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.
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.
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.]
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.
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 EHT used its data to extract the degree of polarization in the radio waves from M87’s black hole. This provides some information about the magnetic field around the accretion disk and jets.
- This in turn has given preliminary indications that the material in the accretion disk is pushed around by the rotating magnetic field [“MAD”], rather than the other way round [“SANE”]. If true, this would likely imply (see the figure below) that
- The accretion disk is probably relatively thin; and
- What is seen in the EHT image likely comes directly from the disk, and not from the regions where the the disk and jet intersect/interact (informally called the “funnel”).
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.