The Event Horizon Telescope (EHT) has just snapped a new image of the massive object lurking at the centre of the Messier 87 (M87) galaxy – this time, in polarised light. Astronomers think this is exciting stuff, because it’s the first time that polarisation – a signature of magnetic fields – has been measured so close to the edge of a black hole.
“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy,” says Monika Mościbrodzka, co-author of the new research and a researcher at Radboud University in the Netherlands.
Back in April 2019, the EHT revealed the first direct visual evidence of a supermassive black hole and its shadow – one of the biggest scientific achievements of recent times. But it wasn’t a point-and-shoot endeavour. The EHT is in fact a massive collaboration, linking precisely timed data from eight ground-based observatories around the world to create a virtual Earth-sized telescope.
The first image, also of M87, was obtained after decades of research, showing the disk of the black hole eclipsing the background glow of high-energy radiation behind it.
Since then, astronomers have realised that a significant amount of the light around the black hole is polarised. You might be most familiar with the concept of polarisation from your sunglasses, where the lenses filter light to reduce glare. In a similar way, magnetic fields can act to polarise the light emitted from a black hole. Measuring polarisation can help astronomers map the magnetic field lines and thus obtain a more accurate view of the region around M87.
Co-author Iván Martí-Vidal, from the University of Valencia in Spain, explains: “The polarisation of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before.”
He adds that “unveiling this new polarised-light image required years of work due to the complex techniques involved in obtaining and analysing the data”.
Astronomers are trying to figure out how the black hole consumes some matter while also launching other matter and energy back out, in incredibly powerful jets that extend out for more than 5,000 light-years. Understanding the magnetic fields will be a key part of this.
This new image, which captures the region just outside the black hole where matter is flowing in and out, can help the team better study these mysterious processes. The observations also reveal that this region is likely filled with strongly magnetised gas.
“The observations suggest that the magnetic fields at the black hole’s edge are strong enough to push back on the hot gas and help it resist gravity’s pull,” explains collaborator Jason Dexter, from the University of Colorado Boulder in the US. “Only the gas that slips through the field can spiral inwards to the event horizon.”
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Lauren Fuge is a science journalist at Cosmos. She holds a BSc in physics from the University of Adelaide and a BA in English and creative writing from Flinders University.
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