In April 2019, the first photo of a black hole caused a worldwide sensation. Now the astronomers of the Event Horizon Telescope Collaboration have stepped up: Comparative analyzes with images dating back a good ten years show that the bright ring around the supermassive black hole M87 * retains its shape, as predicted by the theory of relativity. But over time, the bright ring seems to wobble slightly. This suggests turbulent flows of matter on the black hole’s event horizon and could help refine current models of such gravity giants, the researchers say.
The Event Horizon Telescope (EHT) is a global, virtual alliance of several powerful radio telescopes – from Greenland to the South Pole. For a few days in 2017, these antennas pointed synchronously at the supermassive black hole in the center of the galaxy M87. The coupling of the telescopes using the so-called Very Long Baseline Interferometry (VLBI) enabled astronomers to achieve the resolution of a virtual telescope with a diameter that corresponds to that of the earth. “With the incredible angular resolution of the EHT, we could watch a game of billiards on the moon,” explains first author Maciek Wielgus from the Center for Astrophysics | Harvard & Smithsonian.
This resolution made it possible for the first time to create a photo of a black hole. “The results presented in April 2019 result in the image of a black hole with two essential elements: a ring that shows the plasma swirling around M87 * and a dark inner area in which we suspect the event horizon of the black hole,” says Wielgus .
look in the past
But the 2019 photo only showed a snapshot of the black hole – the astronomers had only targeted M87 * for a few days. “That is far too short to see changes,” says Wielgus. “That’s why we asked ourselves whether this ring-shaped morphology would also be found in archive recordings. Would the ring show a similar size and orientation in them? ”For their investigation, the researchers evaluated data that had already been collected from 2009 to 2013 by the telescopes of the EHT network for M87 *. At that time, the network consisted of only three radio telescopes; the APEX telescope in Chile was added in 2013 as the fourth. Because the resolution of these telescopes was not enough for real photos either, the astronomers used geometric-based statistical modeling methods to reconstruct the appearance of M87 * and to look for changes over time.
The evaluations showed that the black hole M87 * looked similar to 2017 about ten years ago. Even then, there was already the dark shadow, surrounded by a light, slightly asymmetrical ring. The diameter of the shadow already agreed in 2009 with the predictions of Einstein’s general theory of relativity for a black hole of this size. “This is an important confirmation of the theoretical expectations,” says co-author Kazu Akiyama of the MIT Haystack Observatory. “The consistency, even across multiple observations, gives us more confidence than ever that we are right about the nature of M87 * and the origin of its shadow.” The asymmetrical light ring is evidently also stable over years.
The light ring wobbles
But there is also a surprise in the archive data. Because although the general shape and size of M87 * have remained the same, there have been dynamic changes: the light-colored ring changes its position very slightly over time – it wobbles. Astronomers attribute this to the gas circulating around the black hole, which heats up to billions of degrees, becomes plasma and then creates turbulence due to the magnetic fields present on the event horizon. This is the first time that these data reveal the behavior of matter so close to the event horizon of a black hole. “Because this influx of matter is turbulent, the bright ring seems to wobble over time,” explains Wielgus. Co-author Thomas Krichbaum from the Max Planck Institute for Radio Astronomy in Bonn adds: “This allows a first look at the flow of matter that hits the black hole and at its dynamics near the event horizon.”
As the astronomers explain, these observations of the dynamics of the black hole offer new opportunities to test existing theories and models. “Some of this matter is close enough to the black hole to be able to observe the effects of strong gravitational forces,” says co-author Richard Anantua from the Center for Astrophysics | Harvard & Smithsonian. “In some cases, we can also use it to test the predictions of the general theory of relativity more precisely.” But it can also be used to test current models of how matter behaves in the vicinity of a black hole. “For example, we see a lot of variation here – and not all theoretical models of accretion allow so much wobble,” says Wielgus. “This allows us to refute some of these models based on our observations.
For the astronomers in the EHT collaboration, this work is only just beginning: “In the coming years we want to investigate how the structure of M87 * changes over time. We are therefore currently analyzing the EHT data from 2018 and preparing the new observations for 2021, ”reports Anton Zensus, Director at the MPI for Radio Astronomy and founding member of the EHT collaboration. “Then three more telescopes will participate. Our virtual global telescope is therefore getting bigger and more sensitive and the imaging processes are becoming more precise. “
Source: Event Horizon Telescope, Max Planck Institute for Radio Astronomy, Harvard Center for Astrophysics; Technical article:, Astrophysical Journal, doi: 10.3847 / 1538-4357 / abac0d