The first historic image of a black hole unveiled last year has now been turned into a film. The short sequence of frames shows how the appearance of the black hole̵
The images show a lopsided patch of light swirling around the supermassive black hole at the center of the galaxy M87. To create them, the collaboration Event Horizon Telescope (EHT), which leverages a network of planetary observers, dug up old data about the black hole and combined it with an image-based mathematical model released in April 2019 to show how the surroundings have evolved over eight years. Although based in part on assumptions, the result gives astronomers a rich understanding of the behavior of black holes, whose intense gravity sucks up the matter and light around them.
“Because the flow of matter falling on a black hole is turbulent, we can see that the ring oscillates with time,” says lead author Maciek Wielgus, a radio astronomer at Harvard University in Cambridge, Massachusetts.
The work, which appeared on September 23 in The Astrophysical Journal1, offers a glimpse of what the team may be able to do in the near future as its techniques improve. “In a few years, it could really start to look like a movie,” says Wielgus.
The image of the black hole that the EHT collaboration unveiled last year made headlines around the world. It depicted M87 *, the supermassive black hole at the center of the galaxy M87, about 17 megaparsecs (55 million light years) away. The researchers constructed the image by combining radio frequency signals they had collected from observatories on Earth for two nights in April 2017. Their feat has been compared to resolving a donut shape on the surface of the Moon from Earth.
Although blurry, the image matched Albert Einstein’s general theory of relativity predictions of what the immediate proximity of a black hole should look like. Specifically, it provided researchers with the first direct evidence of the shadow of an event horizon, the surface of “no return” that separates a black hole from its surroundings. This darker disc was placed against a ring of light emitted by superheated matter just outside the event horizon.
Surprisingly, one side of the ring appeared brighter. This was expected, due to a combination of effects in the complex dynamics around a black hole. In particular, matter falling into a vacuum should spiral out of the black hole’s equator at high speed, forming what astrophysicists call the accretion disk. The lopsided gaze is partly due to the Doppler effect: on the side of the disk that rotates towards the observer, the movement of matter increases the radiation making it appear brighter; the opposite happens on the elusive side.
Based on these findings, Wielgus wanted to go back and look at the older data from the EHT telescopes to see if he could reinterpret them, using the 2017 image as a guide. The EHT had been observing M87 * since 2009, initially using telescopes in only three positions. As the team added more observers to the EHT network, the quality of the observations improved. In 2017, the collaboration involved eight observers who crossed the globe from Hawaii and Chile to Europe, reaching the level at which the EHT could produce a real image for the first time.
The oldest data consisted of four batches, collected in 2009, 2011, 2012 and 2013, two of which remained unpublished. “To some extent, they have been forgotten, because everyone was extremely excited about the 2017 data,” says Wielgus. With a group of other EHT researchers, he reanalyzed the data and found it consistent with the 2017 campaign results, including the presence of a dark disk and a bright ring. And although, by themselves, the 2009-2013 data batches did not have sufficient resolution to produce images, the team was able to generate synthetic images for each of the years by combining the limited data available with a mathematical model of the black hole constructed. gives 2017 data.
And the results turned out to contain more information than Wielgus expected. Like the 2017 photo, they revealed that one side of the ring was brighter than the other, but the bright spot has shifted. This could be because different regions of the accretion disk have become brighter or dimmer, which may increase or sometimes even cancel Doppler brightness.
This was not unexpected, the authors say: although the black hole M87 * itself does not change from year to year, the surrounding environment changes. On a scale of several weeks, strong magnetic fields are expected to agitate the accretion disk and produce hotter spots that then orbit the black hole. In 2018, a separate team reported evidence of a drop of hot gas surrounding Sagittarius A *, the Milky Way’s central black hole, over the course of about 1 hour. Since M87 *, at 6.5 billion times the mass of the Sun, is more than 1,000 times the size of Sagittarius A *, the dynamics around M87 * take longer to develop.
The EHT collaboration attempts to observe M87 * and Sagittarius A * every year, in late March or early April. This is the time when weather conditions are most likely to be good simultaneously at the numerous sites in its network. The 2020 campaign was halted due to restrictions due to the COVID-19 pandemic, but the team hopes to have another chance in 2021. Hopefully, other observers, including one in Greenland and one in France, will will join the effort.
The team also hopes that next year’s campaign will include its first global observations using shorter wavelength radiation. Although it is more difficult to see through the Earth’s atmosphere, this would improve the resolution of EHT images. “We would get even closer to the shadow of the black hole and get sharper images,” says Sara Issaoun, EHT member, radio astronomer at Radboud University in Nijmegen, the Netherlands.