Back in April 2019, news sites around the world carried pictures of a strange donut of light with headlines stating astronomers had imaged the event horizon of a black hole for the first time. That supermassive black hole is in the heart of the M87 galaxy, located 55 million ly away, and it was imaged by the Event Horizon Telescope, which was actually a massive network of radio telescopes scattered around the world that worked together to image this gravitationally distorted light. In the years since that release, the team has continued to process the data and to obtain even more data, and today we learned that their images allow them to see the effects of the black hole’s magnetic field. According to team member Angelo Ricarte: One of the main science drivers of the EHT is distinguishing different magnetic field configurations around the black hole.

To trace M87’s magnetic field, the team looked at how much light they received that was oriented in different ways. If you’ve ever tried looking at the LCD screen on a camera with polarized sunglasses, you may have noticed the picture completely goes black when you rotate the camera. This is because polarized glasses only allow light that is aligned in a certain direction to pass through. 

Lightwaves are traveling in a specific direction, and their wave component is aligned in a specific way in relation to that direction. Filters, like those in your polarized sunglasses or in a polarized light camera filter, can reveal how the light gets aligned as it scatters off the dust, clouds, and road in front of you, and are in fact designed to reduce the glare of scattered light off the road. The eight telescopes involved in this work, which is published in a trio of papers in The Astrophysical Journal Letters, looked at how magnetic field lines aligned the light as seen via the lights’ polarization.

In the image, which you can see on our website, you can see lines marking where polarized light indicates magnetic field lines are present. According to Andrew Chael: The newly published polarised images are key to understanding how the magnetic field allows the black hole to ‘eat’ matter and launch powerful jets.

Jason Dexter adds: 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. Only the gas that slips through the field can spiral inwards to the event horizon.

This work used data that is now several years old, and with every passing season, our technology improves, and EHT collaboration members are already hoping to repeat these observations with this new tech and with additional telescopes. According to Jongho Park: The EHT is making rapid advancements, with technological upgrades being done to the network and new observatories being added. We expect future EHT observations to reveal more accurately the magnetic field structure around the black hole and to tell us more about the physics of the hot gas in this region.

The thing I personally find most pleasing about this research is that, so far, there are no real surprises. Models that include relativity had been able to accurately predict how the light would be bent and shaped in this enormous gravitational field, and now we’re seeing the details of a magnetic field we thought had to be there all along. As much as we may struggle with theories like planet formation and the stars they formed around, black holes make sense. Isn’t science weird sometimes?

More Information

Center for Astrophysics | Harvard & Smithsonian press release

ESO press release

Event Horizon Telescope press release

NRAO press release

University of Massachusetts Amherst press release

University of the Witwatersrand, Johannesburg press release

“First M87 Event Horizon Telescope Results. VII. Polarization of the Ring,” The Event Horizon Telescope Collaboration et al., 2021 March 24, The Astrophysical Journal Letters