Astronomers Unveil Strong Magnetic Fields Spiraling at the Edge of Milky Way’s Central Black Hole

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*. The results were published today in The Astrophysical Journal Letters.

 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. 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*.

 “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,” said Sara Issaoun, NASA Hubble Fellowship Program Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian and 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.”

A view of the Milky Way supermassive black hole Sagittarius A* in polarized light. The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of our Milky Way black hole released in 2022, has captured a new view of the massive object at the center of our Galaxy: how it looks in polarized light. This is the first time astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of Sagittarius A*. This image shows the polarized view of the Milky Way black hole. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. Credit: EHT Collaboration

 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.

 “By imaging polarized light from hot glowing gas near black holes, we are directly inferring the structure and strength of the magnetic fields that thread the flow of gas and matter that the black hole feeds on and ejects,” said Harvard Black Hole Initiative Fellow and project co-lead Angelo Ricarte. “Polarized light teaches us a lot more about the astrophysics, the properties of the gas, and mechanisms that take place as a black hole feeds.” 

A DYNAMICALLY CHANGING BLACK HOLE

But imaging black holes in polarized light isn’t as easy as putting on a pair of polarized sunglasses, and this is particularly true of Sgr A*, which is changing so fast that it doesn’t sit still for pictures. Imaging the supermassive black hole requires sophisticated tools above and beyond those previously used for capturing M87*, a much steadier target. “Imaging black holes in polarized light unveils far more than what meets the eye, akin to finally reading the story inside a book after only ever seeing its cover”, comments José L. Gómez, Vice-Chair of the EHT Science Council and leader of the EHT group at the Instituto de Astrofísica de Andalucía. “However, because SgrA* is in constant motion, crafting even the unpolarized image proved to be an immense challenge. We were, therefore, relieved to find that obtaining a polarized image was indeed possible, allowing us a first-time glimpse into the magnetic fields swirling around our galaxy’s central black hole”, adds Gómez.

 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.

“The polarization pattern offers a pivotal opportunity to unravel the mysteries of spacetime structure and explore the existence of a faint jet near Sgr A*, addressing a question that has challenged scientists for years.” said Kotaro Moriyama, a postdoctoral researcher in IAA-CSIC and one of the EHT Imaging Working  Group coordinators. “This milestone will significantly impact the improvement of theoretical models and simulations, as well as future EHT observation plans, aiding our deeper understanding of the behavior of matter in the vicinity of black holes.”, adds Moriyama.

The EHT has conducted several observations since 2017 and is scheduled to observe Sgr A* again in April 2024. Each year, the images improve as the EHT incorporates new telescopes, larger bandwidth, and new observing frequencies. Planned expansions for the next decade will enable high-fidelity movies of Sgr A*, may reveal a hidden jet, and could allow astronomers to observe similar polarization features in other black holes. Meanwhile, extending the EHT into space will provide sharper images of black holes than ever before.

A key development for the EHT is the addition of a new antenna at the Teide Observatory in the Canary Islands, a project spearheaded by the Instituto de Astrofísica de Andalucía. This addition is set to significantly improve the array's sensitivity, which is crucial for producing the first-ever movies of black holes. Furthermore, the IAA team is at the forefront of developing cutting-edge algorithms that are essential for the production of these groundbreaking black hole movies. This initiative was recently spotlighted at our inaugural workshop on the topic, held in Granada last February, marking a pivotal step towards realizing this ambitious goal.