Of Particular Significance

Black Hole Announcement Expected Thursday

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 05/10/2022

In 2019, the first image of the surroundings of a black hole was produced, to great fanfare, by the astronomers at the Event Horizon Telescope (EHT). The black hole in question was the enormous one at the center of the galaxy M87. ... We will learn new things from EHT about our own galaxy's black hole on Thursday morning.

In 2019, the first image of the surroundings of a black hole was produced, to great fanfare, by the astronomers at the Event Horizon Telescope (EHT). The black hole in question was the enormous one at the center of the galaxy M87.

The “image” of the surroundings of a black hole in galaxy M87. What does it actually show? It is most likely an image (in radio waves) of an “accretion disk” of material around the black hole, its radio emissions somewhat distorted by the warped geometry around the black hole.

At the time, there was also hope that the EHT would produce an image of the region around the black hole at the center of our own galaxy, the Milky Way. That black hole is thousands of times smaller, but also thousands of times closer, than the one in M87, and so appears about the same size on the sky (just as the Moon and Sun appear the same size, despite the Sun being much further away.)

However, the measurements of the Milky Way’s black hole proved somewhat more challenging, precisely because it is smaller. EHT takes about a day to gather the information needed for an image. M87’s black hole is so large that it takes days and weeks for it to change substantially — even light takes many days to cross from one side of the accretion disk to the other — so EHT’s image is like a short-exposure photo and the image of M87 is relatively clear. But the Milky Way’s galaxy’s black hole can change on the times scale of minutes and hours, so EHT is making a long-exposure image, somewhat like taking a 1-second exposure of a tree on a windy day. Things get blurred out, and it can be difficult to determine the true shape of what was captured in the image.

Apparently, the EHT scientists have now met these challenges, at least in part. We will learn new things about our own galaxy’s black hole on Thursday morning; links to the press conferences are here.

In preparation for Thursday, you might find my non-expert’s guide to a black hole “silhouette” useful. This was written just before the 2019 announcement, when we didn’t yet know what EHT’s first image would show. The title is a double-entendre, because I myself wasn’t entirely expert yet when I wrote it. The vast majority of it, however, is correct, so I still recommend it if you want to be prepared for Thursday’s presentation.

The only thing that’s not correct in the guide (and the offending sections are clearly marked as such) are the statements about the “photon ring”. It took me until my third follow-up post, two months later, to get it straight; that post is accurate, but it is long and very detailed. Most readers probably won’t want to go into that much detail, so what I’ll do here is summarize the correct parts of what I wrote in the weeks following the announcement, repeating a few of the figures that I made at the time, and then tell you about a couple of new things that have been learned since then about M87’s black hole. Hopefully you’ll find this both interesting on its own and useful for Thursday.

A first thing to know about the M87 black hole is that (as we believe to be true for most black holes with matter falling onto them) it has an accretion disk and jets. These happens to be oriented with one of the jets pointing nearly at us; see the figure below. (The picture at left is schematic; the one at right is more to scale, showing the jets more accurately, but may be harder to parse.) The jets presumably point along the axis around which the black hole is spinning.

(Left) Cartoon of the region near the M87 black hole, with an accretion disk of matter spiraling in toward the black hole, and two jets of material being ejected out in opposite directions, presumably along the spin axis of the black hole. It is believed the jets point nearly toward and away from us, as shown. (Right) A somewhat more accurate but harder to visualize cartoon, in which the jets are shown as being as nearly as wide as the disk. The precise correspondence between the cartoon and the actual image is complex; we don’t know exactly which regions glow brightest, and then the radio waves from these regions are warped by the black hole’s geometry (which depends on its mass and spin) and blurred by the limitations of the Event Horizon Telescope.

At the time of the M87 announcement, there were a lot of claims that the image showed the “photon ring” around the black hole, and the dark region between could be used to make a precise estimate of the black hole’s mass. Although I quoted some of these claims in my early posts, they turned out to be badly misleading. I discussed this in the my third follow-up blog post, “A ring of controversy.” The post starts with a relatively short overview, if you just want a brief sketch; then there follows a detailed discussion, with a careful explanation as to where the photon ring comes from, and why, nevertheless, the image that EHT produced doesn’t actually show it. Today I’ll give you a very quick summary of the conclusions.

