Follow-up work has revealed the magnetic field in this accretion disk, as well as details of the highly accelerated jet of matter shooting out at a decent fraction of the speed of light.
But there’s even more to glean from the data: A team of astronomers has just released an image showing the black hole’s photon ring, a narrow ring of light that literally comes from photons orbiting the hole. black before escaping into space. [link to paper].
Black holes are super weird.
Photo: Broderick et al. 2022, ApJ, 935, 61
The black hole in question is M87* (“star M 87”), a supermassive monster at the center of the equally huge neighboring elliptical galaxy M87, which lies about 55 million light-years away from us. We have known about this black hole for decades; M87 is what is called an active galaxy, where a large amount of energy is being expelled from a very small region of its core. This is due to matter falling from the galaxy onto the black hole. As it does, it accumulates outside the event horizon – the point of no return – into a flat disk called the accretion disk. The material becomes incredibly hot due to friction, reaching temperatures of millions of degrees. Material this hot glows brightly, including in high-energy X-rays, a sure sign that a black hole is powering it.
The original M87* image shows material in the accretion disk about 40 billion kilometers in diameter. It’s blurry, which isn’t very surprising given its great distance from us. The ring is incredibly small in our sky, only 40 microseconds of arc. An arcsecond is an angular measurement, where 3600 of them make a degree; the full moon is about 1,800 arc seconds across our sky, or 45 million times larger than the ring of M87*!
The very first image of the “shadow” of a supermassive black hole. This shows the region around a black hole with a mass of 6.5 billion times that of the Sun, located 55 million light-years from Earth at the heart of the galaxy M87. Credit: NSF
But there is more to this. In the image we actually see the combination of two things. One is matter swirling around the black hole, which is expanding; i.e. not very sharp. The other is the photon ring, a narrow, sharp ring of light caused by the black hole’s intense gravity. As light passes near the event horizon, its path is bent by the gravitational pull of the black hole. Because that’s what a lens does, this effect is called gravitational lensing, and it’s no surprise that it gets stronger the closer you get to a black hole.
Light entering at right angles is bent so much that it is sent towards us, forming a rather fuzzy ring called the n=0 ring. But light approaching the black hole that passes just the right distance can actually orbit once before escaping. This is called the n=1 ring, and it is sharply defined. There are other rings where the photons circle twice or more before leaving, which have a diameter a little smaller than the n=1 ring; these photons must approach the black hole more closely to be constrained enough to orbit around it several times.
An annotated version of a black hole simulation explains the different parts of this bizarre object. Credit: NASA Goddard Space Flight Center/Jeremy Schnittman
Theoretical models show that up to 30% of the light in the original image comes from the n=1 ring. The problem is that this light is mixed with the more extensive light. How to separate them?
The astronomers used a method that used observations from the telescope, models of what emission from a ring and an accretion disk should look like using general relativity – necessary because of the very strong region gravity around the black hole – and the very complex mathematics of how ultrahot matter flows, called magnetohydrodynamics. This showed what the two handsets looked like, then they used Bayesian statistics to tell the two apart from each other. It is a kind of statistic that allows you to use prior knowledge of a system to predict how it will behave.
In the end, they were able to separate the two signals, showing the narrow photon ring as well as the more extensive emission. This is the first time this has been done. They even showed that the ring is stable and relatively unchanged over the seven days of observations, as expected, even when the light from the extended accretion disk changed as matter moved around the black hole.
And it’s not just a matter of idle curiosity. The size of the ring depends on the mass of the black hole. Working backwards, they find that the mass of M87* is 7.13 ± 0.39 billion times the mass of the Sun. Earlier measurements found it to be lower, closer to 6.5 billion, but the team believe the new figure is more accurate and, in fact, the biggest source of uncertainty now lies in the distance to M87. himself.
The elliptical galaxy M87 has a supermassive black hole at its core (much too small to see in this Hubble image) that powers a vast jet of matter screaming thousands of light-years away. Credit: NASA, ESA, D. Batcheldor and E. Perlman (Florida Institute of Technology), the Hubble Heritage team (STScI/AURA) and J. Biretta, W. Sparks and FD Macchetto (STScI)
Photo: NASA, ESA, D. Batcheldor and E. Perlman (Florida Institute of Technology), the Hubble Heritage team (STScI/AURA) and J. Biretta, W. Sparks and FD Macchetto (STScI)
Once they separated the extended emission, they saw something else amazing: it is consistent with what is expected from the very base of the matter jet projecting outward tens of thousands of miles away. per second. The theory states that the jet is focused by a complex process where the magnetic field of material in the inner accretion disk extracts energy from the black hole’s rotation, a source of immense power. This causes the jet itself to rotate, and the observed emission is consistent with the jet rotating clockwise from our vantage point, which is also consistent with previous results.
Mind you, this was all in the original data, but it took this new statistical technique to extract it and untangle the sources.
The Event Horizon Telescope is the gift that keeps on giving. In 2022, he took similar images of Sgr A*, the supermassive black hole at the center of our Milky Way. What treasures are also in this data? And what will we find if we point this world-sized telescope at other black holes anchoring the centers of other galaxies?
These kinds of observations were theoretical not so long ago. Now they’re reality, and we’re learning even more about what’s happening just above the infinite plunge into a hole in spacetime.