Astronomers Have Imaged The Magnetic Field At The Edge Of A Black Hole For The First Time

Polarization lines added to the original image of M87's supermassive black hole Image Credit: EHT Collaboration

The incredible first-ever picture of the shadow of a black hole was published almost two years ago. Now, researchers from the Event Horizon Telescope (EHT) collaboration who produced it have added new and important details to that historic image. For the first time, they have measured the magnetic field around the edge of a black hole.

In two papers in The Astrophysical Journal Letters (here and here), astronomers discovered a significant portion of the light around galaxy M87's supermassive black hole is polarized, and describe how they used the polarization of light to study its magnetic field. These observations can help astronomers understand how phenomena like cosmic jets form.

Light is a wave that usually wiggles in all directions. Polarized light waves are light waves whose vibrations occur on a single plane. Light becomes polarized when it goes through certain filters, like sunglasses, but also when it's emitted from hot regions of space where magnetic fields reside.

Polarized light has lots of technological applications, for example, when you watch a 3D movie, the glasses you wear have polarized lenses so that the right eye and the left eye are actually seeing different images, which our brain interprets as three-dimensional. The basic idea is that light is made of electromagnetic waves and if we are looking at a cross-section of these waves, the oscillation can happen at every angle.

But when light is polarized, it is only oscillating in a particular direction. In nature, interactions between light and matter can influence this direction, and magnetic fields are among these interactions. Studying polarization is far from easy but it is very important.  

“This work is a major milestone: 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,” explained Dr Iván Martí-Vidal, one of the coordinators of the EHT Polarimetry Working Group, in a statement. “[U]nveiling this new polarised-light image required years of work due to the complex techniques involved in obtaining and analysing the data.”

polarization oh M87 jet and black hole
Polarization of M87's black hole and jets with a comparison to the scales and the different observatories used to study these regions. Image Credit: EHT Collaboration; ALMA (ESO/NAOJ/NRAO), Goddi et al.; NASA, ESA and the Hubble Heritage Team (STScI/AURA); VLBA (NRAO), Kravchenko et al.; J. C. Algaba, I. Martí-Vidal

Just like sunglasses, polarized light helps reduce glare from bright light sources. This allowed the researchers to have a sharper view of the inner edge of the black hole, and to map the magnetic field lines present there.

The team estimates that up to 666 Earth’s worth of mass falls into the black hole every year. However, some of these particles manage to escape and are blown into space in the form of jets. The observations suggest that the magnetic field lines around the black hole are strong enough to push some of the gas falling into the black hole back, helping it escape the black hole's gravity. 

“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,” Professor Monika Mościbrodzka, coordinator of the EHT Polarimetry Working Group, said.


The Event Horizon Telescope connects radio dishes from across the surface of the planet to act as a single Earth-sized radio observatory. The next iteration of the EHT will include even more telescopes.

Among the observatories involved in the original observations is the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The radio observatory was crucial to the signal collection in polarized light, as well as studying the jet of M87, which extends for 5,000 light-years. 


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