope. It is unfortunate that he chose a false color brightness scale with black at the top intensity. But, that black region does not correspond to the true blackness in the interferometric image which ...

2

The recently announced M87 observations only used 8 of the 10 sites. This paper includes this diagram.Even the South Pole instrument seems to have been used only for calibratiom (the dashed lines).

2

I don't think it is necessary for all the telescopes to view the target simultaneously if you can make the assumption that the source you are observing is only varying on timescales longer than it takes the source to be viewable by all the telescopes. Sure, the maximum baseline you can get will be defined by the most distant pair of simultaneously observing ...

1

This neewpaper article appears to reflect new information from the project. The observations, by the Event Horizon Telescope, are expected to be unveiled in March . . . The team is in the final phase of reviewing data that was gathered in 2017 . . . Nothing there about the 2018 data.

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