Black holes as magnetic jet engines

(Nanowerk News) Super-massive black holes are found at the centers of many galaxies. But all efforts to detect them directly suffer from the fact that no information from their interior can reach us directly. Now, Professor Anton Zensus, director at the Max Planck Institute for Radio Astronomy in Bonn and founding chairman of the Event Horizon Telescope collaboration (EHT), has proposed an independent method to filter out those explanations from the existing ones that are viable.
At the same time, this method may succeed in explaining the high-energy plasma outflows ejected by many black holes. The project is funded with an ERC Advanced Grant worth 2.5 million euros.
The goal of the new method is to precisely observe the magnetic fields in the immediate vicinity of a black hole's event horizon, the distance beyond which light can no longer escape.
"This region is of key importance because it reflects the fundamental physics of a black hole. If we learn more about this region, we will be able to explain how the already observed plasma outflows, also called jets, are formed" says Anton Zensus. "If we understand how magnetic fields behave in the immediate vicinity of a black hole, we will have found an explanation for how the black hole drives these jets. At the same time, we will be able to rule out some alternative explanations of black holes."
Magnetic fields at the central regions of active galactic nuclei
Magnetic fields at the central regions of active galactic nuclei. At the left, polarised emission from the black hole in Messier 87. Middle: simulations of the photosphere around the central black hole and the jet in Messier 87. Right: artist’s view from the central engine in black holes.(Images: Eduardo Ros (EHT Collaboration 2021; Nakamura 2020; Tchekhovskoy 2015))
Active galactic nuclei are the most extreme sources of energy in the universe. They often outshine even the entire galaxy in which they reside. Rotating supermassive black holes, their surrounding rotating gas disks, relativistic plasma jets, and magnetic fields are thought to play a central role in generating these energies and transporting them outward. However, the details of this process are still unexplored.
Relativistic plasma jets have already been imaged, through the interconnection of radio telescopes across the globe. The Event Horizon Telescope (EHT) collaboration has also succeeded in imaging the silhouette of the putative supermassive black hole in the galaxy M 87 with sufficient sensitivity and resolution to confirm theoretical predictions from general relativity.
However, to definitively prove the existence of black holes and determine their physical properties, accurate information about the magnetic field near the event horizon is required. Observing this region precisely is a major challenge that cannot be realized by improvements in EHT imaging alone. Therefore, Anton Zensus, together with researchers at the Max Planck Institute for Radio Astronomy, has developed a novel approach.
The goal of his project, now funded by the ERC with the acronym M2FINDERS, is to further develop technical and radio astronomical methods so that magnetic fields in the vicinity of black holes can be precisely mapped.
In active galactic nuclei, where black holes are suspected, the polarization of the observable radio radiation is to be determined. Polarized means radiation that oscillates in a certain plane, like a guitar string. The effect of polarization is used, for example, by some sunglasses to filter out unwanted reflections.
Polarization observed in astronomical objects is a sure indication of magnetic fields. Magnetic fields determined by more advanced technologies will be combined with novel methods for image analysis and modeling of relativistic flows. This will lead to precise information about the strength and structure of the magnetic field near the event horizon and provide crucial independent evidence for the existence of black holes and their event horizons, which should complement EHT imaging.
Source: Max Planck Institute for Radio Astronomy
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