The photon ring arises from the effect described in the figure below, in the approximation that we are looking straight at one of the poles of the black hole. [This is almost true for M87 but may not be true at all for our galaxy’s black hole.]

(Left) Each part of the disk emits radio waves which both create a direct image [orange band], broadened by gravity to appear larger than the true size of the disk, and an indirect image on the other side of the black hole [green band] which is focused by the black hole’s gravity to be much narrower. (Right) Were the EHT a perfect imager, it would see the full accretion disk from the direct image, and a bright ring (the “photon ring”) from the indirect image. The black hole itself lies inside the dark region, but the dark region, too, appears larger than its true size.

Roughly speaking, the punchline for the M87 image is summarized in the figure below. The photon ring, which reflects details of the black hole’s geometry, would be dramatic in a perfect image, but with the blur that EHT introduces, it is swamped in the glare of the accretion disk itself.

For non-rotating (top) and fast-rotating (bottom) black holes viewed along the rotation axis, showing perfect measurements (left) and realistic ones (right). Imperfect measurements blur the accretion disk and the photon ring into a single blurred ring, dominated by the emission from the accretion disk, and with an inner edge that may lie well inside the photon ring. The M87 black hole image will differ slightly due to our observation point being somewhat off the black hole’s rotation axis.

From the size of the inner dark region in their image (and other information), the EHT folks were able to estimate the mass of the black hole with more accuracy than before.

[Measuring the mass won’t be EHT’s major goal for the Milky Way’s black hole, since we can already measure its mass precisely in other ways (e.g., by watching stars that orbit close to it, part of Andrea Ghez’s Nobel Prize-winning work). But we don’t know how our own galaxy’s black hole is oriented, or how fast it might be spinning. Naively we might expect that the accretion disk is in the same plane as the galaxy as a whole, and that the black hole rotates in the same direction as the galaxy does. However, this may not be the case. Maybe EHT can answer that this week.]

Meanwhile, there have been some developments since then that I didn’t cover. I’m not sure I know all of them, but here are a couple of important ones.

The polarization of the radio waves from M87’s black hole, represented (top) on its own, and (bottom) superposed on the corresponding black hole image, on four separate days of measurements in 2017. See this paper for details.
From this EHT paper, four simulations of the region near a black hole like M87’s; they differ in their assumptions about the magnetic field and the accretion disk’s properties. Each shows a slice through the accretion disk; the black hole is at left (solid half-circle), the disk is horizontal, and the jets are vertical on the image; the direction toward Earth is the green arrow. The bright areas indicate the regions that glow brightest in radio waves. The upper plots show simulations in which magnetic fields push the material [MAD] and thin disks emit the radio waves observed by EHT; these are relatively consistent with EHTs polarization measurement. The lower plots have weaker and less crucial magnetic fields [SANE] and are inconsistent with the polarization measurement. The left plot shows a simulation with emission from a thick accretion disk, while the right one shows one with emission from the region near the meeting of jet and disk.

If there’s more I should have mentioned, EHT experts should feel free to let me know.

I hope these remarks are useful to you in the run-up to Thursday! You can expect a post to follow the announcement, after I’ve had a chance to absorb it and look at the accompanying papers.

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22 Responses

  1. The image is out!  See this page: Exploring Black Holes – How are black holes studied? | Beta site for NSF – National Science Foundation David Schwartz160 W 71st St Apt 12HNew York, NY 10023

  2. Big day today, I hope they have some good information to give us a good direction to follow.

    Prof. Strassler, a simple question, is the BH defining the galaxy or vice versa? My point is, maybe there is nothing in the “BH”, NOTHING, nil, zero mass. So, here’s my conjecture based on that, the BH is a volume at the center of the accretion ring that would require radiation to travel at speeds faster than light, FTL, to get into the volume, hence nothing is in there.

    So, that would mean it’s the spinning galaxy that creates this center of rotation and conditions to create this truly “empty” space.

    On a separate issue, I think, please explain what’s going on at the origin of the Maxwell–Boltzmann statistics?

    Thank you.

  3. Since black holes radically change the direction of incoming light, theoretically, isn’t there an optical path starting at any given star, swinging around a black hole and ending in a telescope on earth?
    Is some portion of the light in the ring around a black hole, an image of every star in the universe?

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