A black hole in a new light

Nineteen observatories worldwide are carefully examining the centre of the giant galaxy M 87 at all possible wavelengths

The black hole at the heart of the giant galaxy Messier 87 is the focus of research. Interest in the object called M87* has been increasing since the first image was published in April 2019. Astronomers are particularly interested in its immediate surroundings. This is because the powerful gravity of this humongous mass drives a high-energy jet that blows particles over huge distances into space at almost the speed of light. Researchers have now published the measurement data from 19 observatories; these shed further light onto M87*. The Max Planck Institute for Radio Astronomy is substantially involved in the project; among other things, it carries out observations with the 100-metre antenna in Effelsberg.

Gigantic black holes produce jets, which are tightly bundled beams of energy and matter. In the case of M87*, these jets extend at least 100,000 light years into space. In doing so, they emit radiation that spans the entire range of the electromagnetic spectrum – from the long-wave radio range to the extremely short-wave gamma range. The pattern of this radiation is different for each black hole and thus provides insights into its properties.

However, the pattern changes over time, thereby making observations difficult. During the measurements with the Event Horizon Telescope (EHT), which provided the first image of M87*, the researchers compensated for such variations by coordinating observations with many of the world’s most powerful telescopes on the ground and in space. This “multi-frequency astronomy” captured radiation across the entire electromagnetic spectrum. It was the most extensive simultaneous measurement campaign to date for a super-massive black hole and its jet.

“This unique data set is crucial for understanding the physical conditions in the immediate surroundings of one of the most massive black holes in our cosmic neighbourhood”, says Stefanie Komossa. The researcher at the Max Planck Institute for Radio Astronomy in Bonn is a team member of the supporting observations of the EHT and one of the lead authors of the current publication in The Astrophysical Journal Letters.

“The combination of radio data with near-real-time measurements at other wavelengths such as near-infrared, visible light, and X-rays and gamma rays provides a huge pool of data for a detailed picture of the physical processes taking place near the black hole and in the launch region of the jet”, adds Komossa’s colleague Thomas P. Krichbaum, also a researcher at the Institute in Bonn and a member of the EHT scientific council.

Two scientists from the Max Planck Institute for Physics also participated in the study. They led the observations of the Magic telescopes, which, together with two others (H.E.S.S. and Veritas), recorded gamma rays in the highest energy range. “One of the goals of the joint observation was to study the focused, high-energy radiation that active black holes hurl into space as jets traveling at the speed of light,” says Alexander Hahn, a PhD student at the Max Planck Institute for Physics.

In their current study, the scientists have released the huge dataset and made it accessible to all interested parties. “Several groups are already investigating whether their models match this extensive set of observations”, says Daryl Haggard of McGill University. She is thrilled that the entire community is now helping to create a better understanding of the close connections between black holes and their jets.

The initial results show that the amount of electromagnetic radiation produced by the matter around M87* is the lowest ever observed. This fact results in ideal conditions for studying the black hole, especially the regions near the event horizon, where the signals encounter fewer obstacles on their way out.

The researchers also plan to improve the tests for Einstein’s general theory of relativity. Such experiments require extremely strong gravitational fields – such as those generated by super-massive black holes. However, the astronomers do not yet know exactly what the material rotating around M87* and emitted in the jet is like. The physical properties of the matter that determine the light emitted also remain a mystery.

“Understanding the particle acceleration is central to our interpretation of both the EHT image and the jet”, says Sera Markoff of the University of Amsterdam. “Because this jet manages to transport the energy released by the black hole to distances larger than the host galaxy – like a giant power cable”. The data now being collected will help researchers to calculate the amount of energy transported and thus the effect that the black hole jet exerts on its surroundings.

The publication coincides with the current EHT observation run, which once again uses a worldwide network of telescopes. This week, the astronomers plan to spend six nights examining the galaxy M87 as well as Sagittarius A*, the super-massive black hole at the centre of the Milky Way.

“In this observation campaign, many telescopes on the Earth and in space have joined forces with the EHT collaboration in order to jointly and simultaneously study the properties of M87* across the electromagnetic spectrum”, says Anton Zensus, founding chairman of the Event Horizon Telescope and Director at the Max Planck Institute for Radio Astronomy. The magnetic fields, cosmic rays, jet structure, and emission and absorption processes as well as the role of general relativity can thus be investigated in greater detail.


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