| Mar 27, 2024 |
Spiral magnetic fields surround black hole Sagittarius A*
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(Nanowerk News) A new image from the Event Horizon Telescope (EHT) collaboration has uncovered strong and organized magnetic fields spiraling from the edge of the supermassive black hole Sagittarius A* (Sgr A*). Seen in polarized light for the first time, this new view of the monster lurking at the heart of the Milky Way Galaxy has revealed a magnetic field structure strikingly similar to that of the black hole at the center of the M87 galaxy, suggesting that strong magnetic fields may be common to all black holes. This similarity also hints toward a hidden jet in Sgr A*.
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Scientists unveiled the first image of Sgr A*— which is approximately 27,000 light-years away from Earth— in 2022, revealing that while the Milky Way’s supermassive black hole is more than a thousand times smaller and less massive than M87’s, it looks remarkably similar. This made scientists wonder whether the two shared common traits outside of their looks. To find out, the team decided to study Sgr A* in polarized light.
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| Event Horizon Telescope: new view of the supermassive black hole at the center of our Galaxy in polarized light. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. (Image: The Event Horizon Telescope Collaboration)
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Previous studies of light around M87* revealed that the magnetic fields around the black hole giant allowed it to launch powerful jets of material back into the surrounding environment. Building on this work, the new images have revealed that the same may be true for Sgr A*.
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“What we’re seeing now is that there are strong, twisted, and organized magnetic fields near the black hole at the center of the Milky Way galaxy,” says Sara Issaoun from the Harvard Center for Astrophysics, co-lead of the project. “Along with Sgr A* having a strikingly similar polarization structure to that seen in the much larger and more powerful M87* black hole, we’ve learned that strong and ordered magnetic fields are critical to how black holes interact with the gas and matter around them.”
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Light is an oscillating, or moving, electromagnetic wave that allows us to see objects. Sometimes, light oscillates in a preferred orientation, and we call it “polarized.” Although polarized light surrounds us, to human eyes it is indistinguishable from “normal” light. In the plasma around these black holes, particles whirling around magnetic field lines impart a polarization pattern perpendicular to the field. This allows astronomers to see in increasingly vivid detail what’s happening in black hole regions and map their magnetic field lines.
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In addition to the overall intensity, the polarisation information in the light reveals much more about the astrophysics, the properties of the gas, and the mechanisms that occur as a black hole is fed. By imaging polarised light from hot, glowing gas near black holes, astronomers can directly infer the structure and strength of the magnetic fields that govern the flow of gas and matter that the black hole ingests and ejects.
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"Visualising black holes in polarised light isn't as easy as putting on a pair of polarised sunglasses. This is especially true for Sgr A*, which exhibits dynamic changes at a rapid rate, defying stillness for imaging purposes. Navigating through these fluctuations was a considerable challenge, but in the end we prevailed," says Maciek Wielgus from MPIfR. Capturing images of the supermassive black hole requires state-of-the-art tools beyond those used for the more stable M87*. Given the elusive nature of Sgr A*, its variable nature complicates the construction of even an unpolarised image. As a result, the initial image was created by stitching together multiple snapshots to account for Sgr A*'s motion. Wielgus explains: "Certain models proved too chaotic and turbulent to decipher a polarised image, but nature proved less harsh and allowed us to succeed."
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Complementary observations with the Global mm-VLBI Array (GMVA), observing at a wavelength of 3.5 mm (cf. 1.3 mm for the EHT), provided an important contribution to the work presented here. Eduardo Ros, also from MPIfR, says: "The first step towards lifting the veil on the Galactic Centre were our GMVA observations together with ALMA, presented in January 2019. We continue to construct, brick by brick, the building of our understanding of black hole physics around supermassive black holes."
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Scientists are excited to have images of both supermassive black holes in polarized light because these images, and the data that come with them, provide new ways to compare and contrast black holes of different sizes and masses. As technology improves, the images are likely to reveal even more secrets of black holes and their similarities or differences.
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Christian M. Fromm, from the University of Würzburg says: "The striking similarity between the magnetic field structure of M87* and that of Sgr A* is remarkable because it raises the possibility that, despite differences in mass, size and environment, the physical mechanisms governing the feeding and jet ejection of a black hole may be common to all supermassive black holes". He continues: "We can now use this result to improve theoretical models and simulations, leading to a better understanding of how matter is affected near a black hole's event horizon."
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The EHT has made several observations since 2017, and is scheduled to observe Sgr A* again in April 2024. Each year the images improve as the EHT adds new telescopes, wider bandwidths and new observation frequencies. Planned upgrades over the next decade will allow high-fidelity movies of Sgr A*, potentially revealing a hidden jet and allowing astronomers to observe similar polarisation features in other black holes. Meanwhile, extending the EHT into space will provide sharper images of black holes than ever before.
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To better understand the physical processes behind the jets in active galactic nuclei and the role of magnetic fields in the vicinity of the central, powering black holes, the European Research Council awarded the M2FINDERS project to J. Anton Zensus, Director at the MPIfR. He says: "The revelation of these magnetic fields opens a window into the innermost regions of Sgr A*, where the interplay of gravity, magnetism and spacetime curvature reaches its zenith. As we delve deeper into this cosmic enigma, we expect further breakthroughs that will illuminate the fundamental nature of black holes and their influence on galactic ecosystems. An important step in understanding the inner workings of these extreme cosmic objects has been the iconic image of the black hole silhouette in Messier 87 and in the Galactic Centre, complemented today by the image of polarised light in the latter." He concludes: "Our M2FINDERS project is also a major contributor to the EHT effort since 2022.”
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Original publication
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The Event Horizon Telescope Collaboration: First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring, 2024, The Astrophysical Journal Letters
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The Event Horizon Telescope Collaboration: First Sagittarius A* Event Horizon Telescope Results. VIII. Physical interpretation of the polarized ring, 2024, The Astrophysical Journal Letters
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