Intense spiral magnetic fields revealed at the edge of the black hole at the centre of our galaxy

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Sagittarius A* supermassive black hole in polarised light.
Sagittarius A* supermassive black hole in polarised light.
The Event Horizon Telescope (EHT) collaboration has released the first polarised light image of SgrA*, the supermassive black hole at the centre of our galaxy. This image reveals the presence of strong, organised magnetic fields that emerge in a spiral pattern from the very edge of the black hole.

This structure is similar to that observed in the central black hole of the galaxy M87, suggesting that such strong magnetic fields may be common to all black holes and pointing to the possible existence of a hidden jet in SgrA*, like the one in M87*. These results were published on Wednesday in The Astrophysical Journal Letters.

In May 2022 the Event Horizon Telescope (EHT) collaboration presented the first image of Sgr A*, the supermassive black hole at the centre of our galaxy, about 27 000 light-years from Earth. That image looked surprisingly similar to the black hole in the galaxy M87, despite being more than a thousand times smaller and less massive than it. Now, the EHT has just released the polarised version of the SgrA* image. Previous studies of M87* in this type of light had confirmed the presence of intense and organised magnetic fields, associated with the emission of powerful jets of material at near-light speeds. Based on this work, the new images of SgrA* have revealed that the same thing may be happening at the centre of our galaxy.

"What we are now observing is the presence of intense, twisted and organised magnetic fields near the black hole at the centre of the Milky Way," says Sara Issaoun, a NASA Einstein Hubble Scholarship Programme researcher at the Centre for Astrophysics/Harvard and Smithsonian, and co-leader of the project. "The fact that Sgr A* exhibits a polarisation structure strikingly similar to that of a much larger and more powerful black hole like M87* has allowed us to deduce that intense and organised magnetic fields play a key role in the interaction of black holes with the gas and matter around them," she added.

Polarised light traces magnetic fields

Light is an electromagnetic wave that sometimes oscillates in a preferred direction. This is when we speak of "polarised light". Although this type of light is common in our everyday lives (from polarised sunglasses or cameras to 3D cinema systems), to human eyes it is indistinguishable from non-polarised light. When there is a strong magnetic field, the plasma particles surrounding black holes emit radiation with a polarisation pattern perpendicular to the field. This makes it possible to reconstruct the magnetic structure in these regions and to observe in detail what is happening in the vicinity of the black holes.

"Polarised light images of hot glowing gas near black holes allow us to directly deduce the structure and strength of the magnetic fields that pass through the flow of gas and matter that feeds the black hole, as well as that which it ejects," says Angelo Ricarte, a researcher at Harvard University’s Black Hole Initiative Institute, and co-leader of the project. "Polarised light offers us valuable insights into the astrophysics, the properties of the gas and the mechanisms that occur as a black hole feeds."

Dynamically changing black hole

But imaging black holes with polarised light is not as easy as putting on polarised sunglasses. The technology behind it has been decades in the making, and finally, in this decade, we are starting to reap the rewards. "We have had to develop pioneering algorithms to recover the faint polarised signal from these black holes. From the Universitat de València, we have provided fundamental calibration data for the analysis of these observations; data that have also helped us to detect the polarised reflection of matter orbiting the black hole" [], says Iván Martí Vidal, Associate Professor in the Department of Astronomy and Astrophysics at the Universitat de València and member of the EHT.

Universal black holes

Having images and data of both supermassive black holes in unpolarised light opens up new opportunities to compare and contrast black holes of different sizes and masses. As technology advances, these images are likely to reveal even more secrets about the black holes and their possible similarities or differences.

"We have long been looking for the possible jet of matter emanating from our galactic centre. This new polarised image tells us that the jet should be there, but we don’t see it yet. It is an intriguing question that remains to be clarified," says Alejandro Mus, PhD in Physics from the Universitat de València and member of the EHT.

The EHT has carried out 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, increased bandwidth and new observing frequencies. The expansions planned for the next decade will provide highly reliable movies of Sgr A*, which could reveal the presence of a hidden jet and allow astronomers to observe similar polarisation features in other black holes. Meanwhile, the extension of the EHT into space will provide sharper images of black holes than ever before.

300 researcheres

The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, and North and South America. This international effort aims to capture images of black holes at an unprecedented level of detail by creating a virtual telescope the size of the Earth. Backed by considerable international investment, the EHT connects existing telescopes through innovative systems, resulting in a completely new instrument with the highest angular resolution power yet achieved.

The telescopes involved in the EHT are ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NoeMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope, and the Greenland Telescope (GLT). The data were correlated at the Max-Planck-Institut für Radioastronomie (MPIfR) and the MIT Haystack Observatory. The post-processing was carried out within the collaboration by an international team at different institutions, with a very prominent participation of the Astrophysics’ Institute of Andalucía (CSIC).

The EHT consortium includes thirteen interested institutions, as well as many other research institutes around the world, including the IAA-CSIC: Academia Sinica Institute for Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the East Asia Observatory, the Goethe University Frankfurt, the Institute for Millimeter Radio Astronomy, the Large Millimeter Telescope, the Max Planck Institute for Radio Astronomy, the MIT Haystack Observatory, the National Astronomical Observatory of Japan, the Perimeter Institute for Theoretical Physics, Radboud University and the Smithsonian Astrophysical Observatory.


This research was presented in two papers by the EHT collaboration published today in The Astrophysical Journal Letters: "First Sagittarius A* Event Horizon Telescope Results. VII. Polarization of the Ring and "First Sagittarius A* Event Horizon Telescope Results. VIII. VIII. Physical Interpretation of the Polarized Ring­ticle/10.3­847/2041-8­213/ad2df0­ticle/10.3­847/2041-8­213/ad2df1