@csm's answer to Why not take a picture of a closer black hole? points out that it's necessary for the supermassive black hole at the center of a galaxy to be actively feeding for it to generate a radio-bright accretion disk that we can image. M87 is always feeding, but our own black hole only nibbles occasionally as a dust cloud passes by.

Question: How will they know when to start taking the picture of the black hole at the center of the Milky Way? Are there bits of food being tracked and they'll have all the EHT's telescopes ready when accretion begins? Or is it long enough that once it starts they can shuffle observing time around and still collect sufficient data?

For background see

The ESA video ESOcast 173: First Successful Test of Einstein’s General Relativity Near Supermassive Black Hole includes a clip of images of stars at the center of our galaxy orbiting around SgrA*, a presumed supermassive black hole.

GIF made from video at around 02:50:

flashing at the center of the Milky Way galaxy and SgrA*

Six annotated frames from GIF highlighting the flashing that I'm seeing.

flares? from Sgr A*


1 Answer 1


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 hole shadow", based on the known sub-mm and mm-wave emission. And in fact the EHT has been observing the Milky Way's SMBH.

As I understand it, the real reason we haven't seen a formal, published detection of the Milky Way's SMBH by the EHT is that its emission is highly variable on short time scales (e.g., minutes to hours). In the case of M87's SMBH, the variability of the (sub-mm and mm-wave) emission is slow (days to weeks), so they could combine observations taken over several hours and two nights in April of 2017 under the assumption that it was all of the same static configuration. Figuring out how to properly account for the short-term variability of the Milky Way's SMBH emission is much more difficult, which is why the (relatively) easier case of M87 was solved and published first.

See also Rob Jeffries' answer to this physics.stackexchange question.

Edited to add: Unfortunately, I don't think there's any validity to the idea that we can track incoming "food" and predict future accretion flares for the Sgr A* SMBH with any useful accuracy. There was some excitement a few years ago when a group reported detection of an apparent gas cloud ("G2") on an orbit that would take it in to about 2000 Schwarzschild radii from the SMBH at pericenter (in 2014), possibly allowing it to be tidally shredded and increasing the accretion rate. But as a review article published in 2013 pointed out, "The actual free-fall time scale from ∼2000 $R_s$ is roughly one month and the viscous time scale could be anywhere between months up to hundred years depending on the viscosity parameter $\alpha$."

And in fact the actual pericenter passage produced... nothing much at all. There's a discussion of the "fizzle" here: "with the majority of the simulation parameters used, only 3–21% of the material Sgr A* accreted from 0–5 years after periapsis is from the cloud".

So in the one case where potential "food" was identified and tracked, one couldn't be sure in advance whether the possible increased accretion would happen on timescales of months to years, and so far nothing significant has happened. I very much doubt the EHT team is basing their observing schedule on this kind of thing.

  • $\begingroup$ Oh I see what happened; when I copied material from the companion question Will the first Event Horizon Telescope image of the Milky Way's black hole be just another orange donut? to here I left out the part mentioning the minute-scale spatial variability. What I'd meant to suggest is that in order to image quickly enough that the pattern in the disk stays still, they would have to wait for some in-falling matter to be bright enough. A longer "exposure" based on background levels of feeding might still show the donut, but smooth not lumpy? $\endgroup$
    – uhoh
    Oct 15, 2020 at 13:00
  • $\begingroup$ So the answer I'm hoping for is how they will know when to observe when an unusual accretion event happens, i.e. when it "blinks". Thus "Are there bits of food being tracked and they'll have all the EHT's telescopes ready when accretion begins? Or is it long enough that once it starts they can shuffle observing time around and still collect sufficient data? It's that kind of observation that I'm asking about here. $\endgroup$
    – uhoh
    Oct 15, 2020 at 13:02
  • $\begingroup$ They've been observing the MW SMBH as part of their general campaign (along with M87 and a few other sources), so they're clearly not waiting for "accretion events". $\endgroup$ Oct 17, 2020 at 12:05
  • $\begingroup$ Well I haven't seen press releases with images, so clearly they are indeed waiting for something! And testing, algorithm development and hunting for appropriate calibration sources and other preparations can all be done before the big event, so I don't think one can say conclusively that no waiting is taking place. $\endgroup$
    – uhoh
    Oct 17, 2020 at 12:34
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    $\begingroup$ @uhoh I've added some text explaining why "waiting for a predicted accretion flare" is probably not at all a feasible thing. (What the EHT team is clearly "waiting" for with respect to Sgr A* is seeing if they can figure out a way to extract plausible images from the data they've already taken.) $\endgroup$ Oct 17, 2020 at 12:54

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