Magnetics Business & Technology - January/February 2025 - 18

RESEARCH & DEVELOPMENT
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 organised magnetic fields spiraling from the edge
of the supermassive black hole Sagittarius A* (Sgr A*). Seen in polarised 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 centre 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 in The Astrophysical Journal Letters.
In 2022 scientists unveiled the first image of Sgr A* at press conferences
around the world, including at the European Southern
Observatory (ESO). While the Milky Way's supermassive black
hole, which is roughly 27 000 light-years away from Earth, is more
than a thousand times smaller and less massive than M87's, the
first-ever black hole imaged, the observations revealed that the
two look 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 polarised light. Previous studies
of light around the M87 black hole (M87*) revealed that the
magnetic fields around it allowed the black hole 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
organised magnetic fields near the black hole at the centre of the
Milky Way galaxy, " said Sara Issaoun, NASA Hubble Fellowship
Program Einstein Fellow at the Center for Astrophysics | Harvard
& Smithsonian, US, and co-lead of the project. " Along with Sgr A*
having a strikingly similar polarisation 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. "
Light is an oscillating, or moving, electromagnetic wave that allows
us to see objects. Sometimes, light oscillates in a preferred
18 Magnetics Business & Technology * January/February 2025
orientation, and we call it 'polarised'. Although polarised 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 polarisation 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 polarised 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. " Polarised light teaches us
a lot more about the astrophysics, the properties of the gas, and
mechanisms that take place as a black hole feeds. "
But imaging black holes in polarised light isn't as easy as putting
on a pair of polarised 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. EHT Project Scientist Geoffrey Bower from
the Institute of Astronomy and Astrophysics, Academia Sinica,
Taipei said, " Because Sgr A* moves around while we try to take
its picture, it was difficult to construct even the unpolarised image, "
adding that the first image was an average of multiple images
owing to Sgr A*'s movement. " We were relieved that polarised
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Magnetics Business & Technology - January/February 2025

